Julia Sell - March 8, 2022
Dissertation Title: EXCITED STATES IN MONOLAYER TRANSITION METAL DICHALCOGENIDES
Date and Time: Tuesday, March 8, 8:00 am
Location: PSC 2136 and Zoom
Dissertation Committee Chair: Mohammad Hafezi
Committee:
Sunil Mittal
Glenn Solomon
Ichiro Takeuchi
You Zhou
Abstract:
Monolayer two-dimensional transition metal dichalcogenides (2D TMDs) represent a class of atomically thin semiconductors with unique optical properties. Similar to graphene, but with a three-layer (staggered) honeycomb lattice, TMDs host direct-gap transitions at their ±K valleys that exhibit circular-dichroism due to their finite Berry curvature. The reduced dimensionality of materials in this system, combined with large effective carrier masses, leads to enhanced Coulomb interaction and extremely tightly bound excitons (EB ≈ 150-300 meV). Here, we seek to exploit the unusually tight binding of the excitons to probe two different types of higher energy exciton species in TMDs.
First, we experimentally probe the magneto-optical properties of 2s Rydberg exciton species in WSe2. The magnetic response of excitons gives information on their spin and valley configurations, nuanced carrier interactions, and insight into the underlying band structure. Recently, there have been several reports of 2s/3s charged excitons in TMDs, but very little is still known about their response to external magnetic fields. Using photoluminescence excitation spectroscopy, we verify the 2s charged exciton and report for the first time its response to an applied magnetic field. We benchmark this response against the neutral exciton and find that both the 2s neutral and charged excitons exhibit similar behavior with $g$-factors of and , respectively.
Second, via theoretical calculations, we investigate the exciton spectrum generated in 2D semiconductors under illumination by twisted light. Twisted light carries orbital angular momentum (OAM) which can act as an additional tunable degree of freedom in the system. We demonstrate that twisted light does not have the ability to modify the exciton spectrum and induce dipole-forbidden excitons, in contrast to atoms. This result stems from the fact that the additional OAM is transferred preferentially to the center-of-mass (COM) of the exciton, without modifying the relative coordinate which would allow dipole-forbidden higher energy excitons to form.
Abby Bull - February 14, 2022
Dissertation Title: Integration and competition in immune cell models
Date and Time: Monday, February 14, 10:00 am
Location: PSC 3150
Dissertation Committee Chair: Wolfgang Losert
Committee:
Kimberly Stroka (Dean's Representative)
Michelle Girvan
Arpita Upadhyaya
Edward Ott
Abstract:
Cell motility plays an integral role in most biological processes. One principle of motility is the protrusions and retractions of cellular membranes called pseudopods. The physical force behind pseudopod formation is actin polymerization. As the physical driver, actin polymerization integrates the cells' upstream biochemical signaling cascades and turns the signals into action as the combined output and readout of the state of the cell. Actin polymerization not only occurs within pseudopods but propagates throughout a cell in waves that can be understood and modeled as excitable media.
This dissertation focuses on actin wave dynamics in the context of directed cell migration using both experimental and numerical techniques. Directed cell migration is essential in physiological processes including embryonic development, cancer metastasis, and wound healing. In this thesis, I analyze how immune-like cells are guided by external stimuli that are common in wound environments: electric fields, chemical gradients, and surface texture. A focus of this work is on the emergent excitable wave behavior of actin, and the pseudopods they generate, in simplified, in vitro environments.
Actin waves have been previously shown to respond to and be guided by the topography of an underlying substrate. Additionally, static quantifications of actin filaments after electric field stimulation become asymmetrically distributed within the cytoplasm. In neutrophils, the combination of cues leads to higher control of cell motility by guiding the internal actin waves. Using an optical flow algorithm, I quantify the actin waves on multiple length scales to ascertain the role of each guidance cue in affecting cell motion. I find that the waves preferentially polymerize and travel along the nanoridge length. Actin waves nucleate preferentially on the cathode side and reorient the cells axis of polarity (i.e., the position of the dominant pseudopod).
The second example of competing guidance cues involves studying the collective motion of cells in response to cell-cell signal relay in competition with surface topography. We use Dictyostelium discoideum cells as a model system for this work, as they migrate collectively due to signal relay. The signal relay of these cells is similar to many immune cell species. Using a combination of image analysis tools and a coarse-grained stochastic model, we find that guidance by nanoridges overrides the chemical signal relay and forces cells to migrate individually, suppressing streaming behavior. We model both the secretion and propagation of chemical signals using an excitable systems framework. This work highlights that bidirectional signals can be effective at suppressing cell-cell attraction and streaming motion.
The response of immune cells to external stimuli in the wound environment is not universal. Macrophages---one of the largest immune cells---are observed to migrate in the opposite direction of the wound upon wound-induced electric field generation. In the third example, I study actin dynamics of M0 (resting) macrophage cells to elucidate the method in which these cells interact with external electric fields. This cell system exhibits oscillatory actin waves at rest. With electric field stimulation, the oscillatory actin waves start to generate protrusions. Without oscillations, the protrusions begin with actin-depleted regions, indicating that contractile elements are involved. This section highlights the different methods in which actin waves integrate external cues into cell responses in different biological contexts.
Hwan Mun Kim - November 11, 2021
Dissertation Title: Engineering Topological Quantum Matter with Patterned Light
Date and Time: Thursday, November 11, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Mohammad Hafezi
Committee:
Maissam Barkeshli
Michael Gullans
Trey Porto
Ronald Walsworth
John Cumings
Abstract:
Topological phases are intriguing phases of matter which cannot be described with traditional theories of the phase transition such as Ginzburg-Landau theory, and numerous efforts has been put to achieve these exotic phases of matter in a variety of quantum platforms. In this thesis, we discuss how topological quantum states of matter can be engineered by utilizing spatially patterned light, which has become available thanks to the recent advances in beam shaping techniques.
First, we discuss a scheme to construct an optical lattice to confine ultracold atoms on the surface of torus. We investigate the feasibility of this construction with numerical calculations as well as propose a supercurrent generation experiment to verify the non-trivial topology of the created surface. We also propose a scheme to construct fractional quantum Hall states which can demonstrate topological degeneracy. We then extend our effort for creation of topologically non-trivial surfaces for ultracold atoms to the surfaces with open boundaries, using a bilayer optical lattice with multiple pairs of twist defects. We explain how a spin-dependent optical lattice can serve as the bilayer optical lattice for this purpose as well as the scheme for preparing and optically manipulating the fractional quantum Hall states.
Then we turn our attention to driven electronic systems. In particular, we investigate a way to imprint the superlattice structure in two-dimensional electronic systems by shining circularly-polarized light. We demonstrate the wide optical tunability of this system allows to a variety of band properties. We show that these tunable band properties lead to exotic physics ranging from the topological transitions to the creation of nearly flat bands, which can allow a realization of strongly correlated phenomena in Floquet systems. We then investigate the Floqut vortex states created by shining light carrying non-zero orbital angular momentum on a 2D semiconductor. We analytically and numerically study the properties of those vortex states and show that such Floquet vortex states exhibit a wide range of tunability and illustrate the potential utility of such tunability with an example of application in quantum state engineering.
Zackery A. Benson - November 9, 2021
Dissertation Title: Reversibility, memory formation, and collective rotations in dense granular media
Date and Time: Tuesday, November 9, 10:00 am
Location: PSC 1136
Dissertation Committee Chair: Professor Wolfgang Losert
Committee:
Prof. Derek C. Richardson
Prof. Michelle Girvan
Prof. Nicole Yunger Halpern
Prof Christine Hartzell, Dean’s Representative
Abstract:
Granular matter is a broad term that describes materials comprised of macroscopic grains. Granular material has unique properties that can mimic either a solid- or fluid-like system and has macroscopic behaviors such as segregation, shear- jamming, reversibility, and compaction. The finite size of the grains implies that tools developed in thermodynamics cannot be readily applied. Instead, research into granular material uses a combination of bulk measurements (density, pressure) with grain-scale tracking of position, orientation, and forces. This thesis presents four main studies utilizing three-dimensional experiments and simulations to probe the dynamics of individual grain subject to cyclic compression.
The first study uses numerical simulations to connect granular rotations to translations in sheared granular packings. It is proposed that rotations play an extensive roll in the formation of shear zones in granular packing, in which the rotations allow for ball-bearing like motion that could reduce the stress from external pressures. In this study, we quantify the effect of friction on the shear-zone rotations and translations. We find a direct connection between average rotations and the vorticity of translations independent of the friction. The second study explores reversibility of grain translations and rotations in the context of memory formation. In granular matter, memory is formed via a response to an external perturbation, ranging from compression and shear to thermal cycling. In this project, we encode and readout memory of compression amplitudes for a cyclically driven granular system. We find that memory is significantly affected by the inter particle friction of each grain and is most readily extracted by quantifying reversible displacements within the sample.
The third study experimentally measures three-dimensional orientations of granular spheres using our refractive index matched scanning setup. We apply a combination of deep learning and image processing to extract the position and orientation of individual grains subject to cyclic compression. Using this, we can quantify the spatial distribution of sliding and rolling motion of contacts. We find that sliding occur deep within the sample where the grains are mostly fixed in place. This occurs with an increase in internal stress within the material.
Finally, we explore collectively rotating states in three-dimensions. We introduce a new measure in which we can identify affine (collective) and non-affine rotations. We find that grain rotations are generated by minimizing sliding motion between all contacts independent of the forces between each contact. Further, we find that the affine component is more directly correlated to irreversible translations than the non-affine. This result identifies that collective rotations are important to reversible states in sheared granular systems.
Jacob Ward - November 5, 2021
Dissertation Title: Experimental Atomic Spectroscopy of Iron Group Elements for Astrophysics
Date and Time: Friday, November 5, 3:00 pm
Location: PSC 2204
Dissertation Committee Chair: Steve Rolston
Committee:
Gillian Nave
Trey Porto
Ronald Walsworth
Wendell Hill
Abstract:
The quality of modern astrophysical spectra has made it clear that there is a lack of sufficiently accurate and robust laboratory atomic reference data sets. Particularly for spectra of the iron-group elements, the growing demand for critically evaluated sets of comprehensive atomic data is a direct result of advancing stellar astrophysics models and fundamental physics problems probing beyond the standard model. My thesis reports on my critical evaluation of the Ni V spectrum and the recent laboratory measurements I have conducted to improve the state of available reference data for astrophysical applications that rely on observations of Ni V. Additionally, I report my laboratory measurements of Fe II branching fraction values in the UV/VUV.
Using high-resolution grating spectroscopy at the National Institute of Standards and Technology, I carried out an analysis of quadruply ionized iron and nickel (Fe V & Ni V) in the vacuum ultraviolet (VUV) region by both recording new spectra and critically evaluating previously published data sets. My analysis resulted in highly accurate wavelengths, presented with calculated oscillator strengths, for roughly 1500 Ni V lines, 200 of which have uncertainties that are almost an order of magnitude lower than in previous publications. Additionally, I present over 300 Ni V energy levels derived from my evaluated wavelengths. My work also strongly supports the previous evaluations of Fe V by another author. With the extreme accuracy requirements of modern astrophysics problems, confirming the wavelength scale and uncertainty evaluation of previous Fe V data sets is still significant.
Additionally my thesis also presents measurements of singly ionized iron (Fe II) branching fractions (BFs) in the VUV using high-resolution Fourier-transform spectroscopy. BFs are essential values for interpreting complex astrophysical spectra, but are notoriously difficult to measure in the VUV; for this reason, VUV BFs of Fe II have only been reported by one other research group for just seven levels. My thesis reports accurate BFs for 10 Fe II levels, involving approximately 150 spectral lines, which roughly doubles the amount of reported Fe II BFs in UV/VUV.
Rodney A. Snyder - November 3, 2021
Dissertation Title: Electronic Transport in PbSnTe Josephson Junctions and ZrTe5 nanowires
Date and Time: Wednesday, November 3, 11:00 am EST
Location: Zoom
Dissertation Committee Chair: Dr. James Williams
Committee:
Dr. Gretchen Campbell
Dr. Alicia J. Kollar
Dr. Frederick C. Wellstood
Dr. Ichiro Takeuchi (Dean's Rep)
Abstract:
Studying topological states of matter offers potential routes to novel excitations with properties beyond that of simple electrons. While the range of materials and techniques for studying topological systems is constantly expanding, only a handful of systems have been thoroughly studied. Topological crystalline insulators (TCI) and the rich states present in ZrTe5 offer alternative routes to studying topological states of matter with surface states of distinct character to those in more common 3d topological insulators.
First, we report on the fabrication of Josephson junctions using MBE-grown candidate TCI material — Pb-doped SnTe — as weak links. We then perform transport characterization measurements on these devices including the effects of DC, RF, and magnetic field biases. Finally, we model these results using a skewed current phase relationship using the resistively shunted josephson junction model.
In the second section, we present the fabrication of devices from mechanically exfoliated ZrTe5 nanowires. After fabrication, we perform magnetoconductance measurements at various temperatures for these devices. We then use existing models for weak antilocalization and universal conductance fluctuations for a quasi-1D system in order to extract a coherence length for this system.
Swarnav Banik - October 27, 2021
Dissertation Title: ANALOGUE COSMOLOGY EXPERIMENTS WITH SODIUM BOSE EINSTEIN CONDENSATES
Date and Time: Wednesday, October 27, 3:00 pm
Location: PSC 1136
Dissertation Committee Chair: Professor Steven Rolston
Committee:
Dr. Gretchen Campbell (Co-Chair)
Dr. Ian Spielman
Professor Mohammad Hafezi
Professor Rajarshi Roy
Abstract:
Due to their high degree of controllability and precise measurement capabilities, ultracold ensembles of neutral atoms have emerged as a promising platform for performing quantum simulations. In this thesis, I will describe the design and construction of an analog quantum simulator based on $^{23}$Na Bose-Einstein Condensates (BEC). Our system can produce and trap BECs in arbitrary-shaped quasi two-dimensional optical dipole traps, which can be dynamically altered during an experimental sequence. Such controlled variation of the BEC's spatial mode enables exploration of open questions in superfluidity, atomtronics, and analogue cosmology. I will describe the implementation of our system to study the inflationary dynamics of the early universe and report our recent results on the simulation of Cosmological Hubble friction. We expand and contract a toroidally shaped BEC and analyze the time evolution of its collective phonon modes. These excitations are analogous to fluctuating scalar fields in an expanding universe. The changing metric of the expanding or contracting background BEC results in dilation of the phonon field through a term dependent on the expansion speed, similar to Hubble friction in inflationary models of the universe. We conclusively demonstrate the analogy by experimentally measuring Hubble attenuation and amplification. Our measured strength of Hubble friction disagrees with recent theoretical work [J. M. Gomez Llorente and J. Plata, Phys. Rev. A 100 043613 (2019) and S. Eckel and T. Jacobson, SciPost Phys. 10 64 (2021)], suggesting inadequacies in the current model.
Christie Trimble - October 21, 2021
Dissertation Title: On the nature of the Josephson effect in topologically nontrivial materials
Date and Time: Thursday, October 21, 12:00 pm
Location: PSC 2148
Dissertation Committee Chair: Prof. James Williams
Committee:
Prof. Alicia Kollàr
Prof. Johnpierre Paglione
Prof. Frederick Wellstood
Prof. Ichiro Takeuchi
Abstract:
A Josephson junction (JJ) couples the supercurrent flowing between two weakly linked superconductors to the phase difference between them via a current-phase relation (CPR). While a sinusoidal CPR is expected for conventional junctions with insulating weak links, devices made from some exotic materials may give rise to unconventional CPRs and unusual Josephson effects. Here, we experimentally investigate three such cases.
In the first part of the thesis, we fabricate JJs with weak links made of the topological crystalline insulator PbSnTe, and additional JJs with weak links made of its topologically trivial cousin, PbTe. In characterizing these junctions, we find that measurements of the AC Josephson effect reveal a stark difference between the two: while the PbTe JJs exhibit Shapiro steps at the expected values of V=nhf/2e, PbSnTe JJs show more complicated subharmonic structure. We present the skewed sinusoidal CPR necessary to reproduce these measurements, and discuss this alteration to the CPR as a consequence of 1D helical channels arising from the topologically nontrivial surface state.
In the second part of the thesis, we investigate the proximity-induced superconductivity in SnTe nanowires by incorporating them as weak links in Josephson junctions. We report on indications of an unexpected breaking of time-reversal symmetry in these devices, detailing the unconventional characteristics that reveal this behavior. These observations include an asymmetric critical current in the DC Josephson effect, a prominent second harmonic in the AC Josephson effect, and a magnetic diffraction pattern with a minimum in critical current at zero magnetic field. We analyze how multiband effects and the experimentally visualized ferroelectric domain walls may give rise to a nonstandard CPR, giving insight into the Josephson effect in materials that possess ferroelectricity and/or multiband superconductivity.
Finally, we fabricate and measure JJs with weak links made of the topological insulator (BiSb)2Te3. Under low frequency RF radiation, we observe suppression of the first and third Shapiro steps, consistent with the fractional AC Josephson effect. This could indicate a signature of 4π periodicity in the junction's CPR, which may in turn suggest the presence of Majorana bound states. However, not all of our measured devices confirm this observation, as some of them show suppression of only the first step, while still others show probabilistic distortions to the AC Josephson effect which might indicate other nonequilibrium effects at play. We discuss the inconsistency of our measurements with topologically trivial sources of step suppression that have been suggested in the literature.
Bin Cao - October 18, 2021
Dissertation Title: Photoexcitation of graphene in the quantum Hall regime
Date and Time: Monday, October 18, 9:00 am
Location: PSC 2136
Dissertation Committee Chair: Prof. Mohammad Hafezi
Committee:
Dr. Glenn Solomon
Dr. Johnpierre Paglione
Dr. Jay Deep Sau
Dr. Thomas E. Murphy, Dean’s Representative
Abstract:
Multipole transitions beyond the dipole approximation apply when the Bohr radius of the quantum state is larger or comparable to the excitation wavelength. This is rarely the case for atoms or quantum dots. However, in the quantum Hall regime, wave functions can be extended to a length scale comparable to optical wavelengths, and the coherence is topologically protected against dephasing. Consequently, multipole transitions become possible. Motivated by this, we study the light-matter interaction in graphene in the quantum Hall regime, manifested as the photocurrent (PC).
In the first part of the thesis, we experimentally study the PC in graphene in the quantum Hall regime. Prominent PC oscillations as a function of gate voltage on samples’ edges are observed with minimal obscurations and noise. These oscillation amplitudes form an envelope which depends on the strength of the magnetic field, as does the PCs’ power dependence and their saturation behavior. We explain these experimental observations through a model using optical Bloch equations, incorporating relaxations through acoustic-, optical-phonons and Coulomb interactions. The simulated PC agrees with our experimental results, leading to a unified understanding of the chiral PC in graphene at various magnetic field strengths, and providing hints for the occurrence of a sizable carrier multiplication.
In the second part, we theoretically study the light-matter interaction beyond dipole, manifested as a PC. Inspired by the seminal gedankenexperiment by Laughlin which describes the charge transport in quantum Hall systems via the pumping of flux, we propose an optical scheme which probes and manipulates quantum Hall systems in a similar way: When light containing orbital angular momentum interacts with electronic Landau levels, it acts as a flux pump which radially moves the electrons through the sample. We investigate this effect for a graphene system with Corbino geometry, and calculate the radial current in the absence of any electric potential bias. Remarkably, the current is robust against the disorder, and in the weak excitation limit, the current shows a power-law scaling with intensity characterized by the novel exponent 2/3.
Linus Feder - August 13, 2021
Dissertation Title: LASER WAKEFIELD ACCELERATION IN OPTICAL FIELD-IONIZED PLASMA WAVEGUIDES
Date and Time: Friday, August 13, 11:00 am
Location: Zoom
Dissertation Committee Chair: Howard Milchberg
Committee:
Drew Baden
Ki-Yong Kim
Giuliano Scarcelli
Phillip Sprangle
Abstract:
Laser wakefield accelerators (LWFA) can support acceleration gradients orders of magnitude higher than conventional radio frequency linear accelerators. This gives them the potential to drive the next generation of accelerators for high energy physics, as well as compact accelerators for many other applications. However, in order to reach higher energies and improve electron beam quality, LWFA requires the development of plasma waveguides.
This thesis demonstrates two new all optical techniques for the creation of plasma waveguides. The first, “two-Bessel” technique uses a Bessel beam to form the core of the waveguide and a higher order Bessel beam to form the cladding. In the second, “self-waveguiding” technique, the guided beam itself forms the cladding of the waveguide. Preliminary electron acceleration results using the self-guiding technique, as well electron acceleration simulations are also presented.
Abhish Dev - July 23, 2021
Dissertation Title: Determining the neutrino lifetime from cosmology
Date and Time: Friday, July 23, 10:00 am
Location: Zoom
Dissertation Committee Chair: Prof. Zackaria Chacko
Committee:
Prof. Alberto Belloni
Prof. Anson Hook
Prof. Rabindra Mohapatra
Prof. Raman Sundrum
Prof. Massimo Ricotti
Abstract:
Neutrinos are the most mysterious particles in the standard model. Many of their fundamental properties such as their masses, lifetimes, and nature (Dirac or Majorana) are yet to be pinned down by experiments. Currently, the strongest bound on neutrino masses comes from cosmology. This bound is obtained by scrutinizing the gravitational effect of the cosmic neutrinos on the evolution of structure in our universe. However, this bound assumes that the neutrinos from the Big Bang have survived until the present day. In this dissertation, the unstable neutrino scenario is studied in light of current and near-future cosmological experiments. We show that the current cosmological bound on the neutrino masses can be relaxed significantly in an unstable neutrino scenario. We further show that near-future experiments offer the possibility of independently measuring both the masses of the neutrinos and their lifetimes.
We consider an elusive scenario in which the cosmic neutrinos decay into invisible radiation after becoming non-relativistic. The Boltzmann equations that govern the cosmological evolution of density perturbations in the case of unstable neutrinos are derived and solved numerically to determine the effects on the matter power spectrum and lensing of the cosmic microwave background (CMB). A Markov-Chain Monte-Carlo (MCMC) analysis is done on the current cosmological data and mock future data to obtain its sensitivity to the neutrino masses and lifetimes. We show that the effect of the neutrino masses on large scale structure is dampened by the decay of neutrinos, which leads to a parameter degeneracy between the neutrino masses and lifetimes inferred from the cosmological data. This degeneracy allows a significant relaxation of the current cosmological upper bound on the sum of neutrino masses from about 0.2 eV in the stable neutrino case to 0.9 eV in the unstable neutrino scenario. This window is important for terrestrial experiments such as KATRIN which are seeking to independently measure the neutrino masses in the laboratory. We further show that near-future large scale structure measurements from the Euclid satellite, when combined with cosmic microwave background data from Planck, may allow an independent determination of both the lifetimes of the neutrinos and the sum of their masses. These parameters can be independently determined because the Euclid data will cover a range of redshifts, allowing the growth of structure over time to be tracked. If neutrinos are stable on cosmological timescales, we show that these observations can improve the lower limit on the lifetimes of the neutrinos by seven orders of magnitude, from O(10) years to 2 x 10^8 years (95 % C.L.), without significantly affecting the measurement of the neutrino masses. On the other hand, if neutrinos decay after becoming non-relativistic but on timescales less than O(100) million years, these observations may allow, not just the first measurement of the sum of neutrino masses, but also the determination of the neutrino lifetime from cosmology.
Aniruddha Bapat - July 16, 2021
Dissertation Title: Design and optimization in near-term quantum computation
Date and Time: Friday, July 16, 9:30 am
Location: Zoom
Dissertation Committee Chair: Professor Zohreh Davoudi
Committee:
Professor Alexey V. Gorshkov, Co-Advisor
Professor Stephen P. Jordan, Co-Advisor
Professor Christopher Monroe
Professor Andrew M. Childs, Dean's Representative
Abstract:
Quantum computers have come a long way since conception, and there is still a long way to go before the dream of universal, fault-tolerant computation is realized. In the near term, quantum computers will occupy a middle ground that is popularly known as the “Noisy, Intermediate-Scale Quantum” (or NISQ) regime. The NISQ era represents a transition in the nature of quantum devices from experimental to computational. There is significant interest in engineering NISQ devices and NISQ algorithms in a manner that will guide the development of quantum computation in this regime and into the era of fault-tolerant quantum computing.
In this thesis, we study two aspects of near-term quantum computation. The first of these is the design of device architectures, covered in Chapters 2, 3, and 4. We examine different qubit connectivities on the basis of their graph properties, and present numerical and analytical results on the speed at which large entangled states can be created on nearest-neighbor grids and graphs with modular structure. Next, we discuss the problem of permuting qubits among the nodes of the connectivity graph using only local operations, also known as routing. Using a fast quantum primitive to reverse the qubits in a chain, we construct a hybrid, quantum/classical routing algorithm on the chain. We show via rigorous bounds that this approach is faster than any SWAP-based algorithm for the same problem.
The second part, which spans the final three chapters, discusses variational algorithms, which are a class of algorithms particular suited to near-term quantum computation. Two prototypical variational algorithms, quantum adiabatic optimization (QAO) and the quantum approximate optimization algorithm (QAOA), are studied for the difference in their control strategies. We show that on certain crafted problem instances, bang-bang control (QAOA) can be as much as exponentially faster than quasistatic control (QAO). Next, we demonstrate the performance of variational state preparation on an analog quantum simulator based on trapped ions. We show that using classical heuristics that exploit structure in the variational parameter landscape, one can find circuit parameters efficiently in system size as well as circuit depth. In the experiment, we approximate the ground state of a critical Ising model with long-ranged interactions on up to 40 spins.
Finally, we study the performance of Local Tensor, a classical heuristic algorithm inspired by QAOA on benchmarking instances of the MaxCut problem, and suggest physically motivated choices for the algorithm hyperparameters that are found to perform well empirically. We also show that our implementation of Local Tensor mimics imaginary-time quantum evolution under the problem Hamiltonian.
Cong Minh Tran - July 13, 2021
Dissertation Title: Locality, Symmetry, and Digital Simulation of Quantum Systems
Date and Time: Tuesday, July 13, 9:30 am
Location: Zoom
Dissertation Committee Chair: Professor Zohreh Davoudi
Committee:
Professor Alexey V. Gorshkov, Co-Advisor
Professor Jacob M. Taylor, Co-Advisor
Professor Alicia J. Kollar
Professor Andrew M. Childs
Abstract:
Besides potentially delivering a huge leap in computational power, quantum computers also offer an essential platform for simulating properties of quantum systems. Consequently, various algorithms have been developed for approximating the dynamics of a target system on quantum computers. But generic quantum simulation algorithms---developed to simulate all Hamiltonians---are unlikely to result in optimal simulations of most physically relevant systems; optimal quantum algorithms need to exploit unique properties of target systems.
The aim of this dissertation is to study two prominent properties of physical systems, namely locality and symmetry, and subsequently leverage these properties to design efficient quantum simulation algorithms.
In the first part of the dissertation, we explore the locality of quantum systems and the fundamental limits on the propagation of information in power-law interacting systems. We prove upper limits on the speed at which information can propagate in power-law systems. We also demonstrate how such speed limits can be achieved by protocols for transferring quantum information and generating quantum entanglement. We then use these speed limits to constrain the propagation of error and improve the performance of digital quantum simulation. Additionally, we consider the implications of the speed limits on entanglement generation, the dynamics of correlation, the heating time, and the scrambling time in power-law interacting systems.
In the second part of the dissertation, we propose a scheme to leverage the symmetry of target systems to suppress error in digital quantum simulation. We first study a phenomenon called destructive error interference, where the errors from different steps of a simulation cancel out each other. We then show that one can induce the destructive error interference by interweaving the simulation with unitary transformations generated by the symmetry of the target system, effectively providing a faster quantum simulation by symmetry protection. We also derive rigorous bounds on the error of a symmetry-protected simulation algorithm and identify conditions for optimal symmetry protection.
Yidan Wang - July 12, 2021
Dissertation Title: Few-body universality in waveguide quantum electrodynamics
Date and Time: Monday, July 12, 11:00 am
Location: Zoom
Dissertation Committee Chair: Prof. Victor Galitski
Committee:
Prof. Alexey V. Gorshkov
Prof. Michael J. Gullans
Prof. Alicia Kollar
Prof. Andrew M. Childs, Dean's Representative
Abstract:
Photons are elementary particles of light, and their interactions in vacuum are extremely weak. The seclusion of photons makes them perfect carriers of classical and quantum information but also poses difficulties for employing them in quantum information technologies. Recent years have seen tremendous experimental progress in the development of synthetic quantum systems where strong and controllable coupling between single photons is achieved. In a variety of solid-state and optical platforms, propagating photons are coupled with local emitters such as atoms, quantum dots, NV centers, or superconducting qubits. Despite the different nature of the platforms, many of these systems can be described using the same theoretical framework called waveguide quantum electrodynamics (WQED).
Dissipation is an inevitable ingredient of many synthetic quantum systems and is a source of error in quantum information applications. Despite its important role in experimental systems, the implications of dissipation in scattering theory have not been fully explored. Chapter 2 discusses our discovery of the dissipation-induced bound states in WQED systems. The appearance of these bound states is in a one-to-one correspondence with zeros in the single-photon transmission. We also formulate a dissipative version of Levinson’s theorem by looking at the relation between the number of bound states and the winding number of the transmission phases.
In chapter 3, we study three-body loss in Rydberg polaritons. Despite past theoretical and experimental studies of the regime with dispersive interaction, the dissipative regime is still mostly unexplored. Using a renormalization group technique to solve the quantum three-body problem, we show how the shape and strength of dissipative three-body forces can be universally enhanced for Rydberg polaritons. We demonstrate how these interactions relate to the transmission through a single-mode cavity, which can be used as a probe of the three-body physics in current experiments.
The high level of control of the synthetic quantum systems behind WQED offers many inspirations for theoretical studies. In Chapter 4 of this dissertation, we explore a new direction of scattering theory motivated by the controllability of dispersion relations in synthetic quantum systems. We study single-particle scattering in one dimension when the dispersion relation is ε(k)=±|d|k^m, where m≥2 is an integer. For a large class of interactions, we discover that the S-matrix evaluated at an energy E→0 converges to a universal limit that is only dependent on m. We also give a generalization of Levinson’s theorem for these more general dispersion relations in WQED systems.
Monica Gutierrez Galan - July 12, 2021
Dissertation Title: Bose Einstein Condensates for Analogue Cosmology Experiments
Date and Time: Monday, July 12, 2:00 pm
Location: Zoom
Dissertation Committee Chair: James Williams, Chair
Committee:
Gretchen Campbell, Co-Chair/Advisor
Ian Spielman
Theodore Jacobson
Ronald Walsworth, Dean's Representative
Abstract:
This thesis presents the construction and characterization of an experimental apparatus to produce sodium Bose-Einstein condensates (BECs) in arbitrary potentials. Particular attention is devoted to the study of toroidal BECs as platforms for analogue cosmology models.
We also report the first results from this apparatus in which we we studied the red-shifting and attenuation of azimuthal phonons in expanding toroidal BECs as well as blue-shifting and amplification of azimuthal phonons in contracting toroidal BECs. The amplification and attenuation of the azimuthal phonons is the result of a fictitious friction term that arises from the changing metric dictated by the background BEC, this fictitious friction is analogous to the Hubble friction term present in models of the inflaton in cosmology.
Abhinav Deshpande - July 9, 2021
Dissertation Title: The complexity of simulating quantum physics: dynamics and equilibrium
Date and Time: Friday, July 9, 10:00 am
Location: Zoom
Dissertation Committee Chair: Prof. Mohammad Hafezi
Committee:
Prof. Alexey Gorshkov, Co-Chair
Prof. Andrew M. Childs, Dean's Representative
Prof. Bill Fefferman
Prof. Christopher Monroe
Abstract:
Quantum computing is the offspring of quantum mechanics and computer science, two great scientific fields founded in the 20th century. Quantum computing is a relatively young field and is recognized as having the potential to revolutionize science and technology in the coming century. The primary question in this field is essentially to ask which problems are feasible with potential quantum computers and which are not. In this dissertation, we study this question with a physical bent of mind. We apply tools from computer science and mathematical physics to study the complexity of simulating quantum systems. In general, our goal is to identify parameter regimes under which simulating quantum systems is easy (efficiently solvable) or hard (not efficiently solvable). This study leads to an understanding of the features that make certain problems easy or hard to solve. We also get physical insight into the behavior of the system being simulated.
In the first part of this dissertation, we study the classical complexity of simulating quantum dynamics. In general, the systems we study transition from being easy to simulate at short times to being harder to simulate at later times. We argue that the transition timescale is a useful measure for various Hamiltonians and is indicative of the physics behind the change in complexity. We illustrate this idea for a specific bosonic system, obtaining a complexity phase diagram that delineates the system into easy or hard for simulation. We also prove that the phase diagram is robust, supporting our statement that the phase diagram is indicative of the underlying physics.
In the next part, we study open quantum systems from the point of view of their potential to encode hard computational problems. We study a class of fermionic Hamiltonians subject to Markovian noise described by Lindblad jump operators and illustrate how, sometimes, certain Lindblad operators can induce computational complexity into the problem. Specifically, we show that these operators can implement entangling gates, which can be used for universal quantum computation. We also study a system of bosons with Gaussian initial states subject to photon loss and detected using photon-number-resolving measurements. We show that such systems can remain hard to simulate exactly and retain a relic of the ``quantumness'' present in the lossless system.
Finally, in the last part of this dissertation, we study the complexity of simulating a class of equilibrium states, namely ground states. We give complexity-theoretic evidence to identify two structural properties that can make ground states easier to simulate. These are the existence of a spectral gap and the existence of a classical description of the ground state. Our findings complement and guide efforts in the search for efficient algorithms.
Kristen Voigt - July 9, 2021
Dissertation Title: Optical and Electrical Response of Superconducting Resonators for a Hybrid Quantum System
Date and Time: Friday, July 9, 11:00 am
Location: Zoom
Dissertation Committee Chair: Professor Frederick Wellstood
Committee:
Professor Christopher Lobb
Professor Steven Rolston
Dr. Benjamin Palmer
Professor Ichiro Takeuchi
Abstract:
I describe my contributions towards a hybrid quantum system that would have 87Rb atoms coupled to a superconducting device. I first discuss my work coupling an optical fiber to a translatable thin-film LC lumped-element superconducting Al microwave resonator operating at 100 mK in a dilution refrigerator. The LC resonators had resonance frequencies f0 of 6.15 GHz, quality factors Q of 1.5 x 105 to 6.5 x 105 at high powers, and were mounted inside a superconducting aluminum 3D cavity with a resonance frequency of 7.5 GHz and Q of 8 x 103. An optical microfiber (60 µm diameter) passed through a hole in the 3D cavity near the LC resonator. The 3D cavity was mounted on an x-z attocube-translation stage that allowed the LC resonator to be moved relative to the fiber.
The resonator’s f0 and Q were affected both by the fiber dielectric perturbing the resonator’s electric field and from scattered light from the fiber. I measured both effects as a function of fiber-resonator position. I modeled the resonator’s optical response by accounting for optical production, recombination, and diffusion of quasiparticles and the non-uniform position-dependent illumination of the resonator. Using the model, I extracted key parameters describing quasiparticles in the resonator.
The hybrid quantum system requires the 87Rb and LC resonator resonance to be tuned to the same frequency. I describe our LC resonator tuning method which moves a superconducting Al pin into the resonator’s electric field, decreasing the resonator capacitance and increasing its resonance frequency up to 137 MHz. This was done at 15 mK using an attocube translation stage. I also investigated two-level system (TLS) defects in an LC resonator by applying a dc voltage. I describe a model in which the TLS causes a capacitive perturbation to the resonator rather than the ‘standard’ electric-dipole coupling model. I use this model of a capacitive TLS or cTLS, to describe intermittent telegraph noise measured in the transmission S21 through the resonator. I measured shifts in f0 of more than 6 kHz corresponding to a cTLS fluctuating its capacitance contribution by 430 zF.
Patrick Becker - June 25, 2021
Dissertation Title: Non-Integrable Dynamics in a Trapped-Ion Quantum Simulator
Date and Time: Friday, June 25, 3:00 pm
Location: Zoom
Dissertation Committee Chair: Prof. Christopher Monroe
Committee:
Prof. Alexey V. Gorshkov
Prof. Zohreh Davoudi
Prof. Chris Jarzynski
Prof. Qudsia Quraishi
Abstract:
Analog quantum simulators, specialized quantum computers limited to applying unitary evolutions instead of digitized gates, are at the forefront of controllable quantum system sizes. In place of digital algorithms, analog quantum simulators excel at studying many-body physics and modeling certain materials and transport phenomena. Here I discuss an analog quantum simulator based on trapped Yb-171 ions as well as its use for studying dynamics and thermalizing properties of the non-integrable long-range Ising model with system sizes near the limit of classical tractability.
In addition to some technical properties of the trapped-ion simulator, I present three experiments run on the machine during my PhD. The first is an observation of a phenomenon in non-equilibrium physics, a dynamical phase transition (DPT). While equilibrium phase transitions follow robust universal principles, DPTs are challenging to describe with conventional thermodynamics. We present an experimental observation and characterization of a DPT with up to 53 qubits.
We also explore the trapped-ion simulator’s ability to simulate physics beyond its own by implementing a quasiparticle confinement Hamiltonian. Here we see that the natural long-range interactions present in the simulator induce an effective confining potential on pairs of domain-wall quasiparticles, which behave similarly to quarks bound into mesons. We measure post-quench dynamics to identify how quasiparticle confinement introduces low-energy bound states and inhibits thermalization in the system.
Lastly, we use the individual-addressing capabilities of our simulator to implement Stark many-body localization (MBL) with a linear potential gradient. Stark MBL provides a novel, disorder-free method for localizing a quantum system that would otherwise thermalize under evolution. We explore how the localized phase depends on the gradient strength and uncover the presence of correlations using interferometric double electron-electron resonance (DEER) measurements.
Kaustubh Deshpande - June 22, 2021
Dissertation Title: Cosmic Inflation and Naturalness
Date and Time: Tuesday, June 22, 11:00 am
Location: Zoom
Dissertation Committee Chair: Raman Sundrum
Committee:
Kaustubh Agashe
Paulo Bedaque
Anson Hook
Rabindra Mohapatra
Richard Wentworth
Abstract:
Cosmic Inflation provides a robust framework to explain the evolution of the very early universe (before the standard Big Bang) but suffers from problems such as a hierarchy problem for the slow-roll inflaton (eta problem) and super-Planckian field displacement. In this thesis, we construct microscopic models for inflation which are theoretically natural, Effective Field Theory controlled, and observationally consistent, while also looking for possible new signals. This talk, in the first part, will present a SUSY bi-axion model of high-scale inflation, in which the axionic structure originates from gauge symmetry in an extra dimension. We show that local SUSY, although necessarily Higgsed during inflation, can readily survive down to the ~TeV scale in the current era in order to resolve the electroweak hierarchy problem. This construction gives observable signals in the form of primordial non-Gaussianities and periodic modulations in the CMB, within the sensitivity of ongoing measurements.
In the face of improving constraints on the tensor-to-scalar ratio, which is a signature of high- scale inflation, we investigate inflation at lower energy scales via the well-motivated mechanism of Hybrid Inflation. This talk, in the second part, will present a natural and EFT-controlled model for this, “TwInflation”, incorporating a discrete “twin” symmetry which reduces quadratic sensitivity in the inflationary potential to UV physics.
Fangli Liu - June 22, 2021
Dissertation Title: Analog quantum simulation and quantum many-body dynamics in atomic, molecular and optical systems
Date and Time: Tuesday, June 22, 3:00 pm
Location: Zoom
Dissertation Committee Chair: Professor Steven L. Rolston
Committee:
Professor Alexey V. Gorshkov, Advisor
Professor Xiaodi Wu, Dean's Representative
Professor Zohreh Davoudi
Professor Christopher Monroe
Abstract:
In recent decades, the rapid development of quantum technologies has led to a new era of programmable platforms, enabling the realizations of quantum simulation and quantum computation. This dissertation is motivated by recent experimental progress on controlling individual quantum degrees of freedom in systems such as trapped ions and Rydberg atom arrays. By tailoring the interactions in these quantum systems, we study analog quantum simulations of various physical phenomena, including non-equilibrium quantum dynamics and nontrivial topological physics.
In the first part of the dissertation, we study slow quantum many-body dynamics in trapped-ion systems and Rydberg atom arrays. We first show that either the long-range interactions or an additional symmetry-breaking field can give rise to a confining potential. Such a potential can couple domain wall quasiparticles into mesonic or baryonic bound states. These confined quasiparticles strongly suppress the quantum information dynamics and lead to slow thermalization. In the limit of strict domain-wall confinement, the full Hilbert space is fragmented into exponentially many disconnected subspaces. Further, we demonstrate that thermalization can be halted by quantum engineering a uniformly increasing field in the trapped-ion quantum simulator.
The second part of the dissertation focuses on topologically relevant phenomena in quantum simulators. We first study the effect of experimentally relevant disorder in 2D Rydberg atom arrays. We find that there are three distinct localization regimes due to the presence of nontrivial topological bands. We further study the non-equilibrium dynamics of Abelian anyons in a one-dimensional system. We show that the interplay of anyonic statistics and interactions can give rise to spatially asymmetric quantum dynamics. Finally, we use Nielsen's geometric approach to quantify the circuit complexity in topological models. We find that circuit complexity of ground states and circuit complexity of non-equilibrium steady states both exhibit nonanalytical behavior at topological transition points.
Majid Ekhterachian - June 17, 2021
Dissertation Title: Cosmological Phase Transition of Composite Higgs Confinement
Date and Time: Thursday, June 17, 11:00 am
Location: Zoom
Dissertation Committee Chair: Kaustubh Agashe
Committee:
Zackaria Chacko
Thomas Cohen
Raman Sundrum
Richard Wentworth
Abstract:
I will discuss the cosmological (de)confinement phase transition (PT) of nearly conformal, strongly coupled large N field theories, applicable to composite Higgs models, using their holographic dual 5D formulation. In this description, the PT is from the high temperature phase in which the IR of the warped extra dimension is covered by a black-brane horizon to the low temperature Randall-Sundrum-1 phase. The PT proceeds by percolation of IR-brane bubbles nucleating from the horizon, and the bubble dynamics during the PT sources a stochastic gravitational wave background that can be detected by future experiments. I will show how to construct a smooth configuration that interpolates between the two phases, allowing for a controlled description of bubble nucleation within 5D EFT. We will see that the cosmological PT in the minimal models can complete only after a large period of supercooling, potentially resulting in excessive dilution of primordial matter abundances. I will then show how generic modifications of the minimal models can result in a much faster completion of the PT. Finally, I will discuss the gravitational wave signal produced by the PT and the implications of the PT for baryogenesis.
Kevin Palm - June 14, 2021
Dissertation Title: Metal hydrides as a platform for reconfigurable photonic and plasmonic elements
Date and Time: Monday, June 14, 10:00 am
Location: Zoom
Dissertation Committee Chair: Jeremy Munday
Committee:
Daniel Lathrop
James Williams
Edo Waks
Thomas Murphy
Abstract:
Metal hydrides often display dramatic changes in optical properties upon hydrogenation. These shifts make them prime candidates for many tunable optical devices from optical hydrogen sensors and switchable mirrors to physical encryption schemes. In order to design and fabricate optimized devices for any of these applications, we need to determine the optical and structural properties of these materials. In this dissertation, we design and implement an apparatus that dynamically measures the gravimetric, stress, calorimetric, and optical properties of metal hydrides as they are exposed to H2. We use this apparatus to measure the properties of 5 different pure metal hydrides (Pd, Mg, Ti, V, and Zr) and then use these properties to design tunable color filters and switchable perfect absorbers, among other devices. To widen our parameter space and to combine desirable characteristics of different metal systems, we use the same apparatus to investigate the properties of different metal alloy hydride systems including Pd-Au, Mg-Ni, Mg-Ti, and Mg-Al. We demonstrate many improved nanophotonic designs with these materials, including a thin film physical encryption scheme with Pd-Au and a switchable solar absorber with Mg-Ti.
Many of these photonic devices can be further enhanced by tailoring the substrate of the device along with the metal hydride. In this dissertation, we also investigate combining the switchable optical properties of metal hydrides with near-zero-index substrates to further enhance the optical device changes. Near-zero-index materials are ones where the refractive index is below 1 and can lead to a variety of interesting optical effects, including high absorption in surrounding materials and enhanced non-linear effects. By combining an ITO substrate with a near-zero-index resonance at ~1250 nm with a thin Pd capped Mg film, we demonstrate a switchable absorption device with >76% absorption change at 1335 nm illumination. To further explore the possibility of large-scale fabrication of these devices, we survey the properties of commercially available near-zero-index materials and report the range of attainable optical properties, showing its feasibility.
Allison Carter - May 20, 2021
Dissertation Title: Design and Construction of a Three-Node Quantum Network
Date and Time: Thursday, May 20, 2:00 pm
Location: Zoom
Dissertation Committee Chair: Prof. Christopher Monroe
Committee:
Prof. Alicia Kollar
Prof. Norbert Linke
Prof. Steven Rolston
Prof. Ronald Walsworth
Abstract:
Quantum computers have wide-ranging potential applications, many of which will require thousands or even millions of quantum bits to be useful. Current state-of-the-art universal quantum computers, on the other hand, contain only several tens of qubits, and scaling to larger system sizes remains one of the primary challenges. Among current quantum computing platforms, trapped ions are a leading hardware option. One proposal for scaling such systems consists of a modular architecture.
The architecture consists of multiple nodes, each with an ion trap containing a communication qubit (138Ba+) and a memory qubit (171Yb+). The communication qubit is responsible for generating photons that link the remote nodes together via entanglement swapping while the memory qubits are used for storing information and performing local computations. We report progress towards demonstration of the remote entanglement of two barium ions. The creation of this link is a probabilistic process and fails much more often than it succeeds. The success rate does not impact the fidelity of the resulting entangled state but imposes significant constraints on the utility of this protocol. We examine the current limitations on both the fidelity of the resulting entangled state and the success probability.
In addition to the two-node experiment, we have designed and built a new ion trap system that should yield much higher photon collection rates. This design represents a significant shift from previous systems because of the inclusion of optical elements inside the vacuum chamber and their resulting proximity to the ions. We incorporate two objective lenses with a numerical aperture of 0.8, each of which can collect twice as much light as the objectives used for the remote entanglement experiment. We present preliminary results characterizing the performance of this system and discuss how it could be incorporated into a three-node network, which has not yet been demonstrated using trapped ions.
Elizabeth Friedman - April 30, 2021
Dissertation Title: All-Sky Search for Neutrinos Correlated with Gamma-Ray Bursts in Extended Time Windows Using Eight Years of IceCube data
Date and Time: Friday, April 30, 12:00 pm
Location: Zoom
Dissertation Committee Chair: Dr. Kara Hoffman
Committee:
Dr. Gregory Sullivan
Dr. Erik Blaufuss
Dr. Regina Caputo
Dr. Suvi Gezari
Abstract:
GRBs have long been considered as potential sources of hadronic acceleration of ultra-high energy cosmic rays and, more recently, as potential sources of the diffuse neutrino flux measured by IceCube. This thesis presents a search for neutrinos correlated with 2,091 gamma-ray bursts (GRBs) in prompt and extended time windows using data from the IceCube Neutrino Observatory ranging from May 2011 to October 2018. Ten time windows, ranging from 10 seconds to -1/+14 days around the time of the GRB's gamma-ray emission, were searched for coincident neutrino emission and a p-value was assigned based on the most significant time window. The results for all 2,091 GRBs were divided by region of the sky and observed gamma-ray emission time, and were evaluated with a Binomial test, which was found to be consistent with background. The 23 most significant GRBs were examined in more detail and they were also found to be consistent with background. Limits were set assuming two flux models: equal neutrino flux at Earth for every GRB and standard candle neutrino emission. The equal flux at Earth model led to similar constraints as previous IceCube prompt GRB studies, namely that GRBs are responsible for, at most, a few percent of the diffuse flux. The results of the standard candle analysis, however, indicate GRBs may be contributing up to 11% of the diffuse neutrino flux up to 1,000 second timescales, which leaves the door open to GRBs as neutrino sources and hadronic accelerators of at least some of the ultra-high energy cosmic rays.
Laird Egan - April 7, 2021
Dissertation Title: Scaling Quantum Computers with Long Chains of Trapped Ions
Date and Time: Wednesday, April 7, 2:00 pm
Location: Zoom
Dissertation Committee Chair: Prof. Christopher Monroe
Committee:
Prof. Norbert Linke
Prof. Alicia Kollar
Prof. Vladimir Manucharyan
Prof. Christopher Jarzynski
Abstract:
Quantum computers promise to solve models of important physical processes, optimize complex cost functions, and challenge cryptography in ways that are intractable using current computers. In order to achieve these promises, quantum computers must both increase in size and decrease error rates.
To increase the system size, we report on the design, construction, and operation of an integrated trapped ion quantum computer consisting of a chain of 15 171Yb+ ions with all-to-all connectivity and high-fidelity gate operations. In the process, we identify a physical mechanism that adversely affects gate fidelity in long ion chains. Residual heating of the ions from noisy electric fields creates decoherence due to the weak confinement of the ions transverse to a focused addressing laser. We demonstrate this effect in chains of up to 25 ions and present a model that accurately describes the observed decoherence. To mitigate this noise source, we first propose a new sympathetic cooling scheme to periodically re-cool the ions throughout a quantum circuit, and then demonstrate its capability in a proof-of-concept experiment.
One path to suppress error rates in quantum computers is through quantum error correction schemes that combine multiple physical qubits into logical qubits that robustly store information within an entangled state. These extra degrees of freedom enable the detection and correction of errors. Fault-tolerant circuits contain the spread of errors while operating the logical qubit and are essential for realizing error suppression in practice. We demonstrate fault-tolerant preparation, measurement, rotation, and stabilizer measurement of a distance-3 Bacon-Shor logical qubit in our quantum computer. The result is an encoded logical qubit with error rates lower than the error of the entangling operations required to operate it.
Antonios Kyprianidis - March 31, 2021
Dissertation Title: Simulating many-body quantum spin models with trapped ions
Date and Time: Wednesday, March 31, 12:00 pm
Location: Zoom
Dissertation Committee Chair: Christopher Moroe
Committee:
Norbert Linke
Alexey Gorshkov
Ronald Walsworth
Mohammad Hafezi
Abstract:
Richard Feynman in 1981 suggested using a quantum machine to simulate quantum mechanics.
Peter Shor in 1994 showed that a quantum computer could factor numbers much more efficiently than a conventional one. Since then, the explosion of the quantum information field is attesting to how motivation and funding work miracles. Naturally, this expansion has led to diversification of the devices being developed. The quantum information systems that cannot simulate an arbitrary evolution, but are specialized in a specific set of Hamiltonians, are called quantum simulators. Several such experimental platforms exist, harnessing the luxury of being able to surpass computational abilities of classical computers right now, at the expense of only doing so for a narrow type of problem. Among those systems, ions trapped in vacuum by electric fields and manipulated with light have proved to be a leading platform in emulating quantum magnetism models. In this thesis, I will present experiments that use trapped ions to realize a prethermal discrete time crystal. This exotic phase occurs in non-equilibrium matter subject to an external periodic drive. Normally, the ensuing Floquet heating maximizes the system entropy, leaving us with nothing but a trivial, infinite-temperature state. However, we can parametrically slow down this heating by tuning the drive frequency. During the time window of slow thermalization, we define an order parameter and observe two different regimes, based on whether it spontaneously breaks the discrete time translation symmetry of the drive or it preserves it.
Furthermore, I demonstrate a simple model of electric field noise classically heating an ion in an anharmonic confining potential. As ion traps shrink, this kind of noise may become more significant. And finally, I discuss a handful of error sources relevant in recent simulation experiments. Most notably, these were AC Stark shifts on the qubit levels, manifesting themselves as additional time-dependent terms in our Hamiltonian evolution. As quantum simulation experiments progress to more qubits and complicated sequences, accounting for system imperfections is becoming an integral part of the process.
Wen Lin Tan - March 22, 2021
Location: Zoom
Dissertation Committee Chair: Prof. Christopher Monroe
Committee:
Professor Norbert Linke
Professor James Williams
Professor Alexey Gorshkov
Professor Christopher Jarzynski
Abstract:
One of the useful applications of a quantum computer is quantum simulation. While the quest for a universal quantum computer is still undergoing research, analog quantum simulators can study specific quantum models that are classically challenging or even intractable. These quantum simulators provide the opportunity to test particular quantum models and the possibility to scale up the system size to gain insight into more exciting physics. The analog quantum simulator featured in this thesis is a cryogenic trapped-ion system. It serves the purpose of a large-scale quantum simulation by reducing the background pressure for storing a large ion chain with a long lifetime. This work presents the construction and characterization of this cryogenic apparatus and its performance as a trapped-ion quantum simulator.
Quantum information is encoded in the atomic state of the ion chain. The entangling operation in trapped ions uses the collective motion of the ion chain for quantum simulation. Therefore, it is imperative to develop a cooling mechanism to prepare the ion chain to near motional ground-state for achieving high fidelity operations. Here, with this system, we explore another ground-state cooling mechanism with electromagnetically induced transparency (EIT) in a four-level system ($^{171}\text{Yb}^{+}$) EIT cooling allows simultaneous ground-state cooling across a bandwidth of motional modes, which it is useful in a large ion chain. Finally, we report the observation of magnetic domain-wall confinement in interacting spins chains. Such confinement is analogous to the color confinement in quantum chromodynamics (QCD), where hadrons are produced by quark confinement. We study the implications of such confinement in many-body spin system by observing the information propagation after applying a quantum quench of the confinement Hamiltonian. We also measure the excitation energy of domain-wall bound states from non-equilibrium quench dynamics. At the end of this experiment, we explore the number of domain wall excitations created with different quench parameters, which can be challenging to model with classical computers.
David A. Garcia - February 26, 2021
Date and Time: Friday, February 26, 10:00 am
Location: Zoom
Dissertation Committee Chair: Professor Arpita Upadhyaya
Committee:
Professor Michelle Girvan
Dr. Gordon L. Hager
Professor Wolfgang Losert
Professor Giuliano Scarcelli
Abstract:
Different proteins and complexes work together at multiple time scales to orchestrate the activation and silencing of genes in a process called transcription. Understanding transcriptional regulation is of utmost importance to reveal mechanisms behind cell homeostasis and pathologies. The transcription machinery needs to be perfectly tuned in space and time to control the expression of genes to carry out cellular and physiological processes in the noisy and highly heterogeneous nuclear microenvironment. Transcription factors (TF), specialized proteins that bind to specific DNA sequences to regulate mRNA production, are central players in transcriptional regulation. TFs need to navigate the intricate nuclear microenvironment to bind to specific regulatory elements with binding times critically determining their regulatory functions. Recent advances in super-resolution microscopy have allowed us to investigate the dynamics of the transcriptional machinery at the single molecule level, revealing the essential features of transcriptional control. However, how TFs dynamically navigate the nuclear microenvironment and interact with chromatin to activate or silence genes remains poorly understood. I used state of the art microscopy and genomic techniques to show that binding times of TFs to chromatin are power-law distributed. I proposed a new theoretical framework to demonstrate the broad distribution of binding affinity arises from heterogeneity in TF- chromatin interactions and the nuclear microenvironment, contrary to the current paradigm of well-defined and distinguishable TF binding times to specific and non-specific chromatin sites. These studies reconciled discrepancies between genomics, gene expression and TF mobility. I used statistical modeling to show that TFs exhibit two distinguishable low mobility states in the nucleus. One state is related to chromatin binding while the second arises due to protein-protein interactions mediated by intrinsically disordered regions of the TF and potentially controls the initiation rate of transcription. Finally, I studied transcriptional regulation on substrates of different stiffness revealing a connection between the physical properties of the cell microenvironment and TF dynamics. I demonstrated that substrate stiffness activates the estrogen receptor even in the absence of its ligand, with implications for our understanding and treatment of breast cancer. The evidence presented here shows that TF binding times are finely tuned to regulate gene expression.
Natalia Pankratova - January 14, 2021
Dissertation Title: Multi-terminal Josephson effect
Date and Time: Thursday, January 14, 12:30 pm
Location: Zoom
Dissertation Committee Chair: Vladimir Manucharyan
Committee:
Christopher Lobb
James Williams
Steven Anlage
Ichiro Takeuchi (Dean's Representative)
Abstract:
Conventionally, a Josephson junction is an ubiquitous quantum device formed by a weak link between a pair of superconductors. In this work, we demonstrate the dc Josephson effect in mesoscopic junctions of more than two superconducting terminals. We report fabrication and characterization of the 3- and 4-terminal Josephson junctions built in a top-down fashion from hybrid semiconductor-superconductor InAs/Al epitaxial heterostructures. In general, the critical current of an N-terminal junction is an (N-1)-dimensional hypersurface in the space of bias currents, which can be reduced to a set of critical current contours (CCCs). The CCC is a key ground state characteristic of a multi-terminal Josephson junction, which is readily available from regular electron transport measurements.
We investigate nontrivial modifications of the CCC's geometry in response to electrical gating, magnetic field, and phase bias. All observed effects are described by the scattering formulation of the Josephson effect generalized to the case of N>2. Our observations indicate superconducting phase coherence between all the terminals which establishes the Josephson effect in mesoscopic junctions of more than two superconductors. Such multi-terminal junctions could find their applications in a broad range of fields from topologically protected quantum computation to quantum metrology.
Harry Arnold - December 15, 2020
Dissertation Title: Electron Acceleration during Macroscale Non-Relativistic Magnetic Reconnection
Date and Time: Tuesday, December 15, 3:00 pm
Location: Zoom
Dissertation Committee Chair: Professor James Drake
Committee:
Professor Adil Hassam
Professor William Dorland
Professor Marc Swisdak
Professor Cole Miller
Abstract:
In this thesis we developed the new model kglobal for the purpose of studying nonthermal electron acceleration in macroscale magnetic reconnection. Unlike PIC codes we can simulate macroscale domains, and unlike MHD codes we can simulate particles that feedback onto the fluids so that the total energy of the system is conserved. This has never been done before. We have benchmarked the model by simulating Alfv\'en waves with electron pressure anisotropy, the growth of the firehose instability, and the growth of electron acoustic waves. We then studied the results of magnetic reconnection and found clear power-law tails that can extend for more than two decades in energy with a power-law index that decreases with the strength of the guide field. Reconnection in systems with guide fields approaching unity produce practically no nonthermal electrons. For weak guide fields the model is extremely efficient in producing nonthermal electrons. The nonthermals contain up to ~80% of the electron energy in our lowest guide field simulation. These results are generally consistent with flare observations and specifically the measurements of the September 10, 2017, flare.
Tristin Metz - December 15, 2020
Dissertation Title: Low Temperature Transport and Thermodynamic Studies of Unconventional Superconductors
Date and Time: Tuesday, December 15, 1:30 pm
Location: Zoom
Dissertation Committee Chair: Professor Johnpierre Paglione
Committee:
Professor Steven Anlage
Professor Richard Greene
Professor Christopher Lobb
Professor Ichiro Takeuchi
Abstract:
Thermal conductivity and heat capacity have been established as powerful bulk probes for studying superconducting states which go beyond the standard BCS theory of superconductivity. In this thesis I present transport and thermodynamic measurements in ultra-low temperatures (down to 20 mK) on two different unconventional superconducting systems: AFe2As2 (A=K,Rb,Cs) and UTe2.
In the first study, I present measurements of charge and thermal transport in the iron based superconductor KFe2As2 in magnetic fields aligned precisely in the ab-plane in order to study thermal transport in a Pauli limited superconductor with the quasiparticle mean free path controlled with disorder induced by electron irradiation. In KFe2As2 with a high level of disorder, a residual quasiparticle thermal conductivity which varies linearly in field is observed which is consistent with predictions for a paramagnetically limited superconductor. None of the measured samples display signatures of a first order superconducting transition or FFLO phase.
In the field induced normal state in a pristine sample of KFe2As2, we found non-Fermi liquid temperature dependence of the charge and thermal resistivity. Furthermore, the thermal resistivity indicates the persistence of strong inelastic quasiparticle scattering in the superconducting state for fields below Hc2. The resistivity was measured in fields up to 15 T along both the a and b axes in the pristine and irradited KFe2As2 samples as well as in RbFe2As2 and CsFe2As2. No indication of field tuning of the non-Fermi liquid behavior was observed. Instead, the field dependent resistivity in these samples follows a generalized Kohler scaling, indicating that the scattering mechanism giving rise to non-Femi liquid resistivity in zero field is unaffected by the application of magnetic field.
Finally, I present measurements of thermal transport and heat capacity in UTe2 to resolve the superconducting gap structure. Surprisingly, no residual quasiparticle thermal conductivity is observed despite a large residual density of states observed in heat capacity. The rapid increase of residual quasiparticle thermal conductivity in magnetic field as well as the low temperature power law dependence of both heat capacity and magnetic penetration depth suggest a point nodal gap structure in UTe2. Furthermore, careful measurements of the heat capacity in the vicinity of the superconducting transition uncover a splitting of the superconducting transition temperature indicative of two nearly degenerate superconducting phases present in UTe2.
Rui Zhang - November 24, 2020
Dissertation Title: Loss in Superconducting Quantum Devices from Non-Equilibrium Quasiparticle and Inhomogeneity in Energy Gap
Date and Time: Tuesday, November 24, 2:30 pm
Location: Zoom
Dissertation Committee Chair: Professor Frederick C. Wellstood
Committee:
Dr. Benjamin S. Palmer
Dr. Christopher J. K. Richardson
Professor Christopher J. Lobb
Professor Ichiro Takeuchi
Abstract:
This dissertation describes energy dissipation and microwave loss due to non-equilibrium quasiparticles in superconducting transmon qubits and titanium nitride coplanar waveguide resonators. During the measurements of transmon relaxation time and resonator quality factor , I observed reduced microwave loss as the temperature increased from 20 mK to approximately at which the loss takes on a minimum value. I argue that this effect is due to non-equilibrium quasiparticles.
I measured the temperature dependence of the relaxation time of the excited state of an Al/AlOx/Al transmon and found that, in some cases, increased by almost a factor of two as the temperature increased from 30 mK to 100 mK with a best of 0.2 ms. I present an argument showing this unexpected temperature dependence occurs due to the behavior of non-equilibrium quasiparticles in devices in which one electrode in the tunnel junction has a smaller volume, and slightly smaller superconducting energy gap, than the other electrode. At sufficiently low temperatures, non-equilibrium quasiparticles accumulate in the electrode with the smaller gap, leading to a relatively high density of quasiparticles at the junction and a short . Increasing the temperature gives the quasiparticles enough thermal energy to occupy the higher gap electrode, reducing the density at the junction and increasing . I present a model of this effect, extract the density of quasiparticles and the two superconducting energy gaps, and discuss implications for increasing the relaxation time of transmons.
I also observed a similar phenomenon in low temperature microwave studies of titanium nitride coplanar waveguide resonators. I report on loss in a resonator at temperatures from 20 mK up to 1.1 K and with the application of infrared pair breaking radiation ( ). With no applied IR light, the internal quality factor increased from = 800,000 at T < 70 mK up to at 600 mK. The resonant frequency increased by 2 parts per million over the same temperature range. Above 600 mK both and decreased rapidly, consistent with the increase in the density of thermally generated quasiparticles. With the application of IR light and for intensities below and T < 400 mK, increased in a similar way to increasing the temperature before beginning to decrease with larger intensities. I show that a model involving non-equilibrium quasiparticles and two regions of different superconducting gaps can explain this unexpected behavior.
Yuchen Yue - November 2, 2020
Dissertation Title: Enhanced transport of spin-orbit coupled Bose gases in disordered potentials
Date and Time: Monday, November 2, 1:00 pm
Location: Zoom
Dissertation Committee Chair: Prof. Steven Rolston
Committee:
Prof. Ian Spielman, Advsior
Prof. Edo Waks, Dean's Representative
Prof. Trey Porto
Prof. Jay Sau
Abstract:
Anderson localization is a single particle localization phenomena in disordered media that is accompanied by an absence of diffusion. Spin-orbit coupling (SOC) describes an interaction between a particle's spin and its momentum that directly affects its energy dispersion, for example creating dispersion relations with gaps and multiple local minima. We show theoretically that combining one-dimensional spin-orbit coupling with a transverse Zeeman field suppresses the effects of disorder, thereby increasing the localization length and conductivity.
This increase results from a suppression of back scattering between states in the gap of the SOC dispersion relation. Here, we focus specifically on the interplay of disorder from an optical speckle potential and SOC generated by two-photon Raman processes in quasi-1D Bose-Einstein condensates. We first describe back-scattering using a Fermi's golden rule approach, and then numerically confirm this picture by solving the time-dependent 1D Gross Pitaevskii equation for a weakly interacting Bose-Einstein condensate with SOC and disorder.
We find that on the 10's of millisecond time scale of typical cold atom experiments moving in harmonic traps, initial states with momentum in the zero-momentum SOC gap evolve with negligible back-scattering, while without SOC these same states rapidly localize.
Nicholas Grabon - October 30, 2020
Dissertation Title: Interacting photons in circuit quantum electrodynamics: Decay of the collective phase mode in one-dimensional Josephson junction arrays due to quantum phase-slip fluctuations
Date and Time: Friday, October 30, 12:00 pm
Location: Zoom
Dissertation Committee Chair: Vladimir Manucharyan
Committee:
Professor Jay Deep Sau
Professor Steven Anlage
Professor Victor Galitski
Professor Ichiro Takeuchi
Abstract:
Light does not typically scatter light, as witnessed by the linearity of Maxwell's equations. In this work, we demonstrate two superconducting circuits, in which microwave photons have well-defined energy and momentum, but their lifetime is finite due to decay into lower energy photons. The circuits we present are formed with Josephson junction arrays where strong quantum phase-slip fluctuations are present either in all of the junctions or in only a single junction. The quantum phase-slip fluctuations are shown to result in the strong inelastic photon-photon interaction observed in both circuits.
The phenomenon of a single photon decay provides a new way to study multiple long-standing many-body problems important for condensed matter physics. The examples of such problems, which we cover in this work include superconductor to insulator quantum phase transition in one dimension and a general quantum impurity problem. The photon lifetime data can be treated as a rare example of a verified and useful quantum many-body simulation.
Yongbin Feng - October 29, 2020
Dissertation Title: A New Deep-Neural-Network--Based Missing Transverse Momentum Estimator, and its Application to W Recoil
Date and Time: Thursday, October 29, 4:00 pm
Location: Zoom
Dissertation Committee Chair: Prof. Alberto Belloni
Committee:
Prof. Sarah Eno
Prof. Manuel Franco Sevilla
Prof. Anson Hook
Prof. Timothy Koeth
Prof. Massimo Ricotti, Dean’s Representative
Abstract:
This dissertation presents the first Deep-Neural-Network--based missing transverse momentum (pTmiss) estimator, called ``DeepMET''. It utilizes all reconstructed particles in an event as input, and assigns an individual weight to each of them. The DeepMET estimator is the negative of the vector sum of the weighted transverse momenta of all input particles. Compared with the pTmiss estimators currently utilized by the CMS Collaboration, DeepMET is found to improve the pTmiss resolution by 10-20%, and is more resilient towards the effect of additional proton-proton interactions accompanying the interaction of interest. DeepMET is demonstrated to improve the resolution on the recoil measurement of the W boson and reduce the systematic uncertainties on the W mass measurement by a large fraction compared with other pTmiss estimators.
Mike Martin - October 28, 2020
Dissertation Title: Turbulent and Collisional Transport in Optimized Stellarators
Date and Time: Wednesday, October 28, 10:00 am
Location: Zoom
Dissertation Committee Chair: William Dorland
Committee:
Thomas Antonsen
Ramani Duraiswami
Adil Hassam
Matthew Landreman
Abstract:
The stellarator is a fusion energy concept that relies on fully three-dimensional shaping of magnetic fields to confine particles. Stellarators have many favorable properties, including, but not limited to, the ability to operate in steady-state, many optimizable degrees of freedom, and no strict upper limit on the plasma density. Due to the three-dimensional character of stellarators, theoretical and computational studies of stellarator physics are challenging and they possess some disadvantages compared to tokamaks. Namely, particle confinement and impurity control are problems in generic stellarator magnetic fields that must be addressed with optimized magnetic fields. Further, simulations will require a substantial increase in grid points because of the three-dimensional structure, leading to more expensive computations. This thesis will address both topics by first exploring the behavior of impurity particle transport in optimized stellarators, and then introducing a boundary condition to reduce the cost of stellarator turbulence simulations.
Impurity temperature screening is a favorable neoclassical phenomenon involving an outward radial flux of impurity ions from the core of fusion devices. Quasisymmetric magnetic fields lead to intrinsically ambipolar neoclassical fluxes that give rise to temperature screening for low enough $\eta^{-1}\equiv d\ln n/d\ln T$. Here we examine the impurity particle flux in a number of approximately quasisymmetric stellarator configurations and parameter regimes while varying the amount of symmetry-breaking in the magnetic field. Results indicate that achieving temperature screening is possible, but unlikely, at low-collisionality reactor-relevant conditions in the core. A further look at the magnitude of these fluxes when compared to a gyro-Bohm turbulence estimate suggests that neoclassical fluxes are small in configurations optimized for quasisymmetry when compared to turbulent fluxes.
Calculating these turbulent fluxes is generally done by solving the gyrokinetic equation in a flux tube simulation domain, which employs coordinates aligned with the magnetic field lines. The standard ``twist-and-shift'' formulation of the boundary conditions \cite{Beer95} was derived assuming axisymmetry and is widely used because it is efficient, as long as the global magnetic shear is not too small. A generalization of this formulation is presented, appropriate for studies of non-axisymmetric, stellarator-symmetric configurations, as well as for axisymmetric configurations with small global shear. The key idea of this generalization is to rely on integrated local shear, allowing one significantly more freedom when choosing the extent of the simulation domain in each direction. Simulations of stellarator turbulence that employ the generalized parallel boundary conditions allow for lower resolution to be used compared to simulations that use the (incorrect, axisymmetric) standard parallel boundary condition.
Levon Dovlatyan - October 7, 2020
Dissertation Title: Study and mitigation of transverse resonances with space charge effects at the University of Maryland Electron Ring
Date and Time: Wednesday, October 7, 2:00 pm
Location: Zoom
Dissertation Committee Chair: Prof. Thomas Antonsen and Dr. Brian Beaudoin
Committee:
Prof. Timothy Koeth
Prof. Daniel Lathrop
Prof. Irving Haber
Abstract:
Research at the intensity frontier of particle physics has led to the consideration of accelerators that push the limits on achievable beam intensities. At high beam intensities Coulomb interactions between charged particles generate a space charge force that complicates beam dynamics. The space charge force can lead to a range of nonlinear, intensity- limiting phenomena that result in degraded beam quality and current loss. This is the central issue faced by the next generation of high-intensity particle accelerators. An improved understanding of the interaction of the space charge forces and transverse particle motion will help researchers better design around these limiting issues. Furthermore, any scheme able to mitigate the impacts of such destructive interactions for space charge dominated beams would help alleviate a significant limitation in reaching higher beam intensities. Experimental work addressing these issues is presented using the University of Maryland Electron Ring (UMER).
This dissertation presents experimental studies of space charge dominated beams, and in particular the resonant interaction between the transverse motion of the beam and the periodic perturbations that occur due to the focusing elements in a circular ring. These interactions are characterized in terms of the tune shifts, Qx and Qy, that are the number of transverse oscillations (in and out of the plane of the ring) per trip around the ring. Resonances occur for both integer and half-integer values of tune shift. Particle tune measurement tools and resonance detection techniques are developed for use in the experiment. Results show no shift for either the integer (Qx = 7.0, Qy = 7.0) or half-integer (Qx = 6.5, Qy = 6.5) resonance bands as a function of space charge. Accepted theory predicts only a shift in the half-integer resonance case.
A second experiment testing the potential mitigation of transverse resonances through nonlinear detuning of particle orbits from resonance driving terms is also presented. The study included the design, simulation, and experimental test of a quasi-integrable accelerator lattice based on a single nonlinear octupole channel insert. Experiments measured a nonlinear amplitude dependent tune shift within the beam on the order of ∆Qx ≈ 0.02 and ∆Qy ≈ 0.03. The limited tolerances on accelerator steering prevented measuring any larger tune shifts.
Seyed Alireza Seif Tabrizi - October 6, 2020
Dissertation Title: Control and characterization of open quantum systems
Date and Time: Tuesday, October 6, 10:00 am
Location: Zoom
Dissertation Committee Chair: Prof. Mohammad Hafezi (Advisor)
Committee:
Prof. Zohreh Davoudi
Prof. Christopher Jarzynski
Prof. Norbert M. Linke
Prof. Yi-Kai Liu
Abstract:
The study of open physical systems concerns finding ways to incorporate the lack of information about the environment into a theory that best describes the behavior of the system. We consider characterizing the environment by using the system as a sensor, mitigating errors, and learning the physics governing systems out of equilibrium with computer algorithms.
We characterize long-range correlated errors and crosstalk, which are important factors that negatively impacts the performance of noisy intermediate-scale quantum (NISQ) computing devices. We propose a compressed sensing method for detecting correlated dephasing errors, assuming only that the correlations are sparse (i.e., at most s pairs of qubits have correlated errors, where s<<n(n-1)/2, and n is the total number of qubits). Our method uses entangled many-qubit GHZ states, and it can detect long-range correlations whose distribution is completely arbitrary, independent of the geometry of the system. Our method is also highly scalable: it requires only s log n measurement settings, and efficient classical postprocessing based on convex optimization.
To learn about physical systems using computer algorithms, we consider the problem of arrow of time. We show that a machine learning algorithm can learn to discern the direction of time's arrow when provided with a system's microscopic trajectory as input. Examination of the algorithm's decision-making process reveals that it discovers the underlying thermodynamic mechanism and the relevant physical observables. Our results indicate that machine learning techniques can be used to study systems out of equilibrium, and ultimately to uncover physical principles.
Sabyasachi Barik - September 23, 2020
Dissertation Title: Chiral quantum optics using topological photonics
Date and Time: Wednesday, September 23, 11:00 am
Location: Zoom
Dissertation Committee Chair: Prof. Edo Waks
Committee:
Prof. Mohammad Hafezi
Prof. Thomas Murphy
Prof. Rajarshi Roy
Prof. Trey Porto
Abstract:
Topological photonics has opened new avenues to designing photonic devices along with opening a plethora of applications. Recently even though there have been many interesting studies in topological photonics in the classical domain, the quantum regime has remained largely unexplored. In this talk, I will speak on a recently developed a topological photonic crystal structure for interfacing single quantum dot spin with photon to realize light-matter interaction with topological photonic states. Developed on a thin slab of Gallium Arsenide(GaAs) membrane with electron beam lithography, such a device supports two robust counter-propagating edge states at the boundary of two distinct topological photonic crystals at near-IR wavelength. I will show the chiral coupling of circularly polarized lights emitted from a single Indium Arsenide(InAs) quantum dot under a strong magnetic field into these topological edge modes. Owing to the topological nature of these guided modes, I will demonstrate this photon routing to be robust against sharp corners along the waveguide. Additionally, I will introduce a new topological component using valley-Hall topological photonic crystal and study the robustness in the system. This paves paths for fault-tolerant photonic circuits, secure quantum computation, exploring unconventional quantum states of light and chiral spin networks.
Alexander Craddock - August 7, 2020
Dissertation Title: Rydberg Ensembles for Quantum Networking
Date and Time: Friday, August 7, 10:00 am
Location: Zoom
Dissertation Committee Chair: Steve Rolston (Co-advisor)
Committee:
Trey Porto (Co-advisor)
Alexey Gorshkov
Mohammad Hafezi (Dean's Representative)
Norbert Linke
Abstract:
Rydberg ensembles, atomic clouds with one or more atoms excited to a Rydberg state, have proven to be a good platform for the study of photon-photon interactions. This is due to the nonlinearities they exhibit at the single photon level arising from Rydberg-Rydberg interactions. As a result, they have shown promise for use in a multitude of applications, among them quantum networking.
In this thesis I describe the construction and operation of an apparatus for the purpose of cooling, trapping and probing Rydberg ensemble physics in a cloud of Rb 87 atoms. In addition, I describe a pair of projects undertaken with the apparatus. In the first, I report our demonstration of a Rydberg ensemble based on-demand single photon source. Here, we make use of Rydberg blockade to allow us to prepare a single collective Rydberg excitation in the cloud. The spin wave excitation is then retrieved by coherently mapping it onto a propagating photon. Our source is highly pure and efficient, while producing narrow bandwidth and indistinguishable photons.
Such sources are important devices for the purposes of quantum networking, computation and metrology. Following from this, I describe a collaborative project where we show time resolved Hong-Ou-Mandel interference between photons produced by our Rydberg ensemble source, and a collaborators source based on a single trapped barium ion. This demonstration is a critical step in the entanglement, and hybrid quantum networking, of these two disparate systems.
Sarthak Chandra - July 28, 2020
Dissertation Title: Extensions of the Kuramoto Model: From Spiking Neurons to Swarming Drones
Date and Time: Tuesday, July 28, 11:00 am
Location: Zoom
Dissertation Committee Chair: Prof. Michelle Girvan
Committee:
Prof. Edward Ott
Prof. Rajarshi Roy
Prof. Thomas M. Antonsen
Prof. Brian Hunt
Abstract:
The Kuramoto model (KM) was initially proposed by Yoshiki Kuramoto in 1975 to model the dynamics of large populations of weakly coupled phase oscillators. Since then, the KM has proved to be a paradigmatic model, demonstrating dynamics that are complex enough to model a wide variety of nontrivial phenomena while remaining simple enough for detailed mathematical analyses. However, as a result of the mathematical simplifications in the construction of the model, the utility of the KM is somewhat restricted in its usual form. In this thesis we discuss extensions of the KM that allow it to be utilized in a wide variety of physical and biological problems.
First, we discuss an extension of the KM that describes the dynamics of theta neurons, i.e., quadratic-integrate-and-fire neurons. In particular, we study networks of such neurons and derive a mean-field description of the collective neuronal dynamics and the effects of different network topologies on these dynamics. This mean-field description is achieved via an analytic dimensionality reduction of the network dynamics that allows for an efficient characterization of the system attractors and their dependence not only on the degree distribution but also on the degree correlations.
Then, motivated by applications of the KM to the alignment of members in a two-dimensional swarm, we construct a Generalized Kuramoto Model (GKM) that extends the KM to arbitrary dimensions. Like the KM, the GKM in even dimensions continues to demonstrate a transition to coherence at a positive critical coupling strength. However, in odd dimensions the transition to coherence occurs discontinuously as the coupling strength is increased through 0. In contrast to the unique stable incoherent equilibrium for the KM, we find that for even dimensions larger than 2 the GKM displays a continuum of different possible pretransition incoherent equilibria, each with distinct stability properties, leading to a novel phenomenon, which we call `Instability-Mediated Resetting.' To aid the analysis of such systems, we construct an exact dimensionality reduction technique with applicability to not only the GKM, but also other similar systems with high-dimensional agents beyond the GKM.
Dalia Ornelas Huerta - July 15, 2020
Dissertation Title: Experiments with Strongly Interacting Rydberg Atoms
Date and Time: Wednesday, July 15, 3:00 pm
Location: Zoom
Dissertation Committee Chair: Steve Rolston
Committee:
Trey V. Porto
Gretchen K. Campbell
Alica J.Kollár
Thomas E. Murphy
Abstract:
Interacting Rydberg excitations in cold atomic ensembles can exhibit large quantum nonlinearities that enable engineering of strong interactions between individual photons. Consequently, Rydberg ensembles are a promising platform for quantum information applications and the study of more fundamental physics of few- and many-body phenomena with interacting photons. This thesis presents a series of experiments that study and exploit different regimes of Rydberg-mediated interactions.
We report the realization of an efficient on-demand single-photon source. The strong long-range Rydberg interactions allow the excitation of only a single collective Rydberg state within the entire atomic medium. The collective excitation can be subsequently retrieved as a single-photon. We use this scheme to generate highly pure and highly indistinguishable photons, which are suitable for scalable quantum information applications. These photons can be compatible with other atomic systems due to their narrow bandwidth, making building hybrid quantum systems feasible. Here, we demonstrate high visibility two-photon quantum interference between our Rydberg-produced photons, and photons emitted by a remote single-trapped ion.
We also study Rydberg atoms under electromagnetically induced transparency conditions, where coherent superpostions of photons and Rydberg excitations propagate through the atomic medium as lossless dark-state polaritons. In the experiment, we use the external control fields to tune the interactions to a many-body regime where we can observe resonant scattering of dark-state polaritons to lossy channels. We show that the enhanced scattering process arises as a pure three-body effect.
Jonathan Curtis - July 13, 2020
Dissertation Title: Quasiparticles in Superfluids and Superconductors
Date and Time: Monday, July 13, 2:30 pm
Location: Zoom
Dissertation Committee Chair: Professor Victor Galitski
Committee:
Professor Gretchen Campbell
Professor Alexey Gorshkov
Professor Mohammad Hafezi
Professor Jay Deep Sau
Abstract:
We start by considering a two-dimensional s-wave BCS superconductor interacting with microwave cavity photons. First we show how a nonequilibrium distribution of the cavity photons can be used to redistribute Bogoliubov quasiparticles, resulting in a nonequilibrium enhancement of the superconducting spectral gap. The analytic dependence of the nonequilibrium enhancement is presented in terms of the cavity photon spectral function and occupation function, offering a wide parameter space over which enhancement exists. Next, we analyze the equilibrium properties a similar two-dimensional cavity system, now including a strong sub-dominant pairing interaction in the d-wave channel. In this case there is a collective mode known as the Bardasis-Schrieffer mode, which is essentially a Cooper pair with higher orbital angular momentum. We show that by driving an external supercurrent through the sample these Bardasis-Schrieffer modes can be hybridized with the cavity photons, forming exotic Bardasis-Schrieffer-polaritons.
We then proceed on to consider a flowing Bose-Einstein condensate. In the presence of inhomogeneous flows, the long-wavelength propagation of Bogoliubov quasiparticles can be mapped onto the kinematics of relativistic quantum matter fields propagating through a curved spacetime. In particular, this allows for the simulation of a black hole and its interactions with quantum matter via analogy. We show that in the case of a step-like jump in the flow velocity and condensate density the emission of analogue Hawking radiation is accompanied by evanescent modes which are pinned to the event horizon.
Finally, we generalize this setup to include pseudo-spin one-half spinor condensates. In this case, we show that the analogue spacetime the Bogoliubov quasiparticles experience can be of an exotic Newton-Cartan type. Newton-Cartan gravity -- the geometric formulation of Newtonian gravity -- is realized when the Goldstone mode dispersion is quadratic as opposed to linear. The nature of the analogue spacetime is controlled by the presence or absence of an easy-axis anisotropy in the spin-dependent boson interaction. We conclude by arguing that this Newton- Cartan spacetime can be experimentally realized in current platforms.
Yahya Alavirad - June 30, 2020
Dissertation Title: Electromagnetic properties of various topological quantum materials
Date and Time: Tuesday, June 30, 11:00 am
Location: Zoom
Dissertation Committee Chair: Professor Jay D Sau
Committee:
Professor Maissam Barkeshli
Professor Theodore Einstein
Professor Mohammad Hafezi
Professor Christopher Jarzynski (Dean's Representative)
Abstract:
In the past few decades, research into electromagnetic properties of topological quantum materials has been one of the most active research areas in the field of condensed matter physics. Physicists have discovered a large class of materials, e.g. Weyl semimetals, topological insulators, and topological superconductors that can host a plethora of interesting topological properties. In addition to their theoretical value as novel and exotic phases of quantum matter, topological quantum materials provide a promising platform for an array of technological applications, particularly as building blocks of topological quantum computers. Unfortunately, despite great progress in the theoretical understanding of topological phases of matter, practical problems have made it difficult to: (i) identify unambiguous examples of topological quantum material and (ii) harness their potential for technological applications. The overarching goal of this thesis is to understand such difficulties and to find ways to overcome them by studying specific problems.
This thesis is divided into four independent parts, each of which is dedicated to a particular problem: In the first part, we study chiral magnetic effect in Weyl semimetals and discuss whether it can be used to probe topological properties of Weyl semimetals in real experiments. In the second part, we propose an experimental setup to realize a certain type of topological excitation called parafermionic zero mode using a quantum dot array structure from the 2/3 fractional quantum Hall state. Importantly, our proposal does not rely on Andreev backscattering. We argue that this feature makes our proposal suitable for experimental realization. In the third part, we provide a quantitative analysis of supercurrent in superconductor/quantum Hall/superconductor junctions and show that by making critical assumptions about the interface, it is possible to obtain a quantitative agreement between theory and the magnitude of the observed supercurrent. In the fourth part, we study quantum anomalous Hall effect and flavor ferromagnetism in twisted bilayer graphene and argue that the one-magnon spectrum can be used as a numerically accessible tool to study the stability of the quantum anomalous Hall phase in twisted bilayer graphene.
Leonard Campanello - June 26, 2020
Dissertation Title: Quantifying the Organization and Dynamics of Excitable Signaling Networks
Date and Time: Friday, June 26, 11:30 am
Location: Zoom
Dissertation Committee Chair: Professor Wolfgang Losert
Committee:
Professor John Fourkas (Dean's Representative)
Professor Michelle Girvan
Professor Arpita Upadhyaya
Professor Brian Schaefer (USUHS Immunology)
Abstract:
The transmission of extracellular information through intracellular signaling networks is ubiquitous in biology–-from single-celled organisms to complex multicellular systems. Via signal-transduction machinery, cells of all types can detect and respond to biological, chemical, and physical stimuli. Although studies of signaling mechanisms and pathways traditionally involve arrays of biochemical assays, detailed quantification of physical information is becoming an increasingly important tool for understanding the complexities of signaling. With the rich datasets currently being collected in biological experiments, understanding the mechanisms that govern intracellular signaling networks is becoming a multidisciplinary problem at the intersection of biology, computer science, physics, and applied mathematics.
In this dissertation, I focus on understanding and characterizing the physical behavior of signaling networks. Through analysis of experimental data, statistical modeling, and computational simulations, I explore a characteristic of signaling networks called excitability, and show that an excitable-systems framework is broadly applicable for explaining the connection between intracellular behaviors and cell functions.
One way to connect the physical behavior of signaling networks to cell function is through the structural and spatial analyses of signaling proteins. In the first part of this dissertation, I employ an adaptive-immune-cell model with a key activation step that is both promoted and inhibited by a microns-long, filamentous protein complex. In combining super-resolution images and a novel image-based bootstrap-like resampling method, I demonstrate that the spatial organization of signaling proteins is an important contributor to immune-cell self-regulation. To probe the excitable dynamics of the system, I develop a Monte Carlo simulation of nucleation-limited growth and degradation, and show that careful balance between simulation parameters can elicit a tunable response dynamic.
The spatiotemporal dynamics of signaling components are also important mediators of cell function. One key readout of the connection between signaling dynamics and cell function is the behavior of the cytoskeleton. In the second part of this dissertation, I use innate-immune-cell and epithelial-cell models to understand how a key cytoskeletal component, actin, is influenced by topographical features in the extracellular environment. Engineered nanotopographic substrates similar in size to typical extracellular-matrix structures have been shown to bias the flow of actin, a concept known as esotaxis. To measure this bias, I introduce a generalizable optical-flow-based-analysis suite that can robustly and systematically quantify the spatiotemporal dynamics of actin in both model systems. Interestingly, despite having wildly different migratory phenotypes and physiological functions, both cell types exhibit quantitatively similar topography-guidance dynamics which suggests that sensing and responding to extracellular textures is an evolutionarily conserved phenomenon.
The signaling mechanisms that enable actin responses to the physical environment are poorly understood. Despite experimental evidence for the enhancement of actin-nucleation-promoting factors (NPFs) on extracellular features, connecting texture-induced signaling to overall cell behavior is an ongoing challenge. In the third part of this dissertation, I study the topography-induced guidance of actin in amoeboid cells on nanotopographic textures of different spacings. Using optical-flow analysis and statistical modeling, I demonstrate that topography-induced guidance is strongest when the features are similar in size to typical actin-rich protrusions. To probe this mechanism further, I employ a dendritic-growth simulation of filament assembly and disassembly with realistic biochemical rates, NPFs, and filament-network-severing dynamics. These simulations demonstrate that topography-induced guidance is more likely the result of a redistribution, rather than an enhancement, of NPF components.
Overall, this dissertation introduces quantitative tools for the analysis, modeling, and simulations of excitable systems. I use these tools to demonstrate that an excitable-systems framework can provide deep, phenomenological insights into the character, organization, and dynamics of a variety of biological systems.
Bakhrom Oripov - June 25, 2020
Dissertation Title: SUPERCONDUCTING RADIO FREQUENCY MATERIAL SCIENCE THROUGH NEAR-FIELD MAGNETIC MICROSCOPY
Date and Time: Thursday, June 25, 1:00 pm
Location: Zoom
Dissertation Committee Chair: Professor Steven M. Anlage
Committee:
Professor Gianluigi Ciovati
Professor Ichiro Takeuchi
Professor Christopher Lobb
Professor Benjamin Palmer
Abstract:
Superconducing Radio-Frequency (SRF) cavities are the backbone of a new generation of particle accelerators used by the High Energy Physics community. Nowadays, the applications of SRF cavities have expanded far beyond the needs of basic science. The proposed usages include waste treatment, water disinfection, material strengthening, medical applications and even use as high Q resonators in quantum computers. A practical SRF cavity needs to operate at extremely high rf fields while remaining in the low-loss superconducting state. State of the art Nb cavities can easily reach quality factors Q > 2 × 1010 at 1.3 GHz.
Currently, the performance of the SRF cavities is limited by surface defects which lead to cavity breakdown at high accelerating gradients. Also, there are efforts to reduce the cost of manufacturing SRF cavities, and the cost of operation. This will require an R&D effort to go beyond bulk Nb cavities. Alternatives to bulk Nb are Nb-coated Copper and Nb3Sn cavities. When a new SRF surface treatment, coating technique, or surface optimization method is being tested, it is usually very costly and time consuming to fabricate a full cavity. A rapid rf characterization technique is needed to identify deleterious defects on Nb surfaces and to compare the surface response of materials fabricated by different surface treatments. In this thesis a local rf characterization technique that could fulfill this requirement is presented.
First, a Scanning Magnetic Microwave Microscopy technique was used to study SRF grade Nb samples. Using this novel microscope the existence of surface weak-links was confirmed through their local nonlinear response. Time-Dependent Ginzburg-Landau (TDGL) simulations were used to reveal that vortex semiloops are created by the inhomogenious magnetic field of the magnetic probe, and contribute to the measured response.
Also, a system was put in place to measure the surface resistance of SRF cavities at extremely low temperatures, down to T = 70 mK, where the predictions for the surface resistance from various theoretical models diverge. SRF cavities require special treatment during the cooldown and measurement. This includes cooling the cavity down at a rate greater than 1K/minute, and very low ambient magnetic field B < 50 nT. I present solutions to both of these challenges.
Seokjin Bae - June 12, 2020
Dissertation Title: Microwave Study of the Properties of Unconventional Superconducting Systems
Date and Time: Friday, June 12, 10:00 am
Location: Zoom
Dissertation Committee Chair: Professor Steven M. Anlage
Committee:
Professor Frederick C. Wellstood
Professor Ichiro Takeuchi (Dean's Representative)
Professor Nicholas P. Butch
Professor Johnpierre Paglione
Abstract:
In this dissertation, unconventional superconducting systems, such as superconductors with non s-wave pairing symmetry and superconductors with non-trivial topology, are investigated by means of microwave experimental techniques. The thesis consists of two parts. Part 1 of the thesis will discuss a newly developed microwave superconducting gap spectroscopy system. Using a combination of the resonant microwave transmission technique and laser scanning microscopy, it is demonstrated that the new technique can directly image the pairing symmetry of superconductors with unconventional pairing symmetries. During the demonstration with an example d-wave superconductor, a signature of Andreev bound states was also found. A phenomenological model to explain the observed properties of the Andreev bound states is also discussed and compared to data. Lastly, an effort to broaden the applicability of the new technique to samples of more general morphology is discussed.
Part 2 of the thesis will introduce the microwave surface impedance technique and its application to the characterization of topological superconducting systems. A thickness dependent surface reactance study of an artificial topological superconductor SmB6/YB6 (topological insulator / superconductor bilayer) was used to determine the characteristic lengths of the system (normal coherence length, penetration depth, and thickness of the topological surface state), and revealed robust bulk insulating properties of the SmB6 thin films. A surface resistance and reactance study on the candidate intrinsic topological superconductor UTe2 revealed the existence of residual normal fluid and a chiral spin-triplet pairing state, which together point out the possible existence of an itinerant Majorana normal fluid on the surface of chiral superconductors.
Soubhik Kumar - May 12, 2020
Dissertation Title: Cosmological Probes of Physics Beyond the Standard Model
Date and Time: Tuesday, May 12, 3:00 pm
Location: Zoom
Dissertation Committee Chair: Prof. Raman Sundrum
Committee:
Prof. Kaustubh Agashe
Prof. Zackaria Chacko
Prof. Anson Hook
Prof. Richard Wentworth
Abstract:
Future measurements of primordial non-Gaussianity (NG) can reveal the mass and the spin information of cosmologically produced particles with masses of order the inflationary Hubble scale, H_inf, which can be as high as 10^13 GeV. In this talk, I will describe how such NG measurements can be used as an on-shell probe of, a) Grand Unified Theories and, b) low scale gauge theories that get “heavy-lifted” to ~ H_inf scales. I will also discuss a simple alternative to the standard inflationary paradigm, involving a curvaton field, that can allow NG signals orders of magnitude larger compared to standard inflation. This brings various motivated particle physics signatures, such as loops of heavy gauge-charged scalars and fermions, within future observational reach.
Elizabeth Paul - May 6, 2020
Dissertation Title: Adjoint methods for stellarator shape optimization
Date and Time: Wednesday, May 6, 9:00 am
Location: Zoom
Dissertation Committee Chair: Dr. William Dorland
Committee:
Dr. Matthew Landreman
Professor Adil Hassam
Professor Thomas Antonsen
Professor Ricardo Nochetto
Abstract:
Stellarators are a class of device for the magnetic confinement of plasmas without toroidal symmetry. As the confining magnetic field is produced by clever shaping of external electro-magnetic coils rather than through internal plasma currents, stellarators enjoy enhanced stability properties over their two-dimensional counterpart, the tokamak. However, the design of a stellarator with acceptable confinement properties requires numerical optimization of the magnetic field in the non-convex, high-dimensional spaces describing their geometry. Another major challenge facing the stellarator program is the sensitive dependence of confinement properties on electro-magnetic coil shapes, necessitating the construction of the coils under tight tolerances. In this Thesis, we address these challenges with the application of adjoint methods and shape sensitivity analysis.
Adjoint methods enable the efficient computation of the gradient of a function that depends on the solution to a system of equations, such as linear or nonlinear PDEs. Rather than perform a finite-difference step with respect to each parameter, one additional adjoint PDE is solved to compute the derivative with respect to any parameter. This enables gradient-based optimization in high-dimensional spaces and efficient sensitivity analysis. We present the first applications of adjoint methods for stellarator shape optimization.
The first example we discuss is the optimization of coil shapes based on the generalization of a continuous current potential model. We optimize the geometry of the coil-winding surface using an adjoint-based method, producing coil shapes that can be more easily constructed. Understanding the sensitivity of coil metrics to perturbations of the winding surface allows us to gain intuition about features of configurations that enable simpler coils. We next consider solutions of the drift-kinetic equation, a kinetic model for collisional transport in curved magnetic fields. An adjoint drift-kinetic equation is derived based on the self-adjointness property of the Fokker-Planck collision operator. This adjoint method allows us to understand the sensitivity of neoclassical quantities, such as the radial collisional transport and self-driven plasma current, to perturbations of the magnetic field strength. Finally, we consider functions that depend on solutions of the magneto-hydrodynamic (MHD) equilibrium equations. We generalize the well-known self-adjointness property of the MHD force operator to include perturbations of the rotational transform and the currents outside the confinement region. This self-adjointness property is applied to develop an adjoint method for computing the derivatives of such functions with respect to perturbations of coil shapes or the plasma boundary. We present a method of solution for the adjoint equations based on a variational principle used in MHD stability analysis.
Yiming Cai - April 14, 2020
Dissertation Title: New insights into the sign problem of QCD and heavy tetraquarks
Date and Time: Tuesday, April 14, 2:00 pm
Location: Zoom
Dissertation Committee Chair: Professor Thomas Cohen
Committee:
Professor Paulo Bedaque
Professor Sarah Eno
Professor Zackaria Chacko
Professor Alice Mignerey
Abstract:
Scott Lawrence - April 13, 2020
Dissertation Title: Sign Problems in Quantum Field Theory: Classical and Quantum Approaches
Date and Time: Monday, April 13, 10:00 am
Location: Zoom
Dissertation Committee Chair: Paulo Bedaque
Committee:
Andrew Baden
Zackaria Chacko
Massimo Ricotti
Raman Sundrum
Abstract:
Francisco Salces Carcoba - April 6, 2020
Dissertation Title: Microscopy of Elongated Superfluids
Date and Time: Tuesday, March 31, 12:45 pm
Location: Zoom
Dissertation Committee Chair: Luis Orozco
Committee:
Alicia Kollar
Mohammad Hafezi (Dean's Rep)
William D. Phillips
Ian Spielman (Advisor)
Abstract:
Quasi one-dimensional (1D) clouds of ultracold atoms are an ideal platform for quantum simulation and experimental benchmarking of simple 1D physics. I present two experiments with elongated clouds of ultracold alkali Rb-87 in two different 1D regimes, where optical microscopy comprises measurement. In a first experiment, we probe the thermodynamics of individual, strongly interacting 1D Bose gases by measuring their equations of state in-situ. In a second experiment with weakly interacting elongated condensates, we apply digital holographic microscopy to transfer hardware complexity into software by removing the effects of aberrations in our resonant absorption images.
Daniel Woodbury - March 31, 2020
Dissertation Title: APPLICATIONS OF INTENSE MID-INFRARED LASER-PLASMA INTERACTIONS
Date and Time: Tuesday, March 31, 10:00 am
Location: Zoom
Dissertation Committee Chair: Prof. Howard Milchberg
Committee:
Prof. Ki-Yong Kim
Prof. Phillip Sprangle
Prof. Adil Hassam
Dr. Jared Wahlstrand
Abstract:
Intense laser-plasma interactions, generally characterized by focused laser intensities exceeding several TW per cm2, are of basic and applied interest in a number of areas, , including the generation of relativistic charged particle beams, high energy photon generation, and nonlinear optics extending to the relativistic regime . Mid-infrared (mid-IR) lasers provide favorable wavelength scaling of various laser-plasma interaction parameters compared to commonly used near-infrared systems, and in several cases enable entirely new phenomena. In this dissertation we present experimental and computational results relating to three laser-plasma based applications using ultrashort mid-infrared and long-wave-infrared laser pulses. First, we demonstrate self-modulated laser wakefield acceleration driven by mid-IR laser pulses collapsing in near-critical density targets, and discuss scaling from common near-infrared systems. Second, we demonstrate that electron avalanche breakdown driven by picosecond, mid-IR lasers drive discrete breakdowns from individual seed electrons, providing a unique and extremely sensitive method of detecting ultralow plasma densities in gases. We apply this technique to demonstrate standoff detection of radioactive materials, and to measure laser ionization yield in atmospheric pressure range gases over an unprecedented 14 orders of magnitude. Finally, we present a theory of self-guiding for high power mid-infrared and long-wave-infrared multi-picosecond pulses interacting with discrete avalanche breakdown sites, and show through propagation simulations that aerosol-initiated avalanches support self-guiding at moderate intensities.
James Juno - March 27, 2020
Dissertation Title: A Deep Dive into the Distribution Function: Understanding Phase Space Dynamics using Continuum Vlasov-Maxwell Simulations
Date and Time: Friday, March 27, 3:30 pm
Location: Zoom
Dissertation Committee Chair: Professor William Dorland
Committee:
Dr. James TenBarge
Professor James Drake
Professor Adil Hassam
Professor Jacob Bedrossian
Abstract:
In collisionless and weakly collisional plasmas, the particle distribution function is a rich tapestry of the underlying physics. However, actually leveraging the particle distribution function to understand the dynamics of a weakly collisional plasma is challenging. The equation system of relevance, the Vlasov-Maxwell-Fokker-Planck (VM-FP) system of equations, is difficult to numerically integrate, and traditional methods such as the particle-in-cell method introduce counting noise into the distribution function.
In this final public examination, we present a new algorithm for the discretization of VM-FP system of equations for the study of plasmas in the kinetic regime. Using the discontinuous Galerkin (DG) finite element method for the spatial discretization and a third order strong-stability preserving Runge--Kutta for the time discretization, we obtain an accurate solution for the plasma's distribution function in space and time.
We both prove the numerical method retains key physical properties of the VM-FP system, such as the conservation of energy and the second law of thermodynamics, and demonstrate these properties numerically. These results are contextualized in the history of the DG method. We discuss the importance of the algorithm being alias-free, a necessary condition for deriving stable DG schemes of kinetic equations so as to retain the implicit conservation relations embedded in the particle distribution function, and the computational favorable implementation using a modal, orthonormal basis in comparison to traditional DG methods applied in computational fluid dynamics.
A diverse array of simulations are performed which exploit the advantages of our approach over competing numerical methods. We demonstrate how the high fidelity representation of the distribution function, combined with novel diagnostics, permits detailed analysis of the energization mechanisms in fundamental plasma processes such as collisionless shocks. Likewise, we show the undesirable effect particle noise can have on both solution quality, and ease of analysis, with a study of kinetic instabilities with both our continuum VM-FP method and a particle-in-cell method.
Our VM-FP solver is implemented in the Gkeyll framework (https://github.com/ammarhakim/gkyl), a modular framework for the solution to a variety of equation systems in plasma physics and fluid dynamics.
Bo Miao - January 14, 2020
Dissertation Title: Laser wakefield accelerator experiments: coherent injection radiation and optical field ionization-based plasma waveguides
Date and Time: Tuesday, January 14, 10:00 am
Location: ERF 1207
Dissertation Committee Chair: Prof. Howard Milchberg
Committee:
Prof. James Drake
Prof. Julius Goldhar
Prof. Ki-Yong Kim
Prof. Philip Sprangle
Abstract:
Laser wakefield electron accelerators (LWFAs) can support accelerating gradients three orders of magnitude higher than conventional radio frequency linear accelerators, enabling compact laser-driven devices. In this dissertation, I explore two regimes of LWFA physics, one at high plasma density, and the other at low density.
The first part of this thesis characterizes bright broadband coherent radiation emitted during wakefield acceleration driven by femtosecond laser interaction with high, near-critical density plasma. Detailed measurement is presented of the radiation spectrum, polarization and angular distribution. The results are consistent with synchrotron radiation emission from laser-assisted injection into wakefields, with this picture supported by particle-in-cell simulations.
The second part of this thesis demonstrates the use of high intensity Bessel beams of various orders for generating low density plasma waveguides that guide high intensity laser pulses over tens of centimeters. Methods are presented for Bessel beam generation and focus optimization using adaptive optics.
Efim Rozenbaum - January 10, 2020
Dissertation Title: Numerical Studies of Quantum Chaos in Various Dynamical Systems
Date and Time: Friday, January 10, 11:00 am
Location: PSC 3150
Dissertation Committee Chair: Prof. Victor Galitski
Committee:
Prof. Christopher Jarzynski
Prof. Gretchen Campbell
Prof. Jay Deep Sau
Prof. James Williams
Abstract:
We study two classes of quantum phenomena associated with classical chaos in a variety of quantum models: (i) dynamical localization and its extension and generalization to interacting few- and many-body systems and (ii) quantum exponential divergences in high-order correlators and other diagnostics of quantum chaos.
Dynamical localization (DL) is a subtle phenomenon related to Anderson localization. It hinges on quantum interference and is typically destroyed in presence of interactions. DL often manifests as a failure of a driven system to heat up, violating the foundations of statistical physics. Kicked rotor (KR) is a prototypical chaotic classical model that exhibits linear energy growth with time. The quantum kicked rotor (QKR) features DL instead: its energy saturates. Multiple attempts of many-body generalizations faced difficulties in preserving DL. Recently, DL was shown in a special integrable many-body model. We study non-integrable models of few- and many-body QKR-like systems and provide direct evidence that DL can persist there. In addition, we show how a novel related concept of localization landscape can be applied to study transport in rippled channels.
Out-of-time-ordered correlator (OTOC) was proposed as an indicator of quantum chaos, since in the semiclassical limit, this correlator's possible exponential growth rate (CGR) resembles the classical Lyapunov exponent (LE). We show that the CGR in QKR is related, but distinct from the LE in KR. We also show a singularity in the OTOC at the Ehrenfest time tE due to a delay in the onset of quantum interference. Next, we study scaling of OTOC beyond tE. We then explore how the OTOC-based approach to quantum chaos relates to the random-matrix-theoretical description by introducing an operator we dub the Lyapunovian. Its level statistics is calculated for quantum stadium billiard, a seminal model of quantum chaos, and aligns perfectly with the Wigner-Dyson surmise. In the semiclassical limit, the Lyapunovian reduces to the matrix of uncorrelated finite-time Lyapunov exponents, connecting the CGR at early times, when the quantum effects are weak, to universal level repulsion that hinges on strong quantum interference. Finally, we consider quantum polygonal billiards: their classical counterparts are non-chaotic. We show exponential growth of the OTOCs in these systems, sharply contrasted with the classical behavior even before quantum interference develops.Ksenia Sosnova - January 9, 2020
Dissertation Title: Mixed-Species Ion Chains for Quantum Networks
Date and Time: Thursday, January 9, 11:00 am
Location: PSC 3150
Dissertation Committee Chair: Prof. Christopher Monroe
Committee:
Prof. Christopher Jarzynski, Dean's Representative
Prof. Gretchen Campbell
Prof. James Williams
Prof. Norbert Linke
Abstract:
Quantum computing promises solutions to some of the world's most important problems that classical computers have failed to address. The trapped-ion-based quantum computing platform has a lot of advantages for doing so: ions are perfectly identical and near-perfectly isolated, feature long coherent times, and allow high-fidelity individual laser-controlled operations. One of the greatest remaining obstacles in trapped-ion-based quantum computing is the issue of scalability. The approach that we take to address this issue is a modular architecture: separate ion traps, each with a manageable number of ions, are interconnected via photonic links. To avoid photon-generated crosstalk between qubits and utilize advantages of different kinds of ions for each role, we use two distinct species -- 171Yb+ as memory qubits and 138Ba+ as communication qubits. The qubits based on 171Yb+ are defined within the hyperfine “clock” states characterized by a very long coherence time while 138Ba+ ions feature visible-range wavelength emission lines. Current optical and fiber technologies are more efficient in this range than at shorter wavelengths.
We present a theoretical description and experimental demonstration of the key elements of a quantum network based on the mixed-species paradigm. The first one is entanglement between an atomic qubit and the polarization degree of freedom of a pure single photon. To verify the purity of single photons, we measure the second-order correlation function and find g(2)(0)=(8.1±2.3)×10-5 without background subtraction, which is consistent with the lowest reported value in any system. Next, we show mixed-species entangling gates with two ions using the Mølmer-Sørensen and Cirac-Zoller protocols. Finally, we theoretically generalize mixed-species entangling gates to long ion chains and characterize the roles of normal modes there. In addition, we explore sympathetic cooling efficiency in such mixed-species crystals. Besides these developments, we demonstrate new techniques for manipulating states within the D3/2-manifold of zero-nuclear-spin ions -- a part of a protected qubit scheme promising seconds-long coherence times proposed by Aharon et al. in 2013.
Noah Sennett - December 13, 2019
Dissertation Title: Probing fundamental physics with gravitational waves from inspiraling binary systems
Date and Time: Friday, December 13, 2:30 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Alessandra Buonanno
Committee:
Prof. Theodore Jacobson
Prof. Julie McEnery
Prof. Peter Shawhan
Prof. Raman Sundrum
Prof. Massimo Ricotti, Dean’s Representative
Abstract:
The first observations of gravitational waves from the mergers of black holes and/or neutron stars with Advanced LIGO and Virgo have opened a new window to the cosmos. This thesis examines how the gravitational-wave signal produced during the inspiral---the earliest phase of a binary system’s coalescence---can better inform our understanding of the highly dynamical, strong-curvature regime of gravity.
My work addressing this topic is comprised of two major components. First, I examine the behavior of binary black-hole and neutron-star systems in various possible extensions of General Relativity, constructing analytic models of their orbital motion and gravitational-wave production during their inspiral. Particular attention is devoted to alternative theories that admit scalarization, a second-order phase transition that occurs in compact bodies or binary systems that can manifest as non-perturbative phenomenology in a gravitational-wave signal.
The other component of this thesis is the development of a statistical infrastructure suitable for testing General Relativity using gravitational-wave observations. This framework is more flexible and modular approach than existing alternatives, allowing this infrastructure to be immediately employed with a wide range of waveform models. In work done in conjunction with the LIGO Scientific and Virgo Collaborations, I use this statistical framework to place bounds on phenomenological deviations from General Relativity using the binary black-hole and neutron-star events detected during LIGO's first and second observing runs. I also use this infrastructure to constrain certain specific alternative theories of gravity, including Brans-Dicke gravity.
Andrew Glaudell - December 9, 2019
Dissertation Title: Quantum Compiling Methods for Fault-Tolerant Gate Sets of Dimension Greater than Two
Date and Time: Monday, December 9, 12:15 pm
Location: PSC 3150
Dissertation Committee Chair: Andrew Childs
Committee:
Jacob M. Taylor (Advisor)
Norbert Linke
Michael Hicks
Larry Washington
Abstract:
Fault-tolerant gate sets whose generators belong to the Clifford hierarchy form the basis of many protocols for scalable quantum computing architectures. At the beginning of the decade, number-theoretic techniques were employed to analyze circuits over these gate sets on single qubits, providing the basis for a number of state-of-the-art quantum compiling algorithms. In this dissertation, I further this program by employing number-theoretic techniques for higher-dimensional gate sets on both qudit and multi-qubit circuits.
First, I introduce canonical forms for single qutrit Clifford+T circuits and prove that every single-qutrit Clifford+T operator admits a unique such canonical form. I show that these canonical forms are T-optimal and describe an algorithm which takes as input a Clifford+T circuit and outputs the canonical form for that operator. The algorithm runs in time linear in the number of gates of the circuit. Our results provide a higher-dimensional generalization of prior work by Matsumoto and Amano who introduced similar canonical forms for single-qubit Clifford+T circuits. Finally, we show that a similar extension of these normal forms to higher dimensions exists, but do not establish uniqueness.
Moving to multi-qubit circuits, I provide number-theoretic characterizations for certain restricted Clifford+T circuits by considering unitary matrices over subrings of Z[1/√2, i]. We focus on the subrings Z[1/2], Z[1/√2], Z[1/√−2], and Z[1/2, i], and we prove that unitary matrices with entries in these rings correspond to circuits over well-known universal gate sets. In each case, the desired gate set is obtained by extending the set of classical reversible gates {X, CX, CCX} with an analogue of the Hadamard gate and an optional phase gate.
I then establish the existence and uniqueness of a normal form for one of these gate sets, the two-qubit gate set of Clifford+Controlled Phase gate CS. This normal form is optimal in the number of CS gates, making it the first normal form that is non-Clifford optimal for a fault tolerant universal multi-qubit gate set. We provide a synthesis algorithm that runs in a time linear in the gate count and outputs the equivalent normal form. In proving the existence and uniqueness of the normal form, we likewise establish the generators and relations for the two-qubit Clifford+CS group. Finally, we demonstrate that a lower bound of 5 log2(1/ε) + O(1) CS gates are required to ε-approximate any 4 × 4 unitary matrix.
Lastly, using the characterization of circuits over the Clifford+CS gate set and the existence of an optimal normal form, I provide an optimal ancilla-free inexact synthesis algorithm for two-qubit unitaries using the Clifford+SC gate set for Pauli-rotations. These operators require 6 log2(1/ε) + O(1) CS gates to synthesize in the typical case and 8 log2(1/ε) + O(1) in the worst case.
Andrew Smith - December 5, 2019
Dissertation Title: Studies in Nonequilibrium Quantum Thermodynamics
Date and Time: Thursday, December 5, 3:00 pm
Location: IPST Building Room 1116
Dissertation Committee Chair: Christopher Jarzynski
Committee:
Victor Yakovenko
Norbert Linke
Theodore Einstein
Sebastian Deffner
Abstract:
The first part of this thesis focuses on verifying the quantum nonequilibrium work relation in the presence of decoherence. The nonequilibrium work relation is a generalization of the second law of thermodynamics that links nonequilibrium work measurements to equilibrium free energies via an equality. Despite being well established for classical systems, a quantum work relation is conceptually difficult to construct for systems that interact with their environment. We argue that for a quantum system which undergoes decoherence but not dissipation, these conceptual difficulties do not arise and the work relation can be proven similarly to the case of an isolated system. This result is accompanied by an experimental demonstration using trapped ions.
The second part of this thesis examines the relationship between quantum work and coherence by constructing analogous quantities in classical physics. It has recently been shown that quantum coherence can function as a resource for work extraction. Furthermore, it has been suggested that this property could be a truly quantum aspect of thermodynamics with no classical analog. We examine this assertion within the framework of classical Hamiltonian mechanics and canonical quantization. For classical states we define a so called non-uniformity measure and show that it is a resource for work extraction similar to quantum coherence. Additionally, we show that work extracted from non-uniformity and coherence agree in the classical limit. This calls into question the idea that coherence qualitatively separates classical and quantum thermodynamics.
The final part of this thesis explores the connection between decoherence and adiabatic (quasistatic) driving. This topic is inspired by an experiment where it was seen that strong dephasing suppressed energy level transitions. Using a perturbative method we investigate this mechanism in the regime of small to moderate decoherence rate and ask if decoherence can help suppress energy transitions when compared with an adiabatic process without decoherence. We find that strategies that include decoherence are inferior to those where decoherence is absent.
While all of these topics will be touched upon in the defense presentation, the talk will focus on work, quantum coherence, and classical non-uniformity.
AnaValdés-Curiel - December 2, 2019
Dissertation Title: Topological dispersion relations in spin-orbit coupled Bose gases
Date and Time: Monday, December 2, 2:00 pm
Location: PSC 2136
Dissertation Committee Chair: Dr. Alicia Kollár
Committee:
Dr. Ian Spielman
Dr. Mohammad Hafezi
Dr. Norbert Linke
Dr. Gretchen Campbell
Dr. Trey Porto
Abstract:
Quantum degenerate gases have proven to be an ideal platform for the simulation of complex systems. Due to their high level of control it is possible to readily design and implement systems with effective Hamiltonians in the laboratory. This thesis presents new tools for the characterization and control of engineered quantum systems and describes one application in the engineering and characterization of a topological system with Rashba-type spin-orbit coupling.
The underlying properties of these engineered systems depend on their single particle energies. I describe a Fourier transform spectroscopy technique for characterizing the single particle spectrum of a quantum system. We tested Fourier spectroscopy by measuring the dispersion relation of a spin-1 spin-orbit coupled Bose-Einstein condensate (BEC) and found good agreement with our predictions.
Decoherence due to uncontrolled fluctuations of the environment presents fundamental obstacles in quantum science. I describe an implementation of continuous dynamical decoupling (CDD) in a spin-1 BEC. We applied a strong radio-frequency magnetic field to the ground state hyperfine manifold of Rubidium-87 atoms, generating a dynamically protected dressed system that was first-order insensitive to changes in magnetic field. The CDD states constitute effective clock states and we observed a reduction in sensitivity to magnetic field of up to four orders of magnitude. I show that the CDD states can be coupled in a fully connected geometry and thus enable the implementation of new models not possible using the bare atomic states.
Finally, I describe the engineering of Rashba-type SOC using Raman coupled CDD states. Our system had non-trivial topology but no underlying crystalline structure that yields integer valued Chern numbers in conventional materials. We validated our procedure using Fourier transform spectroscopy to measure the full dispersion relation containing only a single Dirac point. We measured the quantum geometry underlying the dispersion relation and obtained the topological index using matter-wave interferometry. In contrast to crystalline materials, where topological indices take on integer values, our continuum system reveals an unconventional half-integer Chern number.
Ali Hamed Moosavian - November 19, 2019
Dissertation Title: INITIAL STATE PREPARATION FOR SIMULATION OF QUANTUM FIELD THEORIES ON A QUANTUM COMPUTER
Date and Time: Tuesday, November 19, 11:00 am
Location: PSC 2136
Dissertation Committee Chair: Professor Andrew Childs (Chair, Co-Advisor)
Committee:
Professor Stephen Jordan (Advisor)
Professor Zohreh Davoudi
Professor Brian Swingle
Professor Mohammad Hafezi
Abstract:
In this thesis, we begin by reviewing some of the most important Hamiltonian simulation algorithms that are applied in simulation of quantum field theories. Then we focus on state preparation which has been the slowest subroutine in previously known algorithms. We present two distinct methods that improve upon prior results. The first method utilizes classical computational tools such as Density Matrix Renormalization Group to produce an efficient quantum algorithm for simulating fermionic quantum field theories in 1+1 dimensions. The second method presented is a heuristic algorithm that can prepare the vacuum of fermionic systems in more general cases and more efficiently than previous methods. With our last method, state preparation is no longer the bottleneck, as its runtime has the same asymptotic scaling with the desired precision as the remainder of the simulation algorithm. We then numerically demonstrate the effectiveness of this last method for the 1+1 dimensional Gross-Neveu model.
Jeremy Young - November 6, 2019
Dissertation Title: Nonequilibrium dynamics in open quantum systems
Date and Time: Wednesday, November 6, 3:00 pm
Location: PSC 1136
Dissertation Committee Chair: Professor Steven Rolston
Committee:
Professor Alexey Gorshkov (Advisor)
Professor Mohammad Maghrebi
Professor Mohammad Hafezi
Professor Maissam Barkeshli
Abstract:
Due to the variety of tools possible to control atomic, molecular, and optical (AMO) systems, they provide a versatile platform for studying many-body physics, quantum simulation, and quantum computation. Although extensive efforts are employed to reduce coupling between the system and the environment, the effects of the environment can never fully be avoided, so it is important to develop a comprehensive understanding of open quantum systems. The system-environment coupling often leads to loss via dissipation, which can be countered by a coherent drive. Open quantum systems subject to dissipation and drive are known as driven-dissipative systems, and they provide an excellent platform for studying many-body nonequilibrium physics.
The first part of this dissertation will focus on Rydberg atoms. In particular, we study how the spontaneous generation of contaminant Rydberg states drastically modifies the behavior of a driven-dissipative Rydberg system due to the resultant dipole-dipole interactions. These interactions lead to a complicated competition of both blockade and antiblockade effects, resulting in strongly enhanced Rydberg populations for far-detuned drive and reduced Rydberg populations for resonant drive.
The second part of this dissertation will focus on driven-dissipative phase transitions. In spite of the nonequilibrium nature of these systems, the corresponding phase transitions tend to exhibit emergent equilibrium behavior. However, we will show that in the vicinity of a multicritical point where multiple phase transitions intersect, genuinely nonequilibrium criticality can emerge, even though the individual phase transitions on their own exhibit equilibrium criticality. These nonequilibrium multicritical points can exhibit a variety of exotic phenomena not possible in their equilibrium counterparts, including the emergence of complex critical exponents, which lead to discrete scale invariance and spiraling phase boundaries. Furthermore, the Liouvillian gap can take on complex values, and the fluctuation-dissipation theorem is violated, corresponding to an effective “temperature” which is scale-dependent.
Sanjukta Krishnagopal - November 4, 2019
Dissertation Title: Machine Learning and Network Science to Unravel Patterns in Complex Temporal Data
Date and Time: Monday, November 4, 12:30 pm
Location: IREAP Seminar room ERF 1207
Dissertation Committee Chair: Dr. Michelle Girvan
Committee:
Dr. Brian Hunt
Dr. Edward Ott
Dr. Raj Roy
Dr. James Reggia
Abstract:
We uncover underlying patterns in complex data and forecast their temporal evolution through machine learning and network science. With the availability of more data, machine learning for data analysis has advanced rapidly. We primarily use a dynamical network architecture called reservoir computing to solve the ‘chaos’ version of the cocktail party problem, i.e., where a party-goer hears several signals but is interested in separating out only one signal. We separate superimposed signals from the chaotic Lorenz system and forecast the separated signals. We assume no knowledge of the system that produce the signals, and require only that the training data consist of finite time samples of the component signals. We find that our method significantly outperforms the optimal linear solution to the separation problem, the Wiener filter.
While machine learning is a useful data processing tool, often systems with multiple interacting components are best modeled as a network. Tools that identify properties on networks of temporal multi-variate data (such as disease data) are limited in literature. We close this gap by introducing a novel data-driven, network-based Trajectory Profile Clustering (TPC) algorithm for 1) identification of disease subtypes and 2) early prediction of subtype/disease progression patterns. TPC identifies subtypes by clustering patients with similar disease trajectory profiles derived from bipartite patient-variable networks. For the Parkinson’s dataset, we show that TPC identifies 3 distinct disease subtypes and predicts subtype in test patients 4 years in advance with 74% accuracy. While we implement it on Parkinson's datasets to identify patient subtypes and predict disease progression in new patients, this network medicine approach can be extended to any complex dataset.
Jaideep Pathak - October 18, 2019
Dissertation Title: Machine Learning Approaches for Data-Driven Analysis and Forecasting of High-Dimensional Chaotic Systems
Date and Time: Friday, October 18, 10:30 am
Location: A.V. Williams Building, Room 2460 (ECE Conference Room)
Dissertation Committee Chair: Dr. Edward Ott
Committee:
Dr. Michelle Girvan
Dr. Brian Hunt
Dr. Rajarshi Roy
Dr. Thomas Antonsen
Abstract:
We consider problems in the forecasting of large, complex, spatiotemporal chaotic systems and the possibility that machine learning might be a useful tool for significant improvement of such forecasts. Focusing on weather forecasting as perhaps the most important example of such systems, we note that physics-based weather models have substantial error due to various factors including imperfect modeling of subgrid-scale dynamics and incomplete knowledge of physical processes. In this thesis, we ask if machine learning can potentially correct for such knowledge deficits.
First, we demonstrate the effectiveness of using machine learning for model-free prediction of spatiotemporally chaotic systems of arbitrarily large spatial extent and attractor dimension purely from observations of the system's past evolution. We present a parallel scheme with an example implementation based on the reservoir computing paradigm and demonstrate the scalability of our scheme using the Kuramoto-Sivashinsky equation as an example of a spatiotemporally chaotic system.
We then demonstrate the use of machine learning for inferring fundamental properties of dynamical systems, namely the Lyapunov exponents, purely from observed data. We obtain results of unprecedented fidelity with our novel technique, making it possible to find the Lyapunov exponents of large systems where previously known techniques have failed.
Next, we propose a general method that combines a physics-informed knowledge-based model and a machine learning technique to build a hybrid forecasting scheme. We further extend our hybrid forecasting approach to the difficult case where only partial measurements of the state of the dynamical system are available. For this purpose, we propose a novel technique that combines machine learning with a data assimilation method called an Ensemble Transform Kalman Filter (ETKF).
We conclude by describing our ongoing collaboration with atmospheric science researchers on applying the above proposed techniques to a real-world weather forecasting scenario and also remark on the potential of machine learning assisted numerical weather prediction.
Aaron Ostrander - October 17, 2019
Dissertation Title: Quantum Algorithms for Differential Equations
Date and Time: Thursday, October 16, 10:00 am
Location: PSC 1136
Dissertation Committee Chair: Prof. Andrew Childs (Advisor)
Committee:
Prof. Chris Monroe (Co-Advisor)
Prof. Alexey Gorshkov
Prof. Mohammad Hafezi
Prof. Xiaodi Wu
Abstract:
This thesis describes quantum algorithms for Hamiltonian simulation, ordinary differential equations (ODEs), and partial differential equations (PDEs).
Product formulas are used to simulate Hamiltonians which can be expressed as a sum of terms which can each be simulated individually. By simulating each of these terms in sequence, the net effect approximately simulates the total Hamiltonian. We find that the error of product formulas can be improved by randomizing over the order in which the Hamiltonian terms are simulated. We prove that this approach is asymptotically better than ordinary product formulas and present numerical comparisons for small numbers of qubits.
The ODE algorithm applies to the initial value problem for time-independent first order linear ODEs. We approximate the propagator of the ODE by a truncated Taylor series, and we encode the initial value problem in a large linear system. We solve this linear system with a quantum linear system algorithm (QLSA) whose output we perform a post-selective measurement on. The resulting state encodes the solution to the initial value problem. We prove that our algorithm is asymptotically optimal with respect to several system parameters.
The PDE algorithms apply the finite difference method (FDM) to Poisson's equation, the wave equation, and the Klein-Gordon equation. We use high order FDM approximations of the Laplacian operator to develop linear systems for Poisson's equation in cubic volumes under periodic, Neumann, and Dirichlet boundary conditions. Using QLSAs, we output states encoding solutions to Poisson's equation. We prove that our algorithm is exponentially faster with respect to the spatial dimension than analogous classical algorithms. We also consider how high order Laplacian approximations can be used for simulating the wave and Klein-Gordon equations. We consider under what conditions it suffices to use Hamiltonian simulation for time evolution, and we propose an algorithm for these cases that uses QLSAs for state preparation and post-processing.
Daniel Campbell - October 16, 2019
Dissertation Title: Electronic and Magnetic Properties of MnP-Type Binary Compounds
Date and Time: Wednesday, October 16, 1:00 pm
Location: PSC 2126
Dissertation Committee Chair: Prof. Johnpierre Paglione
Committee:
Prof. Richard L. Greene
Prof. Nicholas P. Butch
Prof. James R. Williams
Prof. Efrain Rodriguez (Dean's Representative)
Abstract:
The interactions between electrons, and the resulting impact on physical properties, are at the heart of present-day materials science. This thesis looks at this idea through the lens of several compounds from a single family: the MnP-type transition metal pnictides. FeAs and FeP show long range magnetic order with some similarities to the high temperature, unconventional iron-based superconductors. CoAs lies on the border of magnetism, with strong fluctuations but no stable ordered state. CoP, in contrast, shows no strong magnetic fluctuations but serves as a useful baseline in determining the origin (from composition, structure, or magnetic order) of behavior in the other materials.
For this work, single crystals were grown with two different techniques: solvent flux and chemical vapor transport. In the case of FeAs the flux method resulted in the highest quality crystals yet produced. Extensive work was then performed on these samples at the University of Maryland and the National High Magnetic Field Laboratory. Quantum oscillations observed in high magnetic fields, in combination with density functional theory calculations, give insight into the Fermi surfaces of these materials. Large magnetoresistance in the phosphides, but not the arsenides, demonstrates differences in the choice of pnictogen atom that cannot be simply a product of electron count. Angle-dependent linear magnetoresistance in FeP is a sign of a possible Dirac dispersion and topological physics, as has been hinted it in other MnP-type materials. Ultimately, it is possible to examine results for all four compounds and draw conclusions on the role of each of the two elements in the formula, which can be extended to other members of this family.
Harvey Kaplan - September 25, 2019
Dissertation Title: Many-Body Dephasing in a Trapped Ion Quantum Simulator with a Cryogenic Apparatus
Date and Time: Wednesday, September 25, 3:00 pm
Location: PSC 2136
Dissertation Committee Chair: Professor Christopher Monroe
Committee:
Prof. James Williams
Prof. Norbert Linke
Prof. Trey Porto
Prof. Christopher Jarzynski
Abstract:
While realizing a fully functional quantum computer presents a long term technical goal, in the present, there are small to mid-sized quantum simulators (up to ~100 qubits), that are capable of approaching specialized problems. The quantum simulator discussed here is housed in a cryogenically cooled vacuum chamber in order to reduce the background pressure, thereby increasing ion chain length and life-time. The details of performance and characterization of this cryogenic apparatus are discussed, and this system is used to study many-body dephasing in a finite-sized quantum spin system.
How a closed quantum many-body system relaxes and dephases as a function of time is important to understand when dealing with many-body spin systems. In this work, the first experimental observation of persistent temporal fluctuations after a quantum quench is presented with a tunable long-range interacting transverse-field Ising Hamiltonian. The fluctuations in the average magnetization of a finite-size system of spin-1/2 particles are measured presenting a direct measurement of relaxation dynamics in a non-integrable system. This experiment is in the regime where the properties of the system are closely related to the integrable Hamiltonian with global coupling. The system size is varied in order to investigate the dependence on finite-size scaling, and the system size scaling exponent extracted from the measured fluctuations is consistent with theoretical prediction.
Connor Roncaioli - September 10, 2019
Dissertation Title: Topological Behavior in Rare Earth Half-Heusler HoPtBi
Date and Time: Tuesday, September 10, 3:00 pm
Location: PSC 3150
Dissertation Committee Chair: Professor Johnpierre Paglione
Committee:
Professor Richard Greene
Professor Christopher Lobb
Adjunct Associate Professor Nicholas Butch
Professor Ichiro Takeuchi, Dean's Representative
Abstract:
Magnetic HoPtBi is created and characterized as a new half-Heusler Weyl-semimetal candidate. By analogy with the well-studied GdPtBi system we undertake measurements intended to understand the normal state of this material, before extending our study to search for characteristics of Weyl behavior. We find a material with semiconducting properties as well as a low temperature antiferromagnetic transition below 1.25K as well as a Curie-Weiss paramagnetic system above. Analysis of the magnetoresistance in HoPtBi finds multiple Weyl-like characteristics, including potential chiral anomaly and anomalous Hall angle components. Finally, we found significant anisotropic magnetoresistance in HoPtBi dependent on field alignment relative to the crystalline axes of the material, which is unexpected for a paramagnetic compound.
Donggeun Tak - August 29, 2019
Dissertation Title: Temporal and spectral evolutionary features of gamma-ray bursts detected by the Fermi Gamma-Ray Space Telescope
Date and Time: Thursday, August 29, 10:00 am
Location: PSC 3150
Dissertation Committee Chair: Professor Peter Shawhan
Committee:
Professor Julie McEnery
Professor Jordan Goodman
Professor Kara Hoffman
Professor Suvi Gezari
Abstract:
Gamma-ray bursts (GRBs) are the most powerful electromagnetic events in universe. GRBs are powered by either core-collapse of massive stars or binary mergers of two compact objects. These progenitor systems are believed to launch a relativistic, collimated jets, which produce short, bright gamma-ray flashes (prompt emission) and long-lived, fading emission (afterglow) in the broad energy band from radio to gamma-rays. Even though the characteristics of the prompt emission and the afterglow have been vigorously studied, many details of the physics of GRBs remain uncertain. The Fermi Gamma-ray Space Telescope (Fermi) provides invaluable data for studying GRBs with the help of a very wide field of view and broad energy coverage from the hard X-ray to gamma-ray band. Fermi consists of two instruments, the Gamma-ray Burst Monitor (GBM; 8 keV- 40 MeV) and the Large Area Telescope (LAT; 20 MeV - >300 GeV).
Here, I present dedicated analysis results on three bright GRBs: GRB131108A, GRB160709A, and GRB190114C. Each of them shows its own evolution that includes the unusual and general features of GRBs. In addition, I performed two systematic studies using the full 10-year samples of LAT and GBM detected GRBs. For the first, I focused on the high-energy emission (> 100 MeV) and its origin by tracking its temporal and spectral evolution. In the second, focusing on the prompt emission phase, I found an observational signature that originates in the geometry of the relativistic jet, which had been predicted but was previously unobserved.
Zachary Schutz Smith - August 8, 2019
Dissertation Title: BOSE EINSTEIN CONDENSATES IN DYNAMICALLY CONTROLLED OPTICAL LATTICES
Date and Time: Thursday, August 8, 2:00 pm
Location: PSC 2136
Dissertation Committee Chair: Dr. Steven L. Rolston
Committee:
Dr. Gretchen Campbell
Dr. Trey Porto
Dr. Norbert Linke
Dr. Mario Dagenais, Dean's Representative
Abstract:
Megan Smith - July 23, 2019
Dissertation Title: SIMULATIONS OF ACCRETION MECHANISMS AND OBSERVATIONAL SIGNATURES OF BLACK HOLE ACCRETION DISKS
Date and Time: Tuesday, July 23, 2:00 pm
Location: PSC 2136
Dissertation Committee Chair: Dr. Jonathan McKinney and Dr. Peter Shawhan
Committee:
Dr. James Drake
Dr. Steven Rolston
Dr. M. Coleman Miller (Dean's Representative)
Abstract:
Qile Zhang - July 19, 2019
Dissertation Title: PARTICLE HEATING AND ENERGY PARTITION IN RECONNECTION WITH A GUIDE FIELD
Date and Time: Friday, July 19, 10:00 am
Location: AVW 2460
Dissertation Committee Chair: Dr. James Drake
Committee:
Dr. Marc Swisdak
Dr. Thomas Antonsen
Dr. Adil Hassam
Dr. Dennis Papadopoulos
Abstract:
Kinetic Riemann simulations have been completed to explore particle heating during reconnection with a guide field in the low-beta environment of the inner heliosphere and the solar corona. The reconnection exhaust is bounded by two rotational discontinuities (RDs) with two slow shocks (SSs) that form within the exhaust as in magnetohydrodynamic (MHD) models. At the RDs, ions are accelerated by the magnetic field tension to drive the reconnection outflow as well as flows in the out-of-plane direction. The out-of-plane flows stream toward the midplane and meet to drive the SSs. The turbulence at the SSs is weak so the shocks are laminar and produce little dissipation, which differs greatly from the MHD treatment. Downstream of the SSs the counter-streaming ion beams lead to higher density and therefore to a positive potential between the SSs that confines the downstream electrons to maintain charge neutrality. The potential accelerates electrons from upstream of the SSs to downstream and traps a small fraction but only produces modest electron heating. In the low-beta limit the released magnetic energy is split between bulk flow and ion heating with little energy going to electrons.
To firmly establish the laminar nature of reconnection exhausts, we explore the role of instabilities and turbulence in the dynamics. Two-dimensional reconnection and Riemann simulations reveal that the exhaust develops large-amplitude striations resulting from electron-beam-driven ion cyclotron waves. The electron beams driving the instability are injected into the exhaust from one of the RDs. However, in 3D Riemann simulations, the additional dimension results in a strong Buneman instability at the RD, which suppresses electron beam formation. The 3D simulation does reveal a weak ion-ion streaming instability within the exhaust. All these instabilities become weaker with higher ion-to-electron mass ratio due to higher electron thermal speed. We also use a kinetic dispersion relation solver to show that the ion-ion instability will become stable in conditions expected under lower upstream beta. The results suggest that in realistic reconnection exhausts, which have three dimensions and real mass ratio, the kinetic-scale turbulence that develops will be too weak to play a significant role in energy conversion.
Israel Martinez Castellanos - July 10, 2019
Dissertation Title: Search for gamma-ray counterparts of gravitational wave events and other transient signals with HAWC
Date and Time: Wednesday, July 10, 10:00 am
Location: PSC 3150
Dissertation Committee Chair: Dr. Jordan A. Goodman
Committee:
Dr. Peter S. Shawhan
Dr. Julie McEnery
Dr. Gregory Sullivan
Dr. Suvi Gezari
Abstract:
In recent years we have seen major advances in multi-messenger astronomy. A milestone was achieved by identifying the electromagnetic counterpart of the gravitational wave event GW170817 detected by LIGO and Virgo. Similar efforts led to a set of neutrinos detected by IceCube to be associated with the blazar TXS 0506+056. Both demonstrate the potential of using multiple types of probes to study an astrophysical source.
The High-Altitude Water Cherenkov Observatory (HAWC), located in the state of Puebla, Mexico, is a wide field instrument (~2 sr) sensitive to very-high-energy gamma rays (~0.1-100 TeV) which can operate with a large duty cycle (>95%). These characteristics make it well suited to look for transient events correlated with other astronomical messengers. In this work we present a maximum likelihood analysis framework developed to search and analyze signals in HAWC data of arbitrary timescales.
We apply this method to search for very-high-energy gamma-ray counterparts of gravitational waves in short timescales (0.3-1000 s). We show that we would be able to either detect or meaningfully constrain the very-high-energy component of a gamma-ray burst within the binary neutron star merger horizon of current gravitational wave detectors if it occurs in our field of view. We did not find evidence for emission for any of the events analyzed. The source location of GW170817 was not observable by HAWC at the time of the merger.
We also set flux upper bounds for TXS 0506+056 during the periods when the neutrino flares were identified. For the flare between September 2014 and March 2015 these are the only available limits at very high energy, and are consistent with the low state in high-energy gamma rays reported by the Fermi-LAT Collaboration.
Peizhi Du - June 5, 2019
Dissertation Title: Leptogenesis and phase transition in a warped extra dimension
Date and Time: Wednesday, June 5, 11:00 am
Location: PSC 1136
Dissertation Committee Chair: Prof. Kaustubh Agashe
Committee:
Prof. Zackaria Chacko
Prof. Theodore Jacobson
Prof. Raman Sundrum
Prof. Richard Wentworth
Abstract:
Despite the success of the standard model of particle physics, a few unsolved problems call for new physics beyond the SM. In this thesis, we studied a natural embedding of the well-known high scale type I seesaw mechanism in a warped five dimensional (5D) theory. Such model can address both neutrino mass problem and a profound theoretical problem in the SM, called Planck weak hierarchy problem. In contrast to the usual high scale type I seesaw mechanism in four dimensions, 5D warped seesaw model become TeV scale “inverse” seesaw like model after Kaluza-Klein decomposition into 4D theories. We also showed this model predicts attractive signatures at the Large Hadron Collider. Moreover, the baryon asymmetry of the universe can be naturally generated via leptogenesis mechanism in seesaw models. We demonstrated that the simplified warped seesaw can achieve successful leptogenesis and feature an interesting interplay of high scale generation of the asymmetry and TeV scale washouts. To make this mechanism realistic in the full warped seesaw model, we also studied the phase transition from the high temperature black hole phase to low temperature phase with two branes in 5D theories. According to AdS/CFT duality, such phase transition is dual to the de-confining and confining phase transition in strongly coupled nearly conformal 4D theories. It was previously believed that such phase transition rate is too slow at critical temperature, resulting in a large amount of supercooling and diluting all primordial abundance. We proposed a mechanism to achieve fast phase transition around critical temperature and thus the asymmetry generated from leptogenesis can survive till today.
Neill Warrington - May 22, 2019
Dissertation Title: Complex Paths Around the Sign Problem
Date and Time: Wednesday, May 22, 12:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Paulo Bedaque
Committee:
Prof. Tom Cohen
Prof. Chris Jarzynski
Prof. Ian Appelbaum
Prof. Don Perlis
Abstract:
A generic method for taming the sign problem is presented. The sign problem is the name given to the difficult task of numerically integrating a highly oscillatory integral, and the sign problem inhibits our ability to understand ab initio properties of a wide range of systems of interest in theoretical physics. Particularly notably for nuclear physics, the sign problem prevents the calculation of the properties of QCD at finite baryon density, thereby precluding an understanding of the dense nuclear matter found in the center of a neutron star.
The central idea is to use a multidimensional generalization of Cauchy's Integral Theorem to deform the Feynman Path Integral of lattice fields theories into complexified field space to manifolds upon which the phase oscillations which cause the sign problem are gentle. Doing so allows calculations of theories with sign problems.
Two practical manifold deformation methods, the holomorphic gradient flow and the sign-optimized manifold method, are developed. The holomorphic gradient flow, a generalization of the Lefschetz thimble method, continuously deforms the original path integration domain to a complex manifold via an evolution dictated by a complex first order differential equation. The sign-optimized manifold method is a way to generate a manifold with gentle phase oscillations by minimizing the sign problem in a parameterized family of manifolds through stochastic gradient ascent. These methods are general and can be applied to both bosonic and fermionic theories at finite density, as well as Minkowski path integrals describing real-time dynamics.
Gareth Roberg-Clark - May 15, 2019
Dissertation Title: Suppression of electron thermal conduction in the intracluster medium of galaxy clusters
Date and Time: Wednesday, May 15, 2:00 pm
Location: AVW 2460 (ECE)
Dissertation Committee Chair: Prof. James F. Drake
Committee:
Prof. William Dorland
Prof. Adil Hassam
Prof. Thomas Antonsen
Prof. M. Coleman Miller
Abstract:
Understanding the thermodynamic state of the hot intracluster medium (ICM) in a galaxy cluster requires a knowledge of the plasma transport processes, especially thermal conduction. The basic physics of thermal conduction in plasmas with ICM-like conditions has yet to be elucidated, however. We use particle-in-cell simulations and analytic models to explore the dynamics of an ICM-like plasma (with small gyroradius, large mean-free-path, and strongly sub-dominant magnetic pressure) induced by heat fluxes associated with thermal conduction. In this system $\beta >> 1$ where $\beta = P_{thermal}/P_{mag} = 8\pi nT / B^{2}$ is the ratio of thermal pressure to magnetic field energy density.
Linear theory reveals that whistler waves are driven unstable by electron heat flux, even when the heat flux is weak. The resonant interaction of electrons with these waves then plays a critical role in scattering electrons and suppressing the heat flux. In a 1D model where only whistler modes that are parallel to the magnetic field are captured, the only resonant electrons are moving in the opposite direction to the heat flux and the electron heat flux suppression is small. In 2D or more, oblique whistler modes also resonate with electrons moving in the direction of the heat flux. The overlap of resonances leads to effective symmetrization of the electron distribution function and a strong suppression of heat flux.
In a numerical model with continually supplied heat flux in the system, two thermal reservoirs at different temperatures drive an electron heat flux that destabilizes oblique whistlers. The whistlers grow to large amplitude, $\delta B / B_{0} \simeq 1$, and resonantly scatter the electrons. A surprise is that the resulting steady state heat flux is largely independent of the thermal gradient. The rate of thermal conduction is instead controlled by the finite propagation speed of the whistlers, which act as mobile scattering centers that convect the thermal energy of the hot reservoir.
When the plasma $\beta$ is reduced in the numerical model, we find that a transition takes place between whistler-dominated (high-$\beta$) and double-layer-dominated (low-$\beta$) heat flux suppression. Whistlers saturate at small amplitude in the low $\beta$ limit and are unable to effectively suppress the heat flux. Electrostatic double layers suppress the heat flux to a mostly constant factor of the free streaming value once this transition happens. The double layer physics is an example of ion-electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of $\beta$. The range of plasma-$\beta$s studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including not just the intracluster medium but hot accretion flows, stellar and accretion disk coronae, and the solar wind.
Joshua Isaacs - May 14, 2019
Dissertation Title: The Physics of High-Intensity Laser-Matter Interactions and Applications
Date and Time: Tuesday, May 14, 3:30 pm
Location: ERF 1207A
Dissertation Committee Chair: Prof. Phillip Sprangle
Committee:
Prof. Thomas Antonsen
Prof. Howard Milchberg
Prof. Joseph Penano
Prof. Antonio Ting
Prof. Rajarshi Roy (Dean's Representative)
Abstract:
This dissertation consists of three separate research topics:
First, the effect of laser noise on the propagation of high-power and high-intensity short pulse lasers in dispersive and nonlinear media is studied. We consider the coupling of laser intensity noise and phase noise to the spatial and temporal evolution of laser radiation. We show that laser noise can have important effects on the propagation of high-power as well as high-intensity lasers in a dispersive and nonlinear medium such as air. We present atmospheric propagation examples of the spatial and temporal evolution of intensity and frequency fluctuations due to noise for laser wavelengths of 0.85 μm, 1 μm, and 10.6 μm.
Next, a concept for all-optical remote detection of radioactive materials is presented and analyzed. The presence of excess radioactivity increases the level of negative ions in the surrounding air region. This can act as a source of seed electrons for a laser-induced avalanche ionization breakdown process. We model irradiated air to estimate the density of negative ions and use a set of coupled rate equations to simulate a subsequent laser-induced avalanche ionization. We find that ion-seeded avalanche breakdown can be a viable signature for the detection of radioactivity, a conclusion which has been experimentally tested and verified.
Finally, we propose and analyze a mechanism to accelerate protons from close to rest in a laser-excited plasma wave. The beating of two counter-propagating laser pulses in a plasma shock-excites a slow forward-propagating wakefield. The trapping and acceleration of protons is accomplished by tapering both the plasma density and the amplitude of the backward-propagating pulse. We present an example in which protons are accelerated from 10 keV to 10 MeV in a distance of approximately 1 cm.
Chen Li - May 10, 2019
Dissertation Title: Novel Approaches to Control of Surface Reactions in Plasma Etching of Electronic Materials
Date and Time: Friday, May 10, 3:00 pm
Location: IREAP conference room
Dissertation Committee Chair: Prof. Gottlieb S. Oehrlein
Committee:
Prof. Steven Anlage
Prof. William Dorland
Prof. Derek Boyd
Prof. Thomas Antonsen
Abstract:
Advanced semiconductor manufacturing requires precise plasma etching control for patterning complex semiconductor device structures. Pattern transfer into dielectric materials is one of the most frequently performed operation and traditionally done using continuous wave (CW) plasma etching processes based on fluorocarbon (FC) chemistries. Such etching methods are facing challenges when critical dimension (CD) approach 10 nm. Issues include low materials etching selectivity, surface damage, roughness and poor etching profile control. In this work, various aspects of low temperature plasma-based etching approaches are tailored for optimal plasma etching performance, including novel gaseous precursors for better control of gas phase and surface processes, tailoring the relative importance of radicals and ion bombardment at surface by sequential processes, and a new way to input energy to surfaces to stimulate etching reactions. We systematically studied the impact of molecular structure parameters of hydrofluorocarbon (HFC) precursors on plasma deposition of fluorocarbon (FC) and material etching performance. The HFC chemical composition and molecular structure such as ring structure, C=C, C≡C, C-O, C-H and degree of unsaturation have dramatic impacts on FC surface polymerization and material etching performance. Further, we report a new atomic layer etching (ALE) technique which temporally separates chemical reactant supply to a surface from ion bombardment induced etching. By this ALE method, the ion bombardment energy can be reduced to ensure low substrate damage and extremely high etching selectivity of two materials. Finally, we developed a hollow cathode electron beam etching system to reduce the energy and momentum input on the material surface by utilizing an electron-radical synergy effect. This present work has unveiled highly promising elements of a new roadmap of next generation semiconductor etching approaches and is expected to impact multiple areas of nanoscience and technology, including plasma etching of post-silicon materials. The use of specially selected gaseous precursor chemistry, temporal separation of radical exposure and energy-induced etching, and finally using electron bombardment for activation of surface etching, challenge our current understanding of semiconductor plasma processing and presents an important step forward in terms of the further industrial development of these approaches.
Andrew Allocca - May 9, 2019
Dissertation Title: Casimir and Optical Phenomena in Two-dimensional Systems
Date and Time: Thursday, May 9, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Victor Galitski
Committee:
Prof. Theodore Einstein
Prof. Mohammad Hafezi
Prof. Frederick Wellstood
Prof. Victor Yakovenko
Abstract:
The nature of the interaction of light with matter is a subject of great interest in condensed matter physics. Here we study the behavior of three electromagnetic effects arising from the coupling of light to two-dimensional electron systems: the Casimir effect, excitons in an insulator, and the formation of polaritons in a cavity.
We begin by examining how the Casimir effect is affected by material properties. First we consider using the Casimir force as a probe of a change in the topology of a material's Fermi surface, called a Lifshitz transition. We study a spin-orbit coupled semiconducting system which can be made to undergo this sort of transition with an external magnetic field, and find that the signature of this transition is a non-analyticity in the Casimir force at the transition point.
We then consider how the phenomenon of weak localization can be used as a test of the role of disorder when describing the Casimir effect between metallic objects. We show how the sensitive dependence of the conductivity of a two-dimensional disordered metal on both temperature and magnetic field should translate into similar sensitivities of the Casimir force, assuming effects of disorder should be included at all.
Next, we examine excitons formed in the bulk of an insulator as the system transitions between topological and trivial insulating phases, finding that the phases have different signatures in the exciton spectrum. This can be understood as an effect of the Berry curvature of the model giving an indirect glimpse of topological properties. We first construct a semiclassical model of the system to develop a qualitative intuition, then move to a numerical calculation in a full quantum model.
Finally, we consider the formation of polaritons inside of a photonic cavity containing a two-dimensional superconducting layer. We show how a coupling can be engineered between cavity photons resonant with a collective mode of the superconductor called the Bardasis-Schrieffer mode, leading to hybridized superconductor polariton states. Motivated by exciton polaritons condensation, we conjecture that a phase-coherent density of these objects could produce an exotic s+id superconducting state.
Andi Tan - May 7, 2019
Dissertation Title: PANDAX-II DARK MATTER DETECTOR AND ITS FIRST RESULTS
Date and Time: Tuesday, May 7, 9:00 am
Location: PSC 2136
Dissertation Committee Chair: Prof. Carter Hall
Committee:
Prof. Xiangdong Ji (Advisor)
Prof. Eun-Suk Seo
Prof. Thomas Cohen
Prof. Da-lin Zhang
Abstract:
The particle physics nature of dark matter (DM) is one of most fundamental scientific questions nowadays. The leading candidates, weakly interacting massive particles (WIMPs), can be directly detected by looking for WIMP-nucleus scattering events in deep underground laboratories. The Particle and astrophysical Xenon (PandaX) project is a series of xenon-based ultra-low background experiments in the China JinPing underground Laboratory (CJPL) targeting the unknown physics of DM and neutrinos. The first and second stage experiments (PandaX-I and II) both utilize dual-phase xenon time projection chamber (TPC) to carry out direct search for the dark matter particles. PandaX-II, a half-ton scale experiment, is currently under operation, and produced leading limits on DM-nucleon spin independent and spin dependent scattering cross sections in 2016 and 2017.
In the dissertation, I focus on the PandaX-II experiment including the development of the detector and data analysis. In 2017, PandaX-II experiment achieved a background level of 0.8×10−3 event/kg/day/keV which was the lowest among similar detectors at the time. Compared to other dual-phase xenon detector, we drift electrons by applying bias voltages on the electrodes which producing a stronger uniform electric field at a strength about 400 V/cm. After running for more than three years, more than 97% of 110 3-inch photo multiplier tubes (PMTs) perform stably. In the data analysis, I studied an inefficiency raised from zero length encoding (ZLE), a data suppression firmware of data acquisition system, in run periods with relatively low PMT gain. A data-driven algorithm is developed for the X-Y position reconstruction using the hit pattern of the proportional scintillation on the top PMT array which enlarged the fiducial volume for WIMP search by 20%. The background analysis is important for rare-event search experiments. I introduce the investigation on the intrinsic electron recoil background events from krypton, radon and xenon isotopes.
In this talk, I will present an overview of the project, discuss recent results from PandaX-II, and give an outlook into the future.
Christopher Eckberg - May 7, 2019
Dissertation Title: Superconducting Enhancement in a Tunable Electronic Nematic System
Date and Time: Tuesday, May 7, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Dr. Johnpierre Paglione
Committee:
Dr. Frederick Wellstood
Dr. Steven Anlage
Dr. Efrain Rodriguez
Dr. Jay Sau
Abstract:
Inspired by the overarching presence of nematicity in the high Tc superconducting systems, this thesis will discuss the interplay of nematic and superconducting order in a system void of long range magnetism. We present here details of the physical properties of BaNi2As2, Ba(Ni1-xCox)2As2, and Ba1-xSrxNi2As2 intermetallic compounds, including a novel, chemically tunable nematic phase in the Ba1-xSrxNi2As2 series.
Thermodynamic, transport, and magnetic properties of single crystals synthesized using a flux growth technique are reported. Using these probes, we construct the electronic phase diagrams of the Ba(Ni1-xCox)2As2, and Ba1-xSrxNi2As2 series. In both substitution series, increasing x smoothly suppresses a tetragonal-triclinic structural phase transition to absolute zero temperature. At the zero temperature structural phase transition, a large enhancement in superconducting Tc is observed, reminiscent of fluctuation mediated superconductivity in high Tc compounds. Through measurements of the material thermodynamic nematic susceptibility, we observe a crossover between structural and electronically driven rotational symmetry breaking in the Ba1-xSrxNi2As2 series with increasing x. A striking sixfold enhancement in superconductivity is observed near the nematic quantum phase transition, suggesting nematic fluctuation enhanced pairing.
Lance Boyer - April 29, 2019
Dissertation Title: Superconductors that Break Time-Reversal Symmetry
Date and Time: Monday, April 29, 4:00 pm
Location: Toll 2202
Dissertation Committee Chair: Dr. Victor Yakovenko
Committee:
Dr. Steven Anlage
Dr. Maissam Barkeshli
Dr. Jeremy Munday
Dr. Victor Galitski
Abstract:
Since 2006 it has been discovered experimentally that the superconducting state spontaneously breaks time-reversal symmetry (TRS) in several materials, such as Sr2RuO4, UPt3, URu2Si2, PrOs4Sb12, and Bi/Ni bilayers. This dissertation studies three physical phenomena related to time-reversal symmetry breaking (TRSB) in these superconductors.
The experimental evidence for TRSB comes from the magneto-optical polar Kerr effect, which is determined by the high frequency ac Hall conductivity. However, these superconductors are also expected to exhibit a spontaneous dc Hall effect in the absence of an applied magnetic field. In the first part of this dissertation we propose a method for measuring the low frequency Hall conductivity in superconductors with TRSB. The method is based on a Corbino disk geometry where an oscillating co-axial magnetic field induces circular electric field, which, in turn, induces radial charge oscillations due to the Hall conductivity.
In the second part, we propose an explanation for the polar Kerr effect observed in the Hidden-Order phase of the heavy-fermion superconductor URu2Si2. Using a Ginzburg-Landau model for a complex order parameter, we show that the system can have a metastable ferromagnetic state, which produces the Kerr signal, even if the Hidden-Order state respects TRS. We predict that applying a reversed magnetic field should reset the system to the non-magnetic ground state, resulting in zero Kerr signal.
In the third part of the dissertation, we investigate the conditions for the existence of a Majorana bound state on a vortex in a 2D d+id superconductor with strong spin-orbit coupling. This TRSB pairing was proposed earlier for the Ni/Bi bilayer. We find that the Majorana bound state can exist for a d+id pairing under conditions similar to those for s-wave pairing.
Josue Morales-Cifuentes - April 24, 2019
Dissertation Title: Submonolayer Adsorbates: Theoretical Studies of Transient Mobility and Symmetry Breaking
Date and Time: Wednesday, April 24, 11:00 am
Location: PSC 2148
Dissertation Committee Chair: Prof. Theodore L. Einstein
Committee:
Prof. Janice E. Ruett-Robey
Prof. John D. Weeks
Prof. Ellen D. Williams
Prof. Alberto Pimpinelli
Abstract:
Weakly bound submonolayer adsorbates provide important insight into fundamental descriptions of physics that would otherwise be masked, or even suppressed, by strong effects such as chemical binding. We focus on two surface effects: transient mobility at the microscopic scale, and symmetry breaking at the atomic one.
We present a novel island nucleation and growth model that explicitly includes, at the microscopic scale, the behavior of transient (ballistic) monomers. At a deposition rate F, monomers are assumed to be in a hot precursor state before thermalizing. In the limiting regimes of fast (diffusive) and slow (ballistic) thermalization, we recover the expected scaling of the island density, N: N ~ Fα. We construct effective exponents αeff and activation energies to properly characterize the transitional regimes between these limiting regimes. Through these constructs, we describe a rich and complex structure of meta-stable limiting regimes, asymptotic behavior and energetically driven transitions. Application to N(F, T) of recent organic-molecule deposition experiments yields excellent fits. We have also studied, at the atomic scale, an effective potential mechanism that breaks the intrinsic two-fold sublattice (triangular) symmetry of (honeycomb) graphene using DFT calculations (VASP ver 5.3.3).
We choose the specific system CF3Cl substrates on graphene, to benefit from experimental results obtained locally. Using ab initio van der Waals density functionals, we discover a physisorbed phase with binding energies of about 280meV. For low coverages, sublattice symmetry breaking effects are responsible for gap openings of 4meV; contrastingly, in large coverages it is the formation of ordered overlayers that opens gaps nearly 5 times as large, of roughly 15meV. We discover that in both cases, it is the breaking of the symmetry between graphene's two sublattices that induces symmetry breaking by means of adsorbate interactions that favor large ordered regions on a single sublattice, coverage itself is insignificant.
Zachary Raines - April 22, 2019
Dissertation Title: Hybridization and enhancement processes in quasi-two dimensional superconductors
Date and Time: Monday, April 22, 2:00 pm
Location: PSC 2136
Dissertation Committee Chair: Prof. Victor M. Galitski
Committee:
Prof. Alexey Gorshkov
Prof. Richard Greene
Prof. Jay Deep Sau
Prof. John Weeks
Abstract:
Superconductivity is a field with a great many branches and applications. In this dissertation, we focus on two specific processes in superconductors – light-induced enhancement and hybridization of collective modes – in two types of quasi-two dimensional materials – either the loosely coupled planes of a layered superconductor or a superconducting thin film.
Motivated by experiments in the cuprates that have seen evidence of a transient superconducting state upon optical excitation we study the effects of inter-plane tunneling on the competition between superconductivity and charge order. We find that an optical pump can suppress the charge order and simultaneously enhance superconductivity, due to the inherent competition between the two. Taking into account that the charge order empirically shows a broad peak in c-axis momentum, we consider a model of randomly oriented charge ordering domains and study how interlayer coupling affects the competition of this order with superconductivity.
Also in the cuprates, several groups have reported observations of collective modes of the charge order present in underdoped cuprates. Motivated by these experiments, we study theoretically the oscillations of the order parameters, both in the case of pure charge order, and for charge order coexisting with superconductivity. Using a hot-spot approximation we find in the coexistence regime two Higgs modes arising from hybridization of the amplitude oscillations of the different order parameters. We explore the damping channels of these hybrid modes.
As another means of enhancing superconductivity we consider coupling a two-dimensional superconducting film to the quantized electromagnetic modes of a microwave resonator cavity. We find that when the photon and quasiparticle systems are out of thermal equilibrium, a redistribution of quasiparticles into a more favorable non-equilibrium steady-state occurs, thereby enhancing superconductivity in the sample, a fluctuation analog of a phenomenon known as the Eliashberg effect.
Finally, following the recent success of realizing exciton-polariton condensates in cavities, we examine the hybridization of cavity photons with two types of collective modes in superconductors. Enabled by the recently predicted and observed supercurrent-induced linear coupling between these excitations and light, we find that significant hybridization between the superconductor's collective modes and resonant cavity photons can occur.
Wan-Ting Liao - April 4, 2019
Dissertation Title: Investigation of Tunneling in Superconductors in a Millikelvin Scanning Tunneling Microscope
Date and Time: Thursday, April 4, 1:00 pm
Location: PSC 1136
Dissertation Committee Chair: Dr. Christopher Lobb
Committee:
Dr. Frederick Wellstood
Dr. Michael Dreyer
Dr. Kevin Osborn
Dr. John Cummings
Abstract:
I describe my use of a dual-point millikelvin scanning tunneling microscope (STM) to observe tunneling into superconducting TiN and Nb surfaces. After describing the STM, I present tip-sample conductance-voltage characteristics measured on 25 nm and 50 nm thick films of superconducting TiN that show large qualitative variations at 0.5 K. At some locations the characteristics show a clear superconducting gap, as expected for conventional superconductor-normal (S-N) tunneling through a low transparency behavior, while at other locations I see a distinct zero-voltage conductance peak, as expected for S/N Andreev tunneling through high transparency barrier. I fit the Blonder-Tinkham-Klapwijk (BTK) model of Andreev tunneling to the data and use this to construct spatially resolved maps of the superconducting gap Δ, the local temperature T and the barrier transparency Z. I observe an unexpected correlation between variations in Δ, T and Z and discuss some possible causes, including local heating and surface contamination. I next describe I(V) (current-voltage) characteristics obtained as a function of the tunneling resistance Rn by varying the distance between a Nb STM tip and a Nb sample at 50 mK and 1.5 K. Depending on Rn, the junction can be in the phase-diffusion regime, the underdamped small junction limit or the point contact regime. The characteristics show sub-gap current steps that depend strongly on Rn, as expected from multiple Andreev reflection (MAR) effects. To better understand this behavior, I generalized the multiple Andreev reflection (MAR) theory of Averin and Bardas to the case where the junction electrodes can have different Δ. Fitting this MAR theory to the I(V) data, I extract the gap Δsample of the sample, the gap Δtip of the tip, and the barrier transparency Z, all as a function of Rn. I find that Δsample = 1.5 meV, nearly the full gap of bulk Nb, but Δtip = 0.67 meV for Rn> 10kΩ and it decreases for Rn ≤ 10 kΩ. I conclude with a discussion of some of the implications for STM of superconductors.
Zachary Eldredge - April 4, 2019
Dissertation Title: Generation and Uses of Distributed Entanglement in Quantum Information
Date and Time: Thursday, April 4, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Dr. Steven Rolston (Chair)
Committee:
Dr. Alexey Gorshkov (Advisor)
Dr. Andrew Childs
Dr. Mohammad Hafezi
Dr. Brian Swingle
Abstract:
In this thesis, we focus on the questions of how quantum entanglement can be generated between two spatially separated systems and, once generated, how it can be applied in quantum metrology. First we will discuss a protocol for the generation of large entangled states using long range interactions. Next, we will turn our attention to more general questions of how the Lieb-Robinson bound and other limitations on entanglement can be used to inform the design of quantum computers. We will present a proposed graph architecture, the hierarchical product, which we believe provides excellent balance between requiring large amounts of communication interaction and being able to perform computations quickly. Finally, we will look at the scenario of quantum sensing. In particular, we will examine protocols for quantum function estimation, where quantum sensors are available to measure all of the inputs to the function. We will demonstrate that entangled sensors are more capable than non-entangled ones by first deriving a new lower bound on measurement error and then presenting protocols that saturate these bounds.
Zachary Epstein - April 3, 2019
Dissertation Title: High-Intensity Laser-Matter Interactions: Physics and Applications
Date and Time: Wednesday, April 3, 10:00 am
Location: ERF 1207, IREAP Large Conference Center
Dissertation Committee Chair: Dr. Phillip Sprangle
Committee:
Dr. Tom Antonsen
Dr. Howard Milchberg
Dr. Joe Penano
Dr. Robert Lehmberg
Dr. Rajarshi Roy
Abstract:
The following topics will be discussed: (1) High-Power Supercontinuum IR Generation, (2) Remote Optical Magnetometry for the detection of underwater objects, and (3) Spectral Broadening of the NIKE KrF Laser in a Negative Nonlinear Index Medium.
Prasoon Gupta - April 1, 2019
Dissertation Title: Phase measurements with a two-mode squeezed state of light
Date and Time: Monday, April 1, 9:30 am
Location: ATL 3330
Dissertation Committee Chair: Prof. Steven Rolston (Chair)
Committee:
Prof. Paul Lett (Advisor)
Prof. Mohammad Hafezi
Prof. Wendell Hill
Prof. Alan Migdall
Abstract:
Introducing squeezed states of light into interferometers can increase the phase sensitivity of the device beyond the standard quantum limit (SQL). We will discuss an SU(1,1) interferometer, where nonlinear optical elements replace the beam splitters in a Mach-Zehnder interferometer. A two-mode squeezed state of light is generated inside of such an interferometer. We will talk about the phase sensitivities of an SU(1,1) interferometer with different detection schemes and their improvement over the SQL. We will describe a modification of the SU(1,1) interferometer which reduces the experimental complexities while giving the same phase sensitivity. We call the design a truncated SU(1,1) interferometer. We will show our experimental results of 4 dB improvement in phase sensitivity over the SQL using the truncated SU(1,1) interferometer. We will also discuss a vacuum-seeded truncated SU(1,1) interferometer and show our experimental results for its phase sensitivity. We will explain the dependence of phase sensitivity on the measurement of squeezing. Finally, we will talk about the methods to improve the measurement of squeezing in a 4-wave mixing experiment, and our efforts in implementing them.
Dina Genkina - February 13, 2019
Dissertation Title: Measuring topology in a synthetic dimensions lattice
Date and Time: Wednesday, February 13, 1:00 pm
Location: PSC 2136
Dissertation Committee Chair: Dr. Ian Spielman (Advisor)
Committee:
Dr. Trey Porto
Dr. Steven Rolston
Dr. Mohammad Hafezi
Dr. Christopher Lobb
Abstract:
Topology in 2-D materials lies at the heart of the quantum Hall effect and topological insulators. In the limit of high magnetic flux, it gives rise to the fractal Hofstadter butterfly. These high fluxes are inaccessible in traditional condensed matter settings, requiring fields of order 10^4 Tesla. We engineered them in our effective 2-D lattice of ultracold 87Rb atoms. We created this lattice using a synthetic dimensions approach: a real 1-D lattice defined sites along the first, 'real', dimension, while the internal spin states of the atoms served as sites along a second, 'synthetic', dimension. We then took advantage of the hybrid imaging that occurs in this lattice during time of flight: momentum is measured along the 'real' axis, while position is measured along the 'synthetic' axis with single site resolution. This allowed us to map out the position of the atoms in the synthetic direction for every point in the lowest band, ie for every value of the real axis crystal momentum. We then levered a Diophantine equation derived by Thouless, Kohomoto, Nightingale, and den Nijs to extract the topological invariant of our system, in 2-D called the Chern number, and provide an intuitive picture of how this equation arises in our system.
Neal Pisenti - January 14, 2019
Dissertation Title: Isotope Shift Spectroscopy of Ultracold Strontium
Date and Time: Monday, January 14, 3:00 pm
Location: PSC 2136
Dissertation Committee Chair: Dr. Steven Rolston (Chair)
Committee:
Dr. Gretchen K. Campbell (Advisor)
Dr. James (Trey) Porto
Dr. James Williams
Dr. Christopher Jarzynski (Dean's Representative)
Abstract:
We describe the design, construction, and performance of a laser system to probe the ultra-narrow (Γ/2π ≈ mHz) clock transition 1S0 → 3P0 in strontium. We present the first reported spectroscopy of this transition in two of the bosonic isotopes, 84Sr and 86Sr. Furthermore, we measure the complete set of isotope shifts between all four stable isotopes on the clock line and the narrow intercombination line 1S0 → 3P1, permitting a King plot analysis of the isotope shifts. Complications arising from the unambiguous determination of a line center in 87Sr 3P1 prevent us from making claims about the King linearity, but we provide a statistical bootstrap analysis of the isotope shifts 88−84Sr and 88−86Sr to compute a field shift ratio F698/F689 = 0.9995, with a 95% confidence interval [0.9973, 1.0019]). The intercept term K698 − (F698/F689) K689 is similarly determined to be -3.1 GHz-amu, with a 95% confidence interval [−4.6, −1.7] GHz-amu. Finally, we describe the design of a next-generation apparatus that will enable improvements on the results described here, as well as other studies that involve coherent manipulation of strontium atoms on the clock line.
Clayton Crocker - December 13, 2018
Dissertation Title: Improving Ion-Photon Entanglement for Quantum Networks
Date and Time: Tuesday, December 13, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Dr. Christopher Monroe (Advisor)
Committee:
Dr. Alexey Gorshkov
Dr. Steven Rolston
Dr. Ian Spielman
Dr. Mohammad Hafezi (Dean's Representative)
Abstract:
Increasing the number of qubits that can be controlled in a quantum system represents an essential challenge to the field of quantum computing. Quantum networks consisting of nodes for local information processing and photonic channels to distribute entanglement between different nodes represent a promising modular approach to achieve this scaling. Trapped atomic ions are an ideal candidate for quantum network nodes, with long-lived identical qubit memories that can be locally entangled through their Coulomb interaction and remotely entangled through photonic channels. In this work I will show a toolkit for using 171Yb+ and 138Ba+ ions individually or together within a quantum node. I will then demonstrate how we can generate ion-photon entanglement as a resource to connect separate nodes. I will focus some important improvements to this ion-photon entanglement which willallow us to implement it as part of a larger network. These improvements include first the use of separate memory (171Yb+ ) and photon generating (138Ba+ ) ions. Additionally, the use of separate atomic lines within 138Ba+ for excitation and collection allows us to preserve integrity of this photonic interface by ensuring the purity of the single photons that are produced. To this end I demonstrate a single-photon source for quantum networking based on a trapped 138Ba+ ion with a single photon purity of g2(0) = (8.1 ± 2.3) × 10-5 without background subtraction. Trade-offs between the photonic generation rate and the memory-photon entanglement fidelity for the case of polarization photonic qubits are also examined and optimized by tailoring the spatial mode of the collected light. These techniques should be useful in constructing larger ion-photon networks.
Min-A Cho - December 11, 2018
Dissertation Title: Low-Latency Searches for Gravitational Waves and their Electromagnetic Counterparts with Advanced LIGO and Virgo
Date and Time: Tuesday, December 11, 10:30 am
Location: PSC 2136
Dissertation Committee Chair: Dr. Peter Shawhan (Advisor)
Committee:
Dr. Paulo Bedaque
Dr. Sarah Eno
Dr. Jordan Goodman
Dr. M. Coleman Miller (Dean's Representative)
Abstract:
For the first time in history, advanced detectors are available to observe the stretching and squeezing of space---gravitational waves---from violent astrophysical events. This opens up the prospects of joint detections with instruments of traditional astronomy to create the new field of multi-messenger astrophysics. Joint detections allow us to form a coherent picture of the unfolding event as told by the various channels of information: mass and energy dynamics from gravitational waves, charged particle environments from electromagnetic radiation, and thermonuclear reactions from neutrinos.
In this work, I motivate low-latency electromagnetic and neutrino follow-up of sources known to emit gravitational radiation in the sensitivity band of ground-based interferometric detectors, Advanced LIGO and Advanced Virgo. To this end, I describe the low-latency infrastructure I developed with colleagues to select promising candidate events and enable successful follow-up of the first binary black hole merger, named GW150914, and binary neutron star coalescence, named GW170817, during the first and second observing runs.
As a review, I outline the theory behind gravitational waves and explain how the advanced detectors, low-latency searches, and data quality vetting procedures work. To highlight the newness of the field that is gravitational wave astronomy, I also share results from an offline search for a more speculative source of gravitational waves, intersecting cosmic strings, from the second observing run.
Finally, I address how LIGO/Virgo is prepared to adapt to challenges that will arise during the upcoming third observing run, an era that will be inundated with near-weekly binary black hole candidate events and near-monthly binary neutron star candidate events. We made several improvements to our low-latency infrastructure, including a new, streamlined candidate event selection process, expansions I helped develop for temporal coincidence searches with electromagnetic/neutrino triggers, and data quality products on source classification and probability of astrophysical origin to provide to our observing partners for potential compact binary coalescences. These measures will further our prospects for multi-messenger astrophysics and increase our science returns.
Safa Motesharrei - October 29, 2018
Dissertation Title: Carrying Capacity of two-way coupled Earth-Human Systems
Date and Time: Monday, October 29, 2:00 pm
Location: ATL 3425
Dissertation Committee Chair: Dist. Univ. Prof. James A. Yorke
Committee:
Dr. Victor Yakovenko
Dist. Univ. Prof. Eugenia Kalnay
Dr. Bill Dorland
Dr. Jelena Srebric (Dean's Representative)
Abstract:
Over the last two centuries, the impact of the Human System has grown dramatically, becoming strongly dominant within the Earth System in many different ways. Consumption, inequality, and population have increased extremely fast, especially since about 1950, threatening to overwhelm the many critical functions and ecosystems of the Earth System. Changes in the Earth System, in turn, have important feedback effects on the Human System, with costly and potentially serious consequences. However, current models do not incorporate these critical feedbacks. We argue that in order to understand the dynamics of either system, Earth System Models must be coupled with Human System Models through bidirectional couplings representing the positve, negative, and delayed feedbacks that exist in the real systems. In particular, key Human System variables, such as demographics, inequality, economic growth, and migration, are not coupled with the Earth System but are instead driven by exogenous estimates, such as the United Nations population projections. This makes current models likely to miss important feedbacks in real Earth-Human system, especially those that may result in unexpected or courtnerintuitive outcomes, and thus requiring different policy interventions from current models. The importance and imminence of sustainabilty challenges, the dominant role of the Human System in the Earth System, and the essential roles the Earth System plays for the Human System, all call for collaboration of natural scientists, social scientists, and engineers in multidisciplinary research and modeling to develop coupled Earth-Human system models for devising effective science-based policies and measures to benefit current and future generations.
David Meyer - October 26, 2018
Dissertation Title: Magnetic & Electric Field Sensing and Applications Based on Coherent Effects in Neutral Atoms
Date and Time: Friday, October 26, 12:00 pm
Location: Atlantic 3330
Dissertation Committee Chair: Prof. Steven Rolston
Committee:
Dr. Fredrik Fatemi (advisor)
Dr. Frederick Wellstood
Dr. Trey Porto
Dr. Edo Waks
Abstract:
This work encompasses two projects employing coherent probing of neutral atom vapors for sensing applications:
1) exploring previously unexplained features within cold-atom Nonlinear Magneto-Optical Rotation signals with applications for multi-directional magnetometry;
2) using a warm vapor of Rydberg atoms as a radio-frequency communications receiver via Electromagnetically Induced Transparency detection.
Jeffrey Magill - September 21, 2018
Dissertation Title: Probing the Nature of Radiative Processes within Radio Galaxies using the Fermi Gamma-Ray Large Area Telescope
Date and Time: Friday, September 21, 3:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Peter Shawhan
Committee:
Dr. Julie McEnery (advisor)
Dr. Jordan Goodman
Dr. Jeremy Perkins
Dr. M. Coleman Miller
Abstract:
Radio galaxies, active galactic nuclei with misaligned relativistic jets and large diffuse extended lobe structures, are home to radiative processes which are still not well understood. In this defense, I describe my use of gamma-ray photon data from the Fermi Large Area Telescope to investigate these radiative processes in the case of two radio galaxies, Fornax A and Centaurus A. I describe my discovery of the spatially extended nature of the gamma-ray emission from Fornax A, and my observation of a gamma-ray intensity which is not consistent with the predicted process of stray energetic electrons inverse-Compton scattering with extragalactic background light photons. I describe how I positively identified a new gamma-ray spectral component from the core region of Centaurus A jointly with data from the High Energy Stereoscopic System, and how the spectral component can be explained by the addition of a second hidden zone of synchrotron self-Compton emission. I describe my discovery of fine filamentary sub-structures in the gamma-ray lobes of Centaurus A using a new imaging technique which I created, mapping out the unexpected gamma-ray emission farther from the assumed central engine than we have observed in radio. I discuss how my observations of the Centaurus A lobes suggest local re-acceleration or channels of negligible magnetic field allowing long distance high energy particle paths.
Amit Nag - August 30, 2018
Dissertation Title: SIGNATURE OF MAJORANA MODES AND ASPECTS OF THEIR BRAIDING
Date and Time: Thursday, August 30, 2:00 pm
Location: PHY 2205 (CMTC Seminar Room)
Dissertation Committee Chair: Prof. Jay Deep Sau
Committee:
Dr. Mohammad Hafezi
Dr. Theodore Einstein
Dr. Maissam Barkeshli
Dr. Christopher Jarzynski
Abstract:
Majorana zero modes are emergent zero-energy quasiparticle excitations in certain superconducting systems that can be viewed as fractionalized or “half” electrons. These quasiparticles obey non-Abelian braiding statistics which is one manifestation of such half-electron character. Due to such non-Abelian braiding property, Majorana zero mode pairs hold promise as potential qubits for topological quantum computation.
It is somewhat surprising that, at least theoretically, ordinary one-dimensional semiconductor systems can be induced to host such esoteric Majorana modes as edge states if some precise experimental conditions are satisfied. Because of relative simplicity of material and experimental requirements to host Majorana modes, there has been a flurry of experimental effort to realize them in semiconductor nanowire systems. While the experimental efforts have produced preliminary evidence for presence of Majorana zero modes in these systems, a thorough confirmation is lacking. The experimental signature in question is presence of a zero-bias conductance-peak that is albeit necessary, is not a sufficient criteria to establish presence of underlying Majorana modes. Given the importance of Majorana braiding for topological quantum computation and skepticism over presence of Majorana modes in these experimental systems, it would seem natural to attempt braiding these putative Majorana modes in near future. In that case an observation of non-Abelian statistics would provide the necessary sufficient condition in favor of Majorana presence in the studied experimental systems.
This thesis has three distinct parts. First we assume perfect Majorana modes as given that can be successfully braided. In this case, we calculate the diabatic error due to finite speed of braiding when the system is coupled to a Bosonic bath. Next, we grant that the mechanism for zero-bias conductance-peak is indeed topological albeit the underlying Majorana modes are imperfect (the modes are not precisely at zero energy). We study the interplay of dissipation and such imperfect Majorana modes and their effect on probability of successful braiding. Lastly, we propose studying correlation between independent left and right conductance measurements as a means to distinguish between topological versus a non-topological mechanism underlying the observed zero-bias conductance-peak.
Cody Ballard - August 22, 2018
Dissertation Title: VARIABLE QUBIT-QUBIT COUPLING VIA A TUNABLE LUMPED-ELEMENT RESONATOR
Date and Time: Wednesday, August 22, 1:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Frederick Wellstood (co-advisor)
Committee:
Dr. Christopher Lobb (co-advisor)
Dr. Benjamin Palmer
Dr. Kevin Osborn
Dr. Ichiro Takeuchi
Abstract:
This dissertation examines the design, fabrication, and characterization of a device with two transmon qubits coupled through a tunable superconducting resonator, having a tuning range of ≈800 MHz. To achieve tunability, the inductance in the LC resonator incorporated two single Josephson junction superconducting loops. Application of an external magnetic flux to the loops varied the total inductance of the circuit, thereby changing its resonance frequency. To isolate the system and provide a means for reading out the state of the qubit, the device was mounted in a 3D Al microwave cavity. The flux-dependent transition frequencies of the system were measured and fit to results from a coupled Hamiltonian model. I show that, as the resonator is tuned, the qubit-qubit dispersive shift ranged from an “off” value of 2χ = 0.1 MHz, allowing single qubit operations, to an “on” value of 2χ = 6 MHz, providing enough coupling to perform gates.
This dissertation also includes observations of the temperature dependence of the relaxation time T1 of three Al/AlOx/Al transmons. In some cases, T1 increased by almost a factor of two as the temperature increased from 30 to 100 mK. We found that this anomalous effect is consistent with loss from non-equilibrium quasiparticles in a transmon in which one junction electrode has a slightly smaller superconducting gap than the other. I present a model of this effect, use the model to extract the density of non-equilibrium quasiparticles in the device, and find the values of the superconducting energy gaps in the films.
Simón Riquelme Muñoz - August 9, 2018
Dissertation Title: Generalized Natural Inflation and the Quest for Cosmic Symmetry Breaking Patterns
Date and Time: Thursday, August 9, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Zackaria Chacko
Committee:
Dr. Raman Sundrum
Dr. Theodore Jacobson
Dr. Rabindra Mohapatra
Dr. Richard Wentworth
Abstract:
We present a two-field model that generalizes Natural Inflation, in which the inflaton is the pseudo-Goldstone boson of an approximate symmetry that is spontaneously broken, and the radial mode is dynamical. Within this model, which we designate as “Generalized Natural Inflation”, we analyze how the dynamics fundamentally depend on the mass of the radial mode and determine the size of the non-Gaussianities arising from such a scenario.
We also motivate ongoing research within the coset construction formalism that aims to clarify how the spontaneous symmetry breaking pattern of spacetime, gauge, and internal symmetries may allow us to get a deeper understanding, and an actual algebraic classification, of different possible “cosmic states”, which may allow model-independent descriptions of different phases in the evolution of our universe.
Shavindra Premaratne - July 12, 2018
Dissertation Title: Coherent Control of Low Anharmonicity Systems for Superconducting Quantum Computing
Date and Time: Thursday, July 12, 1:00 pm
Location: PSC 2136
Dissertation Committee Chair: Prof. Frederick Wellstood (co-advisor)
Committee:
Dr. Benjamin Palmer (co-advisor)
Dr. Kevin Osborn
Dr. James Williams
Dr. Christopher Jarzynski
Dr. Andrew Childs
Abstract:
This dissertation describes research to coherently control quantum states of superconducting devices. In the first project, the state of an 8 GHz 3D superconducting Al cavity at 20 mK was manipulated to add a single excitation. Preparing a harmonic resonator in a state with a well-defined number of excitations (Fock states) is not possible using an external classical drive. I generated Fock states by transferring a single excitation from a 5.5 GHz transmon qubit to a cavity using Stimulated Raman Adiabatic Passage (STIRAP). I also extended the STIRAP technique to put the cavity in higher Fock states, superpositions of Fock states, and Bell states between the qubit and the cavity. Master-equation simulations of the system's density matrix were in good agreement with the data, and I obtained estimated fidelities of 89%, 68% and 43% for the first three Fock states, respectively.
The second project involved implementing an entangling gate between two Al/AlOx/Al transmon qubits that were mounted in an Al cavity and cooled to 20 mK. Pertinent system frequencies were as follows: one qubit was at 6.0 GHz, the other qubit at 6.8 GHz, the cavity at 7.7 GHz, and the qubit-qubit dispersive shift was -1 MHz. I used an all-microwave technique known as Speeding up Waveforms by Inducing Phases to Harmful Transitions (SWIPHT). The technique implemented a generalized CNOT gate, by applying a specially-shaped pulse of duration 907 ns. Using quantum process tomography, I found that the gate fidelity was 80%-82%, close to the 87% fidelity expected from decoherence in the transmons during the gate time. Details of the device fabrication, device characterization, measurement techniques, and extensive modeling of device behavior are presented, along with chi-matrix characterization of single-qubit gates and SWIPHT gates.
In this talk, I will focus on the implementation of a generalized CNOT gate using the SWIPHT technique.
Chun-Xiao Liu - July 6, 2018
Dissertation Title: Majorana and Andreev bound states in semiconductor-superconductor nanostructures
Date and Time: Friday, July 6, 11:00 am
Location: 2202 PHY (Condensed Matter Theory & Experiment Conference Room)
Dissertation Committee Chair: Prof. Jay Deep Sau
Committee:
Dr. Sankar Das Sarma
Dr. Victor Yakovenko
Dr. James Williams
Dr. Christopher Jarzynski
Abstract:
Majorana bound states have been a topic of active research over the last two decades. In the perspective of theoretical physics, Majorana bound states, which are their own antiparticles, are zero-energy quasi-particle excitations in exotic superconducting systems. From a technological perspective, Majorana bound states can be utilized for the implementation of fault-tolerant quantum computation due to their topological properties. For example, two well-separated Majorana bound states can form a fermionic qubit state, the quantum information of its occupancy is stored in a nonlocal way, being robust against local decoherence. Also since Majorana bound states obey non-Abelian statistics, quantum gates can be implemented by braiding. Such gate operations are robust because small deviations in braiding trajectories do not affect the braiding results.
So far the most promising platform for the realization of Majorana bound states is the one-dimensional semiconductor-superconductor nanostructures. The hallmark of the existence of Majorana bound states in such systems is a quantized zero-bias conductance peak in the tunneling spectroscopy for a normal-metal-superconductor junction. Although quantized zero-bias conductance peaks that resemble the theoretical prediction have been observed in several experimental measurements, confusing aspects of the data muddy the conclusion. One source of confusion results from the existence of another type of excitation in these systems i.e. the topologically trivial near-zero-energy Andreev bound states. These excitations mimick many behaviors of the topological Majorana bound states.
In this work, we first investigate the tunnel spectrsocopy signatures of both Majorana and Andreev bound states, and show that the Andreev-bound-state-induced conductance peak indeed resembles that of Majorana bound state in many ways. We then talk about the physical mechanism for the formation of such trivial Andreev bound states and show that their presence can be very generic. We then give multiple practical proposals to differentiate between Majorana and Andreev bound states that are doable in the context of the current normal-metal-superconductor junctions. Finally, we discuss another theoretical proposal that can directly measure the topological invariant of the superconductor for future experiments.
David Somers - July 5, 2018
Dissertation Title: Casimir-Lifshitz Forces and Torques
Date and Time: Thursday, July 5, 2:00 pm
Location: ERF 1207 (IREAP large conference room)
Dissertation Committee Chair: Prof. Jeremy Munday
Committee:
Dr. Steven Rolston
Dr. Ian Appelbaum
Dr. Victor Yakovenko
Dr. Luz Martinez-Miranda
Abstract:
Quantum electromagnetic field fluctuations result in the well-documented Casimir- Lifshitz force between macroscopic objects. If the objects are anisotropic, theory predicts a corresponding Casimir-Lifshitz torque that causes the objects to rotate and align. In this work, we report the first measurements of the Casimir-Lifshitz torque, which confirm the predictions first made decades ago. The experimental design uses a nematic liquid crystal separated from a birefringent crystal by an isotropic Al2O3 layer with a thickness ≤ 25 nm. The molecular orientation of the liquid crystal is fixed with a rubbed counterplate, and, by varying the rubbing and Al2O3 thickness, we measured the Casimir-Lifshitz torque as a function of angle and distance.
Along the way, I developed a simpler formulation for calculating the Casimir-Lifshitz interaction in planar systems, which facilitated further theoretical study of the Casimir-Lifshitz torque. Using this method, I outline the conditions for a repulsive Casimir-Lifshitz force between birefringent materials that would allow for an angularly-dependent sign of the force. I also report an unexpected enhancement of the torque from two sources: an intermediate dielectric medium and the finite speed of light.
Chiao-Hsuan Wang - June 11, 2018
Dissertation Title: Photon thermalization in driven open quantum systems
Date and Time: Monday, June 11, 11:00 am
Location: PSC 2136
Dissertation Committee Chair: Prof. Christopher Jarzynski
Committee:
Dr. Jacob Taylor (advisor)
Dr. Alexey Gorshkov
Dr. Trey Porto
Dr. Luis Orozco
Dr. Mohammad Hafezi
Abstract:
Light is a paradigmatic quantum field, with individual excitations---photons---that are the most accessible massless particles known. However, their lack of mass and extremely weak interactions mean that typically the thermal description of light is that of blackbody radiation. As the temperature of the light decreases, the overall number of photons approaches zero. Therefore, efforts for quantum optics and optical physics have mostly focused on driving systems far from equilibrium to populate sufficient numbers of photons. While lasers provide a severe example of a nonequilibrium problem, recent interests in the near-equilibrium physics of so-called photon gases, such as in Bose condensation of light or in attempts to make photonic quantum simulators, suggest one re-examine near-equilibrium cases.
In this thesis, we consider peculiar driven open quantum system scenarios where near-equilibrium dynamics can lead to equilibration of photons with a finite number, following a thermal description closer to that of an ideal gas than to black body radiation. Specifically, we show how laser cooling of a well-isolated mechanical mode or atomic motion can provide an effective bath which enables control of both the chemical potential and temperature of the resulting grand canonical ensemble of photon. We then theoretically demonstrate that Bose condensation of photons can be realized by cooling an ensemble of two-level atoms inside a cavity. Finally, we find that the engineered chemical potential for light not only admits future applications in many-body quantum simulations, facilitates preparation of near-equilibrium photonic states, but also enables an analogous voltage bias for photonic circuit elements.
Mark Eichenlaub - May 30, 2018
Dissertation Title: Mathematical Sensemaking Via Epistemic Games
Date and Time: Wednesday, May 20, 2:00 pm
Location: PSC 2136
Dissertation Committee Chair: Prof. Edward Redish
Committee:
Dr. Eric Brewe
Dr. Michelle Girvan
Dr. Ayush Gupta
Dr. Andrew Elby
Abstract:
This thesis studies how students in an introductory physics for life sciences course at the University of Maryland learn to use the mathematical reasoning resources, especially those around understanding and manipulating algebraic expressions and equations, to solve physics problems and gain insight into how physics works. There are both qualitative and quantitative threads to this work. The qualitative work analyzes a series of problem-solving interviews, first to survey the variety of rich cognitive tools students bring to bear on the problem via case studies, then to draw a connection between the ontological metaphors students use for equations and the epistemic games they play while solving problems. The quantitative thread describes the creation and analysis of the Math Epistemic Games Survey, a math test we wrote and administered to study how students’ uptake of problem-solving strategies such as “check the extreme cases” progressed over a year-long physics course.
Yee Lam Elim Cheung - May 2, 2018
Dissertation Title: Measurement of Atmospheric Neutrino Oscillation Parameters Using Three Years of IceCube-DeepCore Data
Date and Time: Wednesday, May 2, 12:00 pm
Location: PSC 2204
Dissertation Committee Chair: Prof. Gregory Sullivan
Committee:
Dr. Rabindra Mohapatra
Dr. Kara Hoffman
Dr. Andrew Baden
Dr. M. Coleman Miller
Abstract:
The story of neutrinos began in 1930 when Pauli proposed a hypothesized particle as a “desperate remedy” to rescue quantum theory. Although Pauli was pessimistic about the detectability of his new particle, Reins and Cowan first discovered (anti) neutrinos in 1956. Soon after, neutrinos became a puzzle for particle physicists due to a persistent deficit in observed rates by multiple experiments. This mystery was partly answered by Pontecorvo who first proposed the idea of neutrino oscillations in 1957. In 1998, the Super-Kamiokande (SK) collaboration provided the first definitive evidence of neutrino oscillations, for which both the SK and the Sudbury Neutrino Observatory (SNO) collaborations were awarded the Nobel Prize in Physics 2015.
While measuring oscillation parameters has long been a focus for numerous neutrino experiments, the IceCube Neutrino Observatory with DeepCore provides a unique window to measure atmospheric oscillation parameters. With an effective volume ~ 300 times larger than SK, DeepCore can detect atmospheric neutrinos between a few and 100 GeV. In addition, IceCube acts as a thick veto shield for DeepCore to better identify atmospheric muon backgrounds. Given that the amplitude of atmospheric neutrino oscillations is expected to be maximal at ~ 25
GeV, IceCube-DeepCore is well suited for studying atmospheric neutrino oscillations by probing this energy window for the first time.
This work presents the latest measurement of atmospheric oscillation parameters using three years of IceCube-DeepCore data. The standard three neutrino mixing and matter effect due to Earth are considered. Under the assumption of a unitary mixing matrix, a binned analysis using a modified chi^2 is performed, and sixteen systematics are taken into account. Preferring a normal neutrino mass ordering, this analysis measures the mass squared difference, Delta m^2_{23} = 2.55^{+0.12}_{-0.11} x 10^-3 eV^2, and the mixing angle, sin^2 \theta_{23} = 0.58^{+0.04}_{-0.13}. The measurement from this work is comparable to the latest measurements from other long baseline neutrino experiments.
Antony Speranza - April 27, 2018
Dissertation Title: Investigations on entanglement entropy in gravity
Date and Time: Friday, April 27, 4:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Theodore Jacobson
Committee:
Dr. Raman Sundrum
Dr. Brian Swingle
Dr. Bei Lok Hu
Dr. Jonathan Rosenberg
Abstract:
Entanglement entropy first arose from attempts to understand the entropy of black holes, and is believed to play a crucial role in a complete description of quantum gravity. This thesis explores some proposed connections between entanglement entropy and the geometry of spacetime. One such connection is the ability to derive gravitational field equations from entanglement identities. I will discuss a specific derivation of the Einstein equation from an equilibrium condition satisfied by entanglement entropy, and explore a subtlety in the construction when the matter fields are not conformally invariant. As a further generalization, I extend the argument to include higher curvature theories of gravity, whose consideration is necessitated by the presence of subleading divergences in the entanglement entropy beyond the area law.
A deeper issue in this construction, as well as in more general considerations identifying black hole entropy with entanglement entropy, is that the entropy is ambiguous for gauge fields and gravitons. The ambiguity stems from how one handles edge modes at the entangling surface, which parameterize the gauge transformations that are broken by the presence of the boundary. The final part of this thesis is devoted to identifying the edge modes in arbitrary diffeomorphism-invariant theories. Edge modes are conjectured to provide a statistical description of the black hole entropy, and this work takes some initial steps toward checking this conjecture in higher curvature theories.
Caroline Figgatt - March 22, 2018
Dissertation Title: Building and Programming a Universal Ion Trap Quantum Computer
Date and Time: Thursday, March 22, 10:00 am
Location: PSC 2136
Dissertation Committee Chair: Prof. Christopher Monroe
Committee:
Dr. Gretchen Campbell
Dr. Trey Porto
Dr. James Williams
Dr. Andrew Childs
Abstract:
Quantum computing represents an exciting frontier in the realm of information processing; it is a promising technology that may provide future advances in a wide range of fields, from quantum chemistry to optimization problems. This thesis discusses experimental results for several quantum algorithms performed on a programmable quantum computer consisting of a linear chain of five or seven trapped 171Yb+ atomic clock ions with long coherence times and high gate fidelities. We execute modular one- and two-qubit computation gates through Raman transitions driven by a beat note between counter-propagating beams from a pulsed laser. The system's individual addressing capability provides arbitrary single-qubit rotations as well as all possible two-qubit XX-entangling gates, which are implemented using a pulse-segmentation scheme. The quantum computer can be programmed from a high-level interface to execute arbitrary quantum circuits, and comes with a toolbox of many important composite gates and quantum subroutines.
We present experimental results for a complete three-qubit Grover quantum search algorithm, a hallmark application of a quantum computer with a well-known speedup over classical searches of an unsorted database, and report better-than-classical performance. The algorithm is performed for all 8 possible single-result oracles and all 28 possible two-result oracles. Two methods of state marking are used for the oracles: a phase-flip method employed by other experimental demonstrations, and a Boolean method requiring an ancilla qubit that is directly equivalent to the state-marking scheme required to perform a classical search. All quantum solutions are shown to outperform their classical counterparts.
Performing parallel operations will be a powerful capability as deeper circuits on larger, more complex quantum computers present new challenges. Here, we perform a pair of 2-qubit gates simultaneously in a single chain of trapped ions. We employ a pre-calculated pulse shaping scheme that modulates the phase and amplitude of the Raman transitions to drive programmable high-fidelity 2-qubit XX gates in parallel by coupling to the collective modes of motion of the ion chain. Ensuring the operation yields only spin-spin interactions between the desired pairs, with neither residual spin-motion entanglement nor "crosstalk" spin-spin entanglement, is a nonlinear constraint problem, and pulse solutions are found using optimization techniques. As an application, we demonstrate the quantum full adder using a depth-4 circuit requiring the use of parallel 2-qubit operations.
Qin Liu - March 14, 2018
Dissertation Title: LARGE-SCALE NEURAL NETWORK MODELING: FROM NEURONAL MICROCIRCUITS TO WHOLE-BRAIN COMPLEX NETWORK DYNAMICS
Date and Time: Wednesday, March 14, 2:00 pm
Location: PSC 2136
Dissertation Committee Chair: Prof. Steven Anlage
Committee:
Dr. Barry Horwitz (advisor)
Dr. Wolfgang Losert
Dr. Edward Ott
Dr. Daniel Butts
Abstract:
Neural networks mediate human cognitive functions, such as sensory processing, memory, attention, etc. Computational modeling has been proved as a powerful tool to test hypothesis of network mechanisms underlying cognitive functions, and to understand better human neuroimaging data. The dissertation presents a large-scale neural network modeling study of human brain visual/auditory processing and how this process interacts with memory and attention.
We first modeled visual and auditory objects processing and short-term memory with local microcircuits and a large-scale recurrent network. We proposed a biologically realistic network implementation of storing multiple items in short-term memory. We then realized the effect that people involuntarily switch attention to salient distractors and are difficult to distract when attending to salient stimuli, by incorporating exogenous and endogenous attention modules. The integrated model could perform a number of cognitive tasks utilizing different cognitive functions by only changing a task-specification parameter. Based on the performance and simulated imaging results of these tasks, we proposed hypothesis for the neural mechanism beneath several important phenomena, which may be tested experimentally in the future.
Theory of complex network has been applied in the analysis of neuroimaging data, as it provides a topological abstraction of human brain. We constructed functional connectivity networks for various simulated experimental conditions. A number of important network properties were studied, including the scale-free property, the global efficiency, modular structure, and explored their relations with task complexity. We showed that these network properties and their dynamics of our simulated networks matched empirical studies, and we were able to relate these network-level phenomena to the underlying model mechanisms, which verifies the validity and importance of our modeling work in testing neural network hypothesis.
Jason Andrews - February 16, 2018
Dissertation Title: Study of the decay B+ → K+ π0 at LHCb and mechanical development for the design of the Upstream Tracker
Date and Time: Friday, February 16, 1:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Hassan Jawahery
Committee:
Dr. Nicholas Hadley
Dr. Zackaria Chacko
Dr. Alberto Belloni
Dr. Richard Mushotzky
Abstract:
The LHCb experiment at the Large Hadron Collider (LHC) is designed to measure the properties of particles containing charm (c) and bottom (b) quarks. This dissertation documents two major studies I have completed, one analyzing data collected by the LHCb detector, and another contributing to the design and development of an extensive upgrade to the detector.
The pattern of CP asymmetry measurements of the B → K π family of decays deviates from expectations derived from the SM, a contradiction known as the "K π puzzle." The present size of the experimental errors are such that more precise measurements in the B+ → K+ π0 decay channel are especially important. An analysis of the B+ → K+ π0 decay using data collected during Run 1 is performed. Despite low reconstruction and trigger efficiencies and enormous combinatorial backgrounds, a signal is found with a statistical significance of 3.7σ. This achievement has led to the creation of a dedicated B+ -> K+ π0 trigger, and has inspired the creation of a number of dedicated triggers for decay modes with similar topologies. A preliminary analysis of data collected during Run 2 demonstrates that the new trigger is a major success, with excellent prospects for making the world’s best measurements in the B+ → K+ π0 decay channel using the entire Run 2 data set.
Run 2 of the LHC will conclude at the end of 2018, and will be followed by Run 3, scheduled to begin in early 2021. In the interim, the LHCb detector will be upgraded to be read-out in real-time at 40 MHz, and to withstand the radiation damage associated with collecting 50 fb^(−1) of integrated luminosity by the conclusion of Run 4. A key part of this upgrade is the design and construction of a new silicon-strip tracking detector—the upstream tracker (UT). Regions at the periphery of the UT suffer from severe electrical and mechanical constraints, making a high-fidelity CAD model a critical element of the design process. The result is a mechanical integration solution that is entirely non-trivial, and which has had significant influences on the UT design. This solution and the constraints that influence it are shown in detail.
Kiersten Ruisard - February 14, 2018
Dissertation Title: Design of a Nonlinear Quasi-Integrable Lattice for Resonance Suppression at the University of Maryland Electron Ring
Date and Time: Wednesday, February 14, 10:00 am
Location: ERF 1207 (IREAP large conference room)
Dissertation Committee Co-Chairs: Profs. Thomas Antonsen and Timothy Koeth
Committee:
Dr. Irving Haber
Dr. Brian Beaudoin
Dr. Andrew Baden
Dr. Patrick O'Shea
Abstract:
Conventional particle accelerators use linear focusing forces for transverse confinement. As a consequence of linearity, accelerating rings are sensitive to a myriad resonances and instabilities. At high beam intensities uncontrolled resonance-driven losses can deteriorate beam quality and cause damage or radio-activation in beam line components and surrounding areas. This is currently a major limitation of achievable current densities in state-of-the-art accelerators. Incorporating nonlinear focusing forces into machine design should provide immunity to resonances through nonlinear detuning of particle orbits from driving terms. A theory of nonlinear integrable beam optics is currently being investigated for use in accelerator rings. Such a system has potential to overcome the limits on achievable beam intensity.
This dissertation presents a plan for implementing a proof-of-principle quasi-integrable octupole lattice at the University of Maryland Electron Ring (UMER). UMER is an accelerator platform that supports the research of high-intensity beam transport. In this dissertation, two designs are presented that differ in both complexity and strength of predicted effects. A configuration with a single, relatively long octupole magnet is expected to be more stabilizing than an arrangement of many short, distributed octupoles.
Preparation for this experiment required the development and characterization of a low-intensity regime previously not operated at UMER. Additionally, required tolerances for the control of beam orbit and focusing in the proposed experiments have been determined on the basis of simulated beam dynamics. In order to achieve these tolerances, a new method for improved orbit correction is developed. Finally, a study of resonance-driven losses in the linear UMER lattice is discussed.
Avinash Kumar - January 26, 2018
Dissertation Title: Experiments with a superfluid BEC ring
Date and Time: Friday, January 26, 1:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Steven Rolston
Committee:
Dr. Gretchen Campbell (advisor)
Dr. Christopher Lobb
Dr. Charles Clark
Dr. Christopher Jarzynski
Abstract:
The dissertation presents multiple results of our experiments with ring-shaped $^{23}$Na Bose-Einstein condensates. First, we measure the effect of temperature on the lifetime of a quantized, persistent current. We find that the persistent current lifetime decreases when the temperature is increased. We also extract the critical velocity by measuring the size of hysteresis loops. The critical velocity is found to be a strong function of temperature. Second, we implement a new technique of measuring the circulation state of a persistent current in-situ, which is minimally-destructive. This technique uses the Doppler effect. Finally, we study the dynamics of rapidly expanding rings, and explore the analogy between our experimental system and the expansion of the universe.
Jon Balajthy - January 19, 2018
Dissertation Title: PURITY MONITORING TECHNIQUES AND ELECTRONIC ENERGY DEPOSITION PROPERTIES IN LIQUID XENON TIME PROJECTION CHAMBERS
Date and Time: Friday, January 19, 3:30 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Carter Hall
Committee:
Dr. Elizabeth Beise
Dr. Xiangdong Ji
Dr. Alberto Belloni
Dr. M. Coleman Miller
Abstract:
Currently, one of the most well motivated models of dark matter is the weakly interacting massive particle (WIMP), and the detector technology that is in the best position to observe these WIMPs is the two-phase liquid xenon time projection chamber (TPC). As liquid xenon WIMP detectors grow larger and more sensitive, the requirements placed on their signals and backgrounds become more and more stringent. We develop a technique for measuring the concentration of the radioactive 85Kr isotope in xenon. We show that we are able to detect natural krypton concentrations down to 7.7 ±2.0 parts per quadrillion (ppQ). On the signals side, we provide a measurement of the charge and light yields of beta recoils in liquid xenon. For these measurements, we use 14C and 3H calibration data collected in the LUX detector after the 2014-2016 WIMP-search run was completed. These measurements span from 43 to 491 V/cm in electric field and from 1 to 140 keVee in recoil energy. We also look for a non-statistical shape factor in the 14C spectrum. We observe a spectrum in the LUX data that is consistent with a purely statistical shape, which disagrees with a recent measurement by Kuzminov et al. by 1.8-s. However, pathologies in the LUX signals prevent us from making any strong claims on this topic.
Jack Wimberley - December 13, 2017
Dissertation Title: Semitauonic B_c decays and quark flavor identification methods
Date and Time: Wednesday, December 13, 1:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Hassan Jawahery
Committee:
Dr. Nicholas Hadley
Dr. Sarah Eno
Dr. Kaustubh Agashe
Dr. Eric Slud
Abstract:
The LHCb experiment at the Large Hadron collider is a unique laboratory for studying the properties of heavy quarks. The physics program of the experiment includes studies of CP violation, measurements of CKM matrix parameters, searches for rare decays, quarkonia studies, and other flavor physics, forward physics, and new physics topics.
This dissertation presents an analysis measuring the ratio of the semileptonic branching fractions B(B_c -> J/ψ τ ν) and B(B_c -> J/ψ μ ν) of the doubly-heavy B_c meson, denoted R(J/ψ). Such semitauonic branching fraction measurements have played an increasingly prominent part of B physics research at BaBar, Belle, and LHCb. These measurements are powerful probes of the universality of the couplings of leptons (e, μ, and τ) in electroweak interactions. Currently, measurements of the quantities R(D) and R(D*) by all three experiments are in excess of precise Standard Model predictions.
A second topic of the dissertation is the creation of a new algorithm for the tagging the flavor of neutral mesons in CP violation studies, and a powerful method for calibrating these flavor tagging algorithms via binomial regression. This work ties in to the increasingly prominent use of machine learning techniques in particle physics, and the need for solid understanding of the behavior of their output.
Stephen Ragole - December 13, 2017
Dissertation Title: Understanding Phase Transitions, Symmetry Breaking, and Interaction Enhanced Sensing in Optomechanical and Cold Atomic Systems
Date and Time: Wednesday, December 13, 2:30 pm
Location: 2115 CSS/ATL
Dissertation Committee Chair: Prof. Victor Galitski
Committee:
Dr. Jacob Taylor (advisor)
Dr. Gretchen Campbell
Dr. Jay Deep Sau
Dr. Mohammad Hafezi
Abstract:
We focus on the interaction of light and matter in atomic and optomechanical systems. These highly controllable and engineerable systems present access to new regimes and research opportunities that often do not exist outside the laboratory. As such, they frequently depart from more commonplace systems which are well understood. We extend our understanding of thermodynamic phase transitions, spontaneous symmetry breaking, and quantum-enhanced sensing to new regimes.
Traditionally, phase transitions are defined in thermodynamic equilibrium. However, inspired by the success of the phase transition paradigm in non-equilibrium fields, we derive an effective thermodynamics for the mechanical excitations of an optomechanical system. Noting the common frequency separation between optical and mechanical components, we study the dynamics of the mechanical modes under the influence of the steady state of the optical modes. We identify a sufficient set of constraints which allow us to define an effective equilibrium for the mechanical system. We demonstrate these constraints by studying the buckling transition in an optomechanical membrane-in-the-middle system, which spontaneously breaks a parity symmetry. Having established a thermodynamic limit, we characterize the nature of the phase transition, which can change order based on system parameters. We extend our framework, proposing an photonic systems which realizes an SO(N) symmetry breaking transition of the same nature as the membrane-in-the-middle system. While we have treated these systems in the classical limit, their open nature has pronounced effects when other noise sources are suppressed. We study the canonical optomechanical system to unravel the origin of the semiclassical force and potential on the mechanics. We find that this force, while conservative with respect to the mechanics, deeply depends on the quantum back-action due to photon loss from the cavity.
Additionally, we study the ability of cold atoms to sense rotation. We consider bosonic atoms confined to a one-dimensional ring. Employing Luttinger liquid theory to study the excitations, we find that in the strongly-repulsive regime, atomic currents can be manipulated and superposed by controlling a laser barrier. These superpositions provide a Heisenberg-limited rotation sensing method. When we include noise, the precision is reduced, but the performance still surpasses the standard quantum limit. We comment on the applicability of such a sensor for inertial sensing.
Erin Sohr - December 11, 2017
Dissertation Title: Student Sensemaking in Quantum Mechanics: Lessons to Teachers from Studies of Groupwork and Representation Use
Date and Time: Monday, December 11, 11:00 am
Location: PSC 3150
Dissertation Committee Co-Chairs: Profs. Andrew Elby and Ayush Gupta (advisor)
Committee:
Dr. Janet Walkoe
Dr. Kara Hoffman
Dr. Patricia Alexander
Abstract:
This dissertation covers two distinct threads of research; both threads focus on understanding student-thinking in quantum mechanics and then draw implications for future research and instruction. The primary goal of this collection of work is, in any way possible, to improve instruction and find ways to better support students in their learning.
The first thread of research focuses on tension negotiation in collaborative group problem-solving. While group-work has become more commonplace in physics classes, this research provides instructors some means of seeing just how complicated group dynamics can be. In particular, I highlight one interactional pattern through which students resolve tension emerging in group interaction by closing conversations or conversational topics. In doing so, students leave some conceptual line of reasoning unresolved. This work provides important insights into helping instructors understand and respond to group dynamics and conversational closings. The second thread of work focuses on flexible representation use. This thread has two similar lines of research. The first focuses on how particular representations (wavefunction and external potential graphs) associated with the infinite-well and finite-well potentials can be used by students as tools to learn with. Adapting these models to new situations can lead to deeper understandings of both the model being adapted and the new situation. In some cases, the process of adaptation is not impeded by the student lacking a sophisticated understanding of the model being adapted.
The second line of research on representation use focuses on the reflexiveness of student inquiry with representations. In reflexive reasoning, the student’s sensemaking shapes, and is shaped by, the representations they draw and animate. This form of inquiry stands in contrast with traditional notions of proficiency in using representations which tend to highlight reproducing standard representational forms and then reading-out information from those forms. In this work, I highlight how this non-linear, reflexive sense-making is supported by the development of coherent, coupled systems of representations and attention to particular figural features, leading to the generation of new meaning.
Jonathan Larson - December 8, 2017
Dissertation Title: Innovative Scanning Probe Methods for Energy Storage Science: Elucidating the Physics of Battery Materials at the Nano-to-Microscale
Date and Time: Friday, December 8, 9:30 am
Location: CHM 0112 (Marker Seminar Room)
Dissertation Committee Chair: Prof. Theodore Einstein
Committee:
Dr. Janice Reutt-Robey
Dr. Ellen Williams
Dr. James Williams
Dr. Sangbok Lee
Abstract:
Energy storage research is uniquely positioned in modern science and technology. Advancements in the field (or lack thereof), will affect the future of humans, ecosystems, environments, and economies in a positive (or negative) way. While there has been decades-long progress in the energy storage solutions of everyday portable electronic devices, major energy storage hurdles persist such as grid-scale storage, and economically palatable, safe vehicular batteries. In an attempt to tackle these massive issues, the research community is looking beyond typical storage concepts, chemistries, electrolytes, and geometries. Many of these approaches make use of nanoscale and mesoscale technologies. For example, one such promising approach replaces conventional planar electrodes with a collection of nanostructures, arraigned in dense mesoscale architectures, aiming to increase key figures of merit, like power density (via nanostructures) and energy density (via dense mesoscale architectures). Regardless of the novel approach, new techniques are needed for characterization and scientific discovery at the nano-to-microscale. In this talk, I will discuss my PhD research which has precisely targeted these needs within the energy storage community. The work has resulted in the devolvement and application of innovative scanning probe approaches for basic energy storage discovery at the nano-to-microscale. Leveraging a new class of scanning probes, invented here, “battery probes” help to enable scanning nanopipette and probe microscopy, pascalammetry with microbattery probes, inverted scanning tunneling spectroscopy, and nanoscale solid-state electrochemistry with nanobattery probes.
Hilary Hurst - December 7, 2017
Dissertation Title: Dynamics of Topological Defects in Hybrid Quantum Systems.
Date and Time: Thursday, December 7, 1:00 pm
Location: PSC 2136
Dissertation Committee Chair: Prof. Victor Galitski
Committee:
Dr. Victor Yakovenko
Dr. Maissam Barkeshli
Dr. Ian Spielman
Dr. John Weeks
Abstract:
This dissertation focuses on dynamics and transport effects of semiclassical topological defects in systems with important quantum degrees of freedom, which we term "hybrid quantum systems". The topological defects under consideration are skyrmions and magnetic vortices in layered heterostructures of three-dimensional (3D) topological insulators (TI) and magnetic materials, and dark solitons in Bose-Einstein condensates (BEC).
We examine the proximity effect between a 3D TI and two types of insulating magnets: a chiral magnet with a single skyrmion in a ferromagnetic background, and an XY-magnet with strong easy-plane anisotropy which undergoes a vortex unbinding transition. The skyrmion magnetic texture leads to confinement of Dirac states at the skyrmion radius, resulting in a charged skyrmion that can be manipulated by an external electric field. We show that the bound states are robust in the presence of an external magnetic field. Magnetic vortices in the XY-magnet affect electronic transport at the TI surface. Scattering at classical magnetic fluctuations influences surface resistivity of the TI, and near the transition temperature we find that the resistivity has a clear maximum and scales linearly with temperature on either side of the transition. We discuss the limits of mapping the TI XY-magnet model to the classic theoretical problem of free Dirac fermions in a random magnetic field.
Secondly, we study dark solitons in a BEC coupled to thermal non-interacting impurity atoms acting as a dissipative bath. We calculate the friction coefficient due to scattering and find that it can be tuned with accessible experimental probes. We develop a general theory of stochastic dynamics of the negative-mass dark soliton and solve the corresponding Fokker-Planck equation exactly. From the time-dependent phase-space probability distribution function we find the soliton can undergo Brownian motion only in the presence of friction and a confining potential. Finally, we numerically study the ground-state properties of a spin-1 BEC gas in the "synthetic dimensions" experimental set-up. Ground state phases depend on the sign of the spin-dependent interaction parameter and the strength of the spin-orbit field. We find "charge"- and spin-density-wave phases related to helical spin order.
Matthew Reed - December 6, 2017
Dissertation Title: An Experimental Realization of a Griffiths Phase in 87Rb in Three Dimensions
Date and Time: Wednesday, December 6, 12:00 pm
Location: PSC 1136
Dissertation Committee Chair: Prof. Steven Rolston
Committee:
Dr. Gretchen Campbell
Dr. Jay Deep Sau
Dr. Ian Spielman
Dr. Mohammad Hafezi
Abstract:
We describe a novel High Bandwidth Arbitrary Lattice Generator (HiBAL) we've created to skirt limits imposed on monochromatic standing waves of light. With its current iteration we can phase and amplitude modulate optical lattices over a broad range of wavevectors simultaneously at MHz frequencies. We characterize its behavior with a multi-Mach-Zehnder interferometer and a 0.5 NA diffraction limited imaging system, both designed and built in-house. We report lattice phase control to within a few parts in a thousand.
Disorder plays an important role in the phase diagrams of many materials. Crystal defects can cause exotic phases to coexist with the mundane in real world systems, and some phase diagrams are even dominated by the effects of disorder. We report the trapping and characterization of a Bose gas in an optical field isotropic in two dimensions and disordered in a third. We evaluate the phase diagram of our system as a function of temperature and disorder depth, and find favorable comparisons with indications of an intermediate Griffiths phase predicted by previous Monte Carlo and Renormalization Group studies separating 2D and 3D superfluid regimes.
Finally, I discuss the possibility of realizing the BKT transition in a non-orientable space. The BKT phase transition an infinite order phase transition in two dimensions from a normal gas to a superfluid mediated by vortices, which are orientable topological phase defects in two dimensions. I discuss the properties of vortices and their intractions on a Mobius strip, and describe how a relay-imaged bichromatic optical potential could be used to form a Mobius strip out of ultracold gases.
Benjamin Reschovsky - November 20, 2017
Dissertation Title: Studies of Ultracold Strontium Gases
Date and Time: Monday, November 20, 2:00 pm
Location: PSC 2136
Dissertation Committee Chair: Prof. Steven Rolston
Committee:
Dr. Gretchen Campbell (advisor)
Dr. Trey Porto
Dr. Christopher Monroe
Dr. Amy Mullin
Abstract:
We describe the operation and performance of an ultracold strontium apparatus that is capable of generating quantum degenerate gases. The experiment has produced Bose-Einstein condensates (BECs) of 84Sr and 86Sr as well as degenerate Fermi gases (DFGs) of 87Sr with a reduced temperature of T/TF = 0.2 at a Fermi temperature of TF = 55 nK. Straightforward modifications could be made to allow for isotopic mixtures and BECs of the fourth stable isotope, 88Sr.
We also report on a technique to improve the continuous loading of a magnetic trap by adding a laser tuned to the 3P1 - 3S1 transition. The method increases atom number in the magnetic trap and subsequent cooling stages by up to 65% for the bosonic isotopes and up to 30% for the fermionic isotope of strontium. We optimize this trap loading strategy with respect to laser detuning, intensity, and beam size. To understand the results, we develop a one-dimensional rate equation model of the system, which is in good agreement with the data. We discuss the use of other transitions in strontium for accelerated trap loading and the application of the technique to other alkaline-earth-like atoms.
Finally, we also report on an updated investigation of photoassociation resonances relative to the 1S0 + 3P1 dissassociation limit in bosonic strontium. Multiple new resonances for 84Sr and 86Sr were measured out to binding energies of -5 GHz and several discrepancies in earlier measurements were resolved. These measurements will allow for the development of a more accurate mass-scaled model and a better theoretical understanding of the molecular potentials near the 3P1 state. We also measure the strength of the 84Sr 0u transitions in order to characterize their use as optical Feshbach resonances.
Andika Putra - November 17, 2017
Dissertation Title: MAGNETIZED PLANE WAVE AND STRIPE-ORDERED PHASES IN SPIN-ORBIT-COUPLED BOSE GASES
Date and Time: Friday, November 17, 9:00 am
Location: 2115 CSS/ATL
Dissertation Committee Chair: Prof. Steven Rolston
Committee:
Dr. Ian B. Spielman (advisor)
Dr. Gretchen Campbell
Dr. Frederick Wellstood
Dr. Mohammad Hafezi
Abstract:
Quantum degenerate gases have provided rich systems to simulate engineered Hamiltonians and to explore quantum many-body problems in laboratory-scale experiments. In this work, we focus our study on spin-orbit-coupled (SOC) Bose-Einstein condensates (BECs) of Rubidium-87 atoms realized using two-photon Raman coupling scheme in which various novel phases are predicted to exist due to competing energies from the atomic internal structure, coupling strength, and many-body collisions.
BECs are observed primarily using the interaction between light and matter, where it is common to probe the atoms with near-resonant light and image their shadow on a camera. This absorption imaging technique measures the integrated column density of the atoms where it is crucial to focus the imaging system. I present a systematic method to bring the ultracold atom systems into an optimal focus using power spectral density (PSD) of the atomic density-density correlation function. The spatial frequency at which the defocus-induced artifacts first appear in the PSD is maximized on focus. The focusing process thus identifies the range of spatial frequencies over which the PSD is uncontaminated by finite-thickness effects.
Next, I describe magnetic phases which exist in spin-1 spin-orbit-coupled condensates in a near-zero temperature. We observe ferromagnetic and unmagnetized phases which are stabilized by the locking between the spin and linear momentum of the system. Our measurements of both the first- and second-order transitions are in agreement with theory.
Finally, I discuss the stripe-ordered phase which occurs in SOC Bose gases favoring the miscibility configuration. The stripe phase is theoretically predicted to have excitation spectrum analogous to that of supersolidity and to exhibit spatial density modulation within specific regions of parameter space. We use the optical Bragg scattering to probe any small density modulation present in the atomic spatial distribution. I present for the very first time the observation of the stripe phase in full phase diagrams. Our measurement results of the phase boundaries are consistent with existing theory and all observations to date.
Nightvid Cole - November 15, 2017
Dissertation Title: CYCLOTRON RESONANCE GAIN IN THE PRESENCE OF COLLISIONS
Date and Time: Wednesday, November 15, 10:00 am
Location: IPT 1116 (IPST Conference Room)
Dissertation Committee Chair: Prof. Thomas Antonsen
Committee:
Dr. Mohammad Hafezi
Dr. Edward Ott
Dr. Thomas E. Murphy
Dr. Wendell T. Hill
Abstract:
The conditions needed for the amplification of radiation by an ensemble of magnetized, relativistic electrons that are collisionally slowing down are investigated. The current study is aimed at extending the work of other researchers in developing solid-state sources of Terahertz radiation. The source type considered here is based on gyrotron-like dynamics of graphene electrons, or it can alternately be viewed as a solid state laser source which uses Landau levels as its band structure and is thus similar to a quantum cascade laser. Such sources are appealing because they offer the potential for a compact, tunable source of Terahertz radiation that could have commercial applications in scanning, communication, or energy transfer. An exploration is undertaken, using linear and nonlinear theories, of the conditions under which such sources might be viable, assuming realistic parameters. Classical physics is used, and the model involves electrons in graphene assumed to be pumped by a laser, follow classical laws of motion with the dissipation represented by a damping force term, and lose energy to the electromagnetic field as well. The graphene is assumed to be in a homogeneous magnetic field, and is sandwiched between two partially-transmissive mirrors so that the device acts as an oscillator.
This thesis incorporates the results of two approaches to the study of the problem. In the first approach, a linear model is derived semi-analytically, which is relevant to the conditions under which there is gain in the device and thus stable operation is possible, versus the regime in which there is no net gain. In the second approach, a numerical simulation is employed to explore the nonlinear regime and saturation behavior of the oscillator. The simulation and the linear model both assume the same original equations of motion for the field and particles that interact self-consistently. The model used here is very simplified, but the aim here is to elucidate the basic principles and scaling behavior of such devices, not necessarily to calculate what the exact dynamics, outputs, and parameters of a fully commercially realized device will be.
Eric Rosenthal - November 3, 2017
Dissertation Title: Energy Deposition in Femtosecond Filamentation: Measurements and Applications
Date and Time: Friday, November 3, 10:00 am
Location: ERF 1207 (IREAP large conference room)
Dissertation Committee Chair: Prof. Howard Milchberg
Committee:
Dr. Phillip Sprangle
Dr. Gregory Nusinovich
Dr. Jared Wahlstrand
Dr. Ki-Yong Kim
Abstract:
Femtosecond filamentation is a nonlinear optical propagation regime of high peak power ultrashort laser pulses characterized by an extended and narrow core region of high intensity whose length greatly exceeds the Rayleigh range corresponding to the core diameter. Providing that a threshold power is exceeded, filamentation can occur in all transparent gaseous, liquid and solid media. In air, filamentation has found a variety of uses, including the triggering of electric discharges, spectral broadening and compression of ultrashort laser pulses, coherent supercontinuum generation, filament-induced breakdown spectroscopy, generation of THz radiation, and the generation of air waveguides. Several of these applications depend on the deposition of energy in the atmosphere by the filament. The main channels for this deposition are the plasma generated in the filament core by the intense laser field and the rotational excitation of nitrogen and oxygen molecules. The ultrafast deposition acts as a delta function-like pressure source to drive a hydrodynamic response in the air. This thesis experimentally demonstrates two applications of the filament-driven hydrodynamic response. One application is the ‘air waveguide’, which is shown to either guide a separately injected laser pulse, or act as a remote collection optic for weak optical signals. The other application is the high voltage breakdown of air, where the effect of filament-induced plasmas and hydrodynamic response on the breakdown dynamics is elucidated in detail. In all of these experiments, it is important to understand quantitatively the laser energy absorption; detailed absorption experiments were performed as a function of laser parameters. Finally, as check on simulations of filament propagation and energy deposition, we measured the axially resolved energy deposition of a filament; in the simulations, this profile is quite sensitive to the choice of the nonlinear index of refraction (n2). We found that using our measured values of n2 in the propagation simulations results in an excellent fit to the measured energy deposition profiles.
Renxiong Wang - October 25, 2017
Dissertation Title: SEARCH FOR NATURAL OCCURRING SUPERCONDUCTIVITY AND NOVEL PHENOMENA: MAGNETIC TRANSITIONS IN NATURAL TRANSITION METAL COMPOUNDS
Date and Time: Wednesday, October 25, 1:00 pm
Location: PHY 2126
Dissertation Committee Chair: Prof. Johnpierre Paglione
Committee:
Dr. Richard Greene
Dr. Nicholas Butch
Dr. Efrain Rodriguez
Dr. Ichiro Takeuchi
Abstract:
Transition metal chalcogenides and transition metal arsenides are important families of natural mineral compounds widely distributed in the natural world. With similar structural and electronic properties of transition metal oxides, natural transition metal compounds are expected to have similar novel phenomena. With an ongoing project for searching natural superconductors in collaboration with Department of Mineral Science, Smithsonian National Museum of Natural History, we had a chance to investigate several natural minerals from the Smithsonian Museum in order to study previously unexpected naturally occurring mineral compounds for interesting ground states.
We found several interesting magnetic transitions in these natural occurring mineral samples. Some of the magnetic transitions are not reported, some of the transitions are associated with other unreported novel quantum phenomena. In this thesis, I will discuss Bornite (Cu5FeS4), Berthierite (FeSb2S4), Nagyagite (Pb5Au(Te,Sb)4S5 8), Maucherite (Ni11As8) and related experiments in detail.
Bornite (Cu5FeS4) has a semiconductor-insulator transition accompanied with an antiferromagnetic transition. As shown by our ability to tune the transition temperature and low-temperature metallicity by applying external pressure, Bornite may be a good candidate for Mott system and searching new superconductors.
Berthierite (FeSb2S4) is a quasi-1-dimensional antiferromagnet. With strong anisotropic physical properties, berthierite may provide a very good system for understanding the low dimensional magnetic material.
A Ferromagnetic order was found in natural Nagyagite (Pb5Au(Te,Sb)4S5 8) samples. The magnetic order, the weak anti-localization property with strong spin-orbital coupling and the 2-dimensional structure of this compound makes it a very interesting system for realizing topological properties in a natural compound.
The magnetic order and transitions in both natural and synthetic Maucherite (Ni11As8) samples show interesting finite-size scale effect. It gives us a different approach to understand the differences in some physical properties between natural and synthetic compounds.
Also, we will present a summary of other magnetic transitions and magnetic properties of more than 40 distinct minerals for this study and show the relation and similarities between strongly correlated transition metal oxide materials and other quantum materials. We will also make a list of other transition metal minerals that are worthy of investigation based on our research experience.
David Wong-Campos - October 16, 2017
Dissertation Title: DEMONSTRATION OF A QUANTUM GATE WITH ULTRAFAST LASER PULSES
Date and Time: Monday, October 16, 3:00 pm
Location: PSC 2136
Dissertation Committee Chair: Prof. Christopher Monroe
Committee:
Dr. Alexey Gorshkov
Dr. Mohammad Hafezi
Dr. Luis A. Orozco
Dr. Howard Milchberg
Abstract:
One of the major problems in building a quantum computer is the development of scalable and robust methods to entangle many qubits. Quantum computers based on trapped atomic ions are one of the most mature and promising platforms for quantum information processing, exhibiting excellent coherence properties, near-perfect qubit detection efficiency and high-fidelity entangling gates. Entangling operations between multiple ions in a chain typically rely on qubit state-dependent forces that modulate their Coulomb-coupled normal modes of motion. However, scaling these operations to large qubit numbers in a single chain must account for the increasing complexity of the normal mode spectrum, and can result in a gate time slowdown or added complexity of the control forces. In this thesis, I present an alternative route to the scalability problem using optical interactions faster than any state evolution. The experiments shown here represent a proof of principle for quantum manipulation of atoms in the strong coupling regime. This work relies on spin dependent forces (SDK) with short laser pulses and use it as our fundamental building block for thermometry and non-trivial motional state preparation. Together with a robust stabilization of the ion trap and high light collection efficiency, we demonstrate two-atom entanglement with ten ultrafast pulses. Due to the nature of the interaction, the demonstrated entangling operation can be made arbitrarily fast only limited by laser engineering.
Ayoti Patra - September 15, 2017
Dissertation Title: Bridging quantum, classical and stochastic shortcuts to adiabaticity
Date and Time: Friday, September 15, 1:00 pm
Location: IPT 1116 (IPST conference room)
Dissertation Committee Chair: Prof. Christopher Jarzynski
Committee:
Dr. Alexey Gorshkov
Dr. Rajarshi Roy
Dr. Victor Yakovenko
Dr. Perinkulam Krishnaprasad
Abstract:
Adiabatic invariants -- quantities that are preserved under the slow driving of a system's external parameters -- are important in classical mechanics, quantum mechanics and thermodynamics. Adiabatic processes allow a system to be guided to evolve to a desired final state. However, the slow driving of a quantum system makes it vulnerable to environmental decoherence, and for both quantum and classical systems, it is often desirable and time-efficient to speed up a process. Shortcuts to adiabaticity are strategies for preserving adiabatic invariants under rapid driving, typically by means of an auxiliary field that suppresses excitations, otherwise generated during rapid driving. Several theoretical approaches have been developed to construct such shortcuts. In this dissertation we focus on two different approaches, namely counterdiabatic driving and fast-forward driving, which were originally developed for quantum systems. The counterdiabatic approach introduced independently by Dermirplak and Rice [J. Phys. Chem. A, 107:9937, 2003], and Berry [J. Phys. A: Math. Theor., 42:365303, 2009] formally provides an exact expression for the auxiliary Hamiltonian, which however is abstract and difficult to translate into an experimentally implementable form. By contrast, the fast-forward approach developed by Masuda and Nakamura [Proc. R. Soc. A, 466(2116):1135, 2010] provides an auxiliary potential that may be experimentally implementable but generally applies only to ground states.
The central theme of this dissertation is that classical shortcuts to adiabaticity can provide useful physical insights and lead to experimentally implementable shortcuts for analogous quantum systems. We start by studying a model system of a tilted piston to provide a proof of principle that quantum shortcuts can successfully be constructed from their classical counterparts. In the remainder of the dissertation, we develop a general approach based on flow-fields which produces simple expressions for auxiliary terms required for both counterdiabatic and fast-forward driving. We demonstrate the applicability of this approach for classical, quanutum as well as stochastic systems. We establish strong connections between counterdiabatic and fast-forward approaches, and also between shortcut protocols required for classical, quantum and stochastic systems. In particular, we show how the fast-forward approach can be extended to highly excited states of quantum systems.
Joseph L. Garrett - August 18, 2017
Dissertation Title: THE EFFECTS OF GEOMETRY AND PATCH POTENTIALS ON CASIMIR FORCE MEASUREMENTS
Date and Time: Friday, August 18, 10:00 am
Location: ERF 1207 (IREAP large conference room)
Dissertation Committee Chair: Prof. Jeremy Munday
Committee:
Dr. Ian Applebaum
Dr. Victor Galitski
Dr. Min Ouyang
Dr. Jan Sengers
Abstract:
Electromagnetic fluctuations of the quantum vacuum cause an attractive force between surfaces, called the Casimir force. In this dissertation, the first Casimir force measurements between two gold-coated spheres are presented. The proximity force approximation (PFA) is typically used to compare experiment to theory, but it is known to deviate from the exact calculation far from the surface. Bounds are put on the size of possible deviations from the PFA by combining several sphere-sphere and sphere-plate measurements.
Electrostatic patch potentials have been postulated as a possible source of error since the first Casimir force measurements sixty years ago. Over the past decade, several theoretical models have been developed to characterize how the patch potentials contribute an additional force to the measurements. In this dissertation, Kelvin probe force microscopy (KPFM) is used to determine the effect of patch potentials on both the sphere and the plate. Patch potentials are indeed present on both surfaces, but the force calculated from the patch potentials is found to be much less than the measured force. In order to better understand how KPFM resolves patch potentials, the artifacts and sensitivities of several different KPFM implementations are tested and characterized. In addition, we introduce a new technique, called tunable spatial resolution (TSR) KPFM, to control resolution by altering the power-law separation dependence of the KPFM signal.
Tian Li - August 10, 2017
Dissertation Title: Optical properties of a quantum-noise limited phase-sensitive amplifier
Date and Time: Thursday, August 10, 2:00 pm
Location: PSC 2136
Dissertation Committee Co-Chairs: Prof. Steven Rolston and Prof. Paul Lett
Committee:
Dr. William Phillips
Dr. Gretchen Campbell
Dr. Julius Goldhar
Abstract:
This thesis is a summary of investigations on the optical properties of the phase-sensitive and phase-insensitive amplifiers. Both optical amplifiers are implemented using four-wave mixing in Rb 85 atomic vapor based on a double-lambda level scheme.
We first study the effects of input phase and amplitude modulation on the output of a quantum-noise-limited phase-sensitive amplifier (PSA). We investigate the dependence of phase modulation imposed on a signal by an acousto-optical modulator on its alignment, and demonstrate a novel approach to quantifying the phase modulation by using the PSA as a diagnostic tool. We then use this method to measure the alignment-dependent phase modulation produced by an optical chopper, which arises due to diffraction effects as the chopper blade passes through the optical beam.
The phase-insensitive amplifier (PIA) has been proven to be a reliable source of two-mode squeezed light, and of quantum entanglement. In this dissertation, the PIA is utilized to generate a two-mode squeezed state, i.e., a twin-beam state, and is also used as a gain-assisted anomalous dispersion medium. We experimentally demonstrate the ability of a PSA to pre-amplify quantum correlations in twin light beams before degradation due to loss and detector inefficiency. By including a PSA before loss, one is able to preserve the correlations as well as the two-mode squeezing level. We compare the results to simulations employing a simple quantum-mechanical model and find a good agreement.
We have demonstrated that the cross-correlation between the two modes of a bipartite entangled state can be advanced by propagation through a fast-light medium. The extra noise added by a PIA has been speculated to be the mechanism that limits the advance of entanglement, preventing the mutual information from traveling superluminally. As an extension of this phase-insensitive gain assisted anomalous dispersion investigation, we explore the advance and delay of information transmitted through the PSA. We start with a two-mode squeezed state created by the PIA and measure the mutual information shared by the correlated quadratures. We then pass one of these two modes through a PSA and investigate the shift of the mutual information as a function of the PSA phase. In the case of a PSA, it is well known that no extra noise will be added to the quadrature with the correct input phase (e.g., the quadrature with the maximal amplification or the maximal deamplification). We find that there is no dispersion-like behavior at these two phases, however, the peak of mutual information could either be delayed or advanced at any other phase. We also observe an almost identical behavior when we input an amplitude modulated signal to the PSA. We are able to explain the physics of this "quasi-fast-and-slow-light" behavior utilizing a theoretical framework with distributed gain on the carrier and both positive and negative side bands but with distributed loss only on the negative side band. We obtain a good agreement between the experimental results and the theoretical simulations.
Ranchu Mathew - August 10, 2017
Dissertation Title: Classical and quantum dynamics of Bose-Einstein condensates
Date and Time: Thursday, August 10, 1:00 pm
Location: CSS 2115
Dissertation Committee Chair: Prof. Jay Deep Sau
Committee:
Dr. Eite Tiesinga (advisor)
Dr. Jacob Taylor
Dr. William Dorland
Dr. Dionisios Margetis
Abstract:
After the first experimental realization of a Bose-Einstein condensate (BEC) in 1995, BECs have become a subject of intense experimental and theoretical study. In this dissertation, I present our results on the classical and quantum dynamics of BECs at zero temperature under different scenarios.
First, I consider the analog of slow light in the collision of two BECs near a Feshbach resonance. The scattering length then becomes a function of the collision energy. I derive a generalization of the Gross-Pitaevski equation for incorporating this energy dependence. In certain parameter regimes, the group velocity of a BEC traveling through another BEC decreases. I also study the feasibility of an experimental realization of this phenomena.
Second, I analyze an experiment in which a BEC in a ring-shaped trap is stirred by a rotating barrier. The phase drop across and current flow through the barrier is measured from spiral-shaped density profiles created by interfering the BEC in the ring-shaped trap and a concentric reference BEC after release from all trapping potentials. I show that a free-particle expansion is sufficient to explain the origin of the spiral pattern and relate the phase-drop to the geometry of a spiral. I also bound the expansion times for which the phase-drop can be accurately determined and study the effect of inter-atomic interactions on the expansion time scales.
Third, I study the dynamics of few-mode BECs when they become dynamically unstable after preparing an initial state at a saddle point of the Hamiltonian. I study the dynamics within the truncated Wigner approximation (TWA) and find that due to phase-space mixing, the expectation value of an observable relaxes to a steady-state value. Using the action-angle formalism, we derive analytical expressions for the steady-state value and the time evolution towards this value. We apply these general results to two systems: a condensate in a double-well potential and a spin-1 (spinor) condensate.
Finally, I study quantum corrections beyond the TWA in the semiclassical limit. I derive general expressions for the dynamics of an observable by using the van Vleck-Gutzwiller propagator and find that the interference of classical paths leads to non-perturbative corrections. As a case study, I consider a single-mode nonlinear oscillator; this system displays collapse and revival of observables. We find that the interference of classical paths, which is absent in the TWA, leads to revivals.
Joyce Coppock - August 1, 2017
Dissertation Title: Optical and magnetic measurements of a levitated, gyroscopically stabilized graphene nanoplatelet
Date and Time: Tuesday, August 1, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Frederick Wellstood
Committee:
Dr. Richard Greene
Dr. Christopher Lobb
Dr. Bruce Kane (advisor)
Dr. Ichiro Takeuchi
Abstract:
Levitation of nanoscale particles is an increasingly popular technique in studies ranging from the investigation of material properties to fundamental tests of quantum mechanics. Two-dimensional materials have been extensively studied while attached to substrates, but have rarely been levitated. This work will discuss the design of a system for levitating a charged, micron-scale, multilayer graphene nanoplatelet in a quadrupole electric field trap in high vacuum, enabling sensitive mechanical and magnetic measurements.
The levitated nanoplatelet is gyroscopically stabilized by locking its frequency of rotation to an applied radio frequency (rf) electric field. The stabilized nanoplatelet is extremely sensitive to external torques, and the residual slow dynamics of the direction of the axis of rotation are determined by an applied magnetic field. Optical data on the interaction of the platelet with the magnetic field are presented. Two mechanisms of interaction are observed: a diamagnetic polarizability and a magnetic moment proportional to the frequency of rotation.
Chang-Hun Lee - July 26, 2017
Dissertation Title: Left-right symmetric model and its TeV-scale phenomenology
Date and Time: Wednesday, July 26, 1:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Rabindra Mohapatra
Committee:
Dr. Raman Sundrum
Dr. Sarah Eno
Dr. Kaustubh Agashe
Dr. Niranjan Ramachandran
Abstract:
The Standard Model of particle physics is a chiral theory with a broken parity symmetry, and the left-right symmetric model is an extension of the SM with the parity symmetry restored at high energies. Its extended particle content allows us not only to find the solution to the parity problem of the SM but also to solve the problem of understanding the neutrino masses via the seesaw mechanism. If the scale of parity restoration is in the few TeV range, we can expect new physics signals that are not present in the Standard Model in planned future experiments. We investigate the TeV-scale phenomenology of the various classes of left-right symmetric models, focusing on the charged lepton flavour violation, neutrinoless double beta decay, electric dipole moments of charged leptons, and leptogenesis.
Sungwoo Hong - July 13, 2017
Dissertation Title: A NATURAL EXTENSION OF STANDARD WARPED HIGHER-DIMENSIONAL COMPACTIFICATIONS: THEORY AND PHENOMENOLOGY
Date and Time: Thursday, July 13, 11:00 am
Location: PSC 3150
Dissertation Committee Chair: Prof. Kaustubh Agashe
Committee:
Dr. Raman Sundrum
Dr. Rabindra Mohapatra
Dr. Sarah Eno
Dr. Zackaria Chacko
Dr. Richard Wentworth
Abstract:
Warped higher-dimensional compactifications with “bulk” standard model, or their AdS/CFT dual as the purely 4D scenario of Higgs compositeness and partial compositeness, offer an elegant approach to resolving the electroweak hierarchy problem as well as the origins of flavor structure. However, low-energy electroweak/flavor/CP constraints and the absence of non-standard physics at LHC Run 1 suggest that a “little hierarchy problem” remains, and that the new physics underlying naturalness may lie out of LHC reach. Assuming this to be the case, we show that there is a simple and natural extension of the minimal warped model in the Randall-Sundrum framework, in which matter, gauge and gravitational fields propagate modestly different degrees into the IR of the warped dimension, resulting in rich and striking consequences for the LHC (and beyond).
The LHC-accessible part of the new physics is AdS/CFT dual to the mechanism of “vectorlike confinement”, with TeV-scale Kaluza-Klein excitations of the gauge and gravitational fields dual to spin-0,1,2 composites. Unlike the minimal warped model, these low-lying excitations have predominantly flavor-blind and flavor/CP-safe interactions with the standard model. In addition, the usual leading decay modes of the lightest KK gauge bosons into top and Higgs bosons are suppressed. This effect permits erstwhile subdominant channels to become significant. These include flavor-universal decays to SM fermions and Higgs bosons, and a novel channel — decay to a radion and a SM gauge boson, followed by radion decay to a pair of SM gauge bosons.
Remarkably, this scenario also predicts small deviations from flavor-blindness originating from virtual effects of Higgs/top compositeness at ∼ O(10) TeV, with subdominant resonance decays into Higgs/top-rich final states, giving the LHC an early “preview” of the nature of the resolution of the hierarchy problem. Discoveries of this type at LHC Run 2 would thereby anticipate (and set a target for) even more explicit explorations of Higgs compositeness at a 100 TeV collider, or for next-generation flavor tests.
Nihal Jhajj - July 7, 2017
Dissertation Title: HYDRODYNAMIC AND ELECTRODYNAMIC IMPLICATIONS OF OPTICAL FEMTOSECOND FILAMENTATION
Date and Time: Thursday, July 7, 10:00 am
Location: ERF 1207 (IREAP large conference room)
Dissertation Committee Chair: Prof. Howard Milchberg
Committee:
Dr. Phillip Sprangle
Dr. James Drake
Dr. Daniel Lathrop
Dr. Ki-Yong Kim
Abstract:
Filamentation is a nonlinear propagation mode of high peak power optical pulses in dielectric media, where nonlinearities arising from the interaction between light and matter can overcome diffraction, resulting in the self-channeling of the pulse. The large optical field present in filaments enables efficient nonlinear conversion for electromagnetic sources spanning from terahertz to x-rays. Filamentation in air is of particular interest for remote applications, where filaments are unique in their ability to deliver high field intensities (up to 10^14 W/cm^2) at kilometer distances, allowing for ranged applications such as LIDAR and laser induced breakdown spectroscopy. This dissertation presents research exploring two topics: i) how to use filaments to inscribe optical guiding structures into the air with millisecond lifetimes, enabling the generation of line-of-sight atmospheric waveguides, and ii), the discovery of the spatiotemporal optical vortex (STOV), a new and previously unobserved type of optical vortex present in all filamenting beams.
George Hine - May 25, 2017
Dissertation Title: COMPACT LASER-DRIVEN ELECTRON AND PROTON ACCELERATION WITH LOW ENERGY LASERS
Date and Time: Thursday, May 25, 3:30 pm
Location: ERF 1207 (IREAP large conference room)
Dissertation Committee Chair: Prof. Howard Milchberg
Committee:
Dr. Gretchen Campbell
Dr. Phillip Sprangle
Dr. Timothy Koeth
Dr. Ki-Yong Kim
Abstract:
Laser-driven particle accelerators offer many advantages over conventional particle accelerators. The most significant of these is the magnitude of the accelerating gradient and, consequently, the compactness of the accelerating structure. In this dissertation, experimental and computational advances in laser-based particle acceleration in three intensity regimes are presented. All mechanisms investigated herein are accessible by "tabletop" ultrashort terawatt-class laser systems found in many university labs, with the intention of making them available to more compact and high repetition rate laser systems. The first mechanism considered is the acceleration of electrons in a preformed plasma "slow-wave" guiding structure. Experimental advances in the generation of these plasma guiding structures are presented. The second mechanism is the laser-wakefield acceleration of electrons in the self-modulated regime. A high-density gas target is implemented experimentally leading to electron acceleration at low laser pulse energy. Consequences of operating in this regime are investigated numerically. The third mechanism is the acceleration of protons by a laser-generated magnetic structure. A numerical investigation is performed identifying operating regimes for experimental realizations of this mechanism.
Jacob Tosado - May 11, 2017
Dissertation Title: Investigation of Graphene and other Low Dimensional Materials
Date and Time: Thursday, May 11, 4:00 pm
Location: PSC 3402
Dissertation Committee Chair: Prof. Ellen Williams
Committee:
Dr. Min Ouyang
Dr. Theodore Einstein
Dr. Michael Fuhrer
Dr. Janice Reutt-Robey
Abstract:
This thesis describes experiments to characterize defects in two-dimensional materials and understand their effect on electrical conductivity. Defects limit the electrical conductivity through a material by scattering electrons. Understanding the physics of defects is therefore essential to building materials and structures with novel electronic properties. This dissertation has focused on low dimensional materials because they are simple thereby allowing for more advanced theory and they will act as a foundation for understanding higher dimensional systems.
High resolution x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption fine structure spectroscopy (NEXAFS) were used to determine the character of vacancy defects in graphene. Vacancies were induced in graphene on a thermally oxidized silicon substrate using argon ion bombardment. XPS of the carbon 1s core level of pristine graphene shows a C 1s spectrum consistent with a single C 1s peak broadened both instrumentally and by a Doniach-Sunjic type effect. As defects are created, the resulting spectrum is deconvolved into two peaks. The first retains the same spectral width as that for the pristine graphene but with a reduced intensity. The second peak, which is broader and at a slightly higher binding energy (~200 meV), increases in intensity with increasing defect concentration. This second peak is identified as the experimental XPS signature of defective graphene. The observation is somewhat at odds with theoretical calculations of XPS spectra for graphene with various vacancy arrangements, which generally produce C 1s peaks shifted to lower binding energy. Instead, the emergence of this second peak, together with the emergence of a single sharp resonance seen near the vacuum level in the NEXAFS spectra, is interpreted as a distribution of molecular-like states forming on the surface.
Preliminary efforts were made to characterize defects in semiconducting single layer MoS2 using scanning tunneling microscopy (STM) and spectroscopy (STS). Techniques for obtaining a clean MoS2 surface suitable for ultra-high vacuum STM were developed, and preliminary characterization of the single layer WS2 surface by STM and STS was carried out. The local density of states of MoS2, as measured by STS, shows the semiconducting bandgap as well as signatures of donor and acceptor states within the gap.
Gina Quan - April 27, 2017
Dissertation Title: Becoming a Physicist: How Identities and Practices Shape Physics Trajectories
Date and Time: Thursday, April 27, 2:30 pm
Location: PHY 1305F (the Toll Room)
Dissertation Committee Co-Chairs: Prof. Andrew Elby and Prof. Chandra Turpen
Committee:
Dr. Edward Redish
Dr. Ayush Gupta
Dr. James Williams
Dr. Derek Richardson
Abstract:
This dissertation studies the relationships and processes which shape students' participation within the discipline of physics. Studying this early disciplinary participation gives insight to how students are supported in or pushed out of physics, which is an important step in cultivating a diverse set of physics students. This research occurs within two learning environments that we co-developed: a physics camp for high school girls and a seminar for undergraduate physics majors to get started in physics research. Using situated learning theory, we conceptualized physics learning to be intertwined with participation in physics practices and identity development. This theoretical perspective draws our attention to relationships between students and the physics community. Specifically, we study how students come to engage in the practices of the community and who they are within the physics community. We find that students' interactions with faculty and peers impact the extent to which students engage in authentic physics practices. These interactions also impact the extent to which students develop identities as physicists. We present implications of these findings for the design of physics learning spaces. Understanding this process of how students become members of the physics community will provide valuable insights into fostering a diverse set of successful trajectories in physics.
Kenneth Wright - April 24, 2017
Dissertation Title: MANIPULATION OF THE QUANTUM MOTION OF TRAPPED ATOMIC IONS VIA STIMULATED RAMAN TRANSITIONS
Date and Time: Monday, April 24, 12:15 pm
Location: CSS 2115 (Atlantic Building)
Dissertation Committee Chair: Prof. Christopher Monroe
Committee:
Dr. Gretchen Campbell
Dr. Luis Orozco
Dr. James Williams
Dr. Andrew Childs
Abstract:
Trapped ions have been a staple resource of quantum simulation for the past several decades. By taking advantage of the spin motion coupling provided by the Coulomb interaction, trapped ions have been used to study quantum phase transitions of highly frustrated spins, many body localization, as well as discrete time crystals. However, all of these simulations involve decoupling the ion motion from spin at the end of the experimental procedure. Here we present progress towards driving bosonic interference between occupied phonon modes of vibration.
This thesis details a tool box for manipulating the motional states of a chain of trapped ions. Taking advantage of spin motion interaction of tightly trapped chains of Yb ions with two photon Raman transition, we will show how to prepare a specific number state of a given normal mode of motion. This is achieved without traditional individual addressing but instead by using composite pulse sequences and ion transport. This will involve a stage of quantum state distillation, and we also show preservation of phonon and spin coherence after this distillation step. This Fock state preparation sets the stage to observe bosonic interference of different phonon modes.
We will use stimulated Raman transitions to create a parametric drive; this drive will couple different normal modes of motion. To observe the bosonic nature of the phonons, we perform a Hong-Ou-Mandel (HOM) interference experiment on two singly occupied normal modes. We use the same spin motion coupling to read out the spin states of individual ions as a witness for this interaction. We also describe a process to use stimulated rapid adiabatic passage (STIRAP) to read out normal mode occupation. The toolbox presented here will be useful for future experiments towards boson sampling using trapped ions.
Nathaniel Steinsultz - April 10, 2017
Dissertation Title: NITROGEN-VACANCY COUPLING IN NANODIAMOND HYBRID NANOSTRUCTURES
Date and Time: Monday, April 10, 2:00 pm
Location: PHY 1305F (John S. Toll Physics Building)
Committee Chair: Prof. Min Ouyang
Committee:
Dr. Steven Anlage
Dr. Christopher Lobb
Dr. Luis Orozco
Dr. John Cumings
Abstract:
Nitrogen-vacancy centers (NVs) are an atomic defect in diamond which possess remarkable fluorescence and spin properties which can be used for multiple metrological applications, particularly when the NV is hosted in nanodiamond, which can be easily integrated with a variety of nanoscale systems. A new class of nanodiamond hybrid nanostructures was developed using bottom-up synthesis methods. In this work, coupling between NV centers and plasmonic, excitonic and magnetic nanoparticles in these nanodiamond hybrid nanostructures is investigated using fluorescence lifetime measurements, spin relaxometry measurements and modeled using finite element method (FEM) and Monte Carlo simulations. This work not only characterizes the properties of these nanodiamond-hybrid nanostructures but also facilitates design guidelines for future hybrid structures with enhanced metrological and imaging capabilities.
Setiawan - April 7, 2017
Dissertation Title: Topological Superconductivity and Majorana Zero Modes
Date and Time: Friday, April 7, 1:00 pm
Location: PHY 2205 (John S. Toll Physics Building)
Committee Chair: Prof. Sankar Das Sarma
Committee:
Dr. Jay Deep Sau
Dr. Maissam Barkeshli
Dr. Theodore Einstein
Dr. Christopher Jarzynski
Abstract:
Recent years have seen a surge interest in realizing Majorana zero modes in condensed matter system. Majorana zero modes are zero-energy quasiparticle excitations which are their own anti-particles. The topologically degenerate Hilbert space and non-Abelian statistics associated with Majorana zero modes renders them useful for realizing topological quantum computation. These Majorana zero modes can be found at the boundary of a topological superconductor. While preliminary evidence for Majorana zero modes in form of zero-bias conductance peaks have already been observed, confirmatory signatures of Majorana zero modes are still lacking.
In this thesis, we theoretically investigate several signatures of Majorana zero modes, thereby suggesting improvement and directions that can be pursued for an unambiguous identification of the Majorana zero modes. We begin by studying analytically the differential conductance of the normal-metal--topological superconductor junction across the topological transition within the Blonder-Tinkham-Klapwijk formalism. We show that despite being quantized in the topological regime, the zero-bias conductance only develops as a peak in the conductance spectra for sufficiently small junction transparencies, or for small and large spin-orbit coupling strength. We proceed to investigate the signatures of the Majorana zero modes in superconductor--normal-metal--superconductor junctions and show that the conductance quantization in this junction is.not robust against increasing junction transparency. Finally, we propose a dynamical scheme to study the short-lived topological phases in ultracold systems by first preparing the systems in its long-lived non-topological phases and then driving it into the topological phases and back. We find that the excitations' momentum distributions exhibit Stuckelberg oscillations and Kibble-Zurek scaling characteristic of the topological quantum phase transition, thus provides a bulk probe for the topological phase.
Pablo Andrés Solano Palma - March 31, 2017
Dissertation Title: Quantum Optics in Optical Nanofibers
Date and Time: Friday, March 31, 9:00 am
Location: PSC 2136
Committee Chair: Prof. Luis Orozco
Committee:
Dr. Mohammad Hafezi
Dr. Steven Rolston
Dr. William Phillips
Dr. Jeremy Munday
Dr. Edo Waks
Abstract:
The study of atom-light interaction is the core of quantum optics and a central part of atomic physics. Systems composed of atoms interacting among each other through the electromagnetic field can be used from fundamental research to practical applications. Experimental realizations of these systems benefit from three distinct attributes: large atom-light coupling, trapping an control of atomic ensembles, and engineering and manipulation of the electromagnetic field. Optical waveguides provide a platform that achieves these three goals. In particular, optical nanofibers are an excellent candidate. They produce a high confinement of the electromagnetic field that improves atom-light coupling, guiding the field that mediates the interactions between atoms, while allowing trapping of the atoms close to it.
This thesis uses an optical nanofiber for quantum optics experiments, demonstrating its possibilities for enabling special atom-light interactions. We trap atoms near the optical nanofiber surface, and characterize the trap in a non-destructive manner. We show how the presence of the nanofiber modifies the fundamental atomic property of spontaneous emission, by altering the electromagnetic environment of the atom. Finally, we use the nanofiber to prepare collective states of atoms around it. These states can radiate faster or slower than a single atom (super and subradiance). The observation of subradiance of a few atoms, a rather elusive effect, evidences nanofibers as a strong candidate for future quantum optics experiments. Moreover, we show how the guided field mediates interaction between atoms hundreds of wavelengths apart, creating macroscopically delocalized collective states.
Thomas Rensink - January 20, 2017
Dissertation Title: Modeling strong-field laser-atom interactions with nonlocal potentials
Date and Time: Friday, January 20, 3:00 pm
Location: PSC 3150
Committee Chair: Prof. Thomas Antonsen
Committee:
Dr. Phillip Sprangle
Dr. Mohammad Hafezi
Dr. Rajarshi Roy
Dr. Ki-Yong Kim
Abstract:
Atom-field interactions in the ionization regime give rise to a wide range of physical phenomena, and their study continues to be an active field of research. Many process are may be explained by modeling the time-dependent Schrodinger equation; however, simulation in the strong field regime is computationally expensive and time-consuming. Here, a nonlocal model potential replaces the Coulomb potential in the time dependent Schrodinger equation, and examined for suitability of modeling strong field-atom dynamics while significantly reducing computation time.
Nonlocal potentials have been used to model many quantum mechanical systems, from multi-electron molecular configurations to semiconductor theory. Despite their relative success, nonlocal potentials are largely unexplored for modeling high field laser-gas interactions in the ionizing regime. This work examines theory and numerical results of a gaussian nonlocal model atom in intense, femtosecond laser pulses, with main findings: Nonlocal potentials are useful for obtaining the photoionization rate in the tunnel and multiphoton regimes, and qualitatively characterize electron wavefunction dynamics of irradiated atoms. The model is used to explore the two-color technique for producing Terahertz (THz) frequency radiation.
Richard Knoche - December 9, 2016
Dissertation Title: Signal Corrections and Calibrations in the LUX Dark Matter Detector
Date and Time: Friday, December 9, 3:00 pm
Location: PSC 3150
Committee Chair: Prof. Carter Hall
Committee:
Dr. Elizabeth Beise
Dr. Alberto Belloni
Dr. Luis Orozco
Dr. Massimo Ricotti
Abstract:
The Large Underground Xenon (LUX) Detector has recently finished a 332 day exposure and placed world-leading limits on the spin-independent WIMP-nucleon scattering cross-section. In this work, I discuss the basic techniques to produce signal corrections, energy scale calibrations, and recoil band calibrations in a dark matter detector. I discuss the nonuniform electric field that was present during LUX's 332 day exposure, and detail how such a field complicates these calibration techniques. Finally, I present novel techniques that account for all of the complications introduced by the nonuniform electric field, and allow a WIMP-nucleon cross scattering limit to be produced from the data.
Jeffery Demers - December 9, 2016
Dissertation Title: Stochastic Processes in Physics: Deterministic Origins and Control
Date and Time: Friday, December 9, 1:00 pm
Location: IPST 1116
Committee Chair: Prof. Christopher Jarzynski
Committee:
Dr. Dionisios Margetis
Dr. Edward Ott
Dr. Charles Levermore
Dr. Perinkulam Krishnaprasad
Abstract:
Stochastic processes are ubiquitous in the physical sciences and engineering. While often used to model imperfections and experimental uncertainties in the macroscopic world, stochastic processes can attain deeper physical significance when used to model the seemingly random and chaotic nature of the underlying microscopic world. Nowhere more prevalent is this notion than in the field of stochastic thermodynamics - a modern systematic framework used describe mesoscale systems in strongly fluctuating thermal environments which has revolutionized our understanding of, for example, molecular motors, DNA replication, far-from equilibrium systems, and the laws of macroscopic thermodynamics as they apply to the mesoscopic world. With progress, however, come further challenges and deeper questions, most notably in the thermodynamics of information processing and feedback control. Here it is becoming increasingly apparent that, due to divergences and subtleties of interpretation, the deterministic foundations of the stochastic processes themselves must be explored and understood.
This thesis presents a survey of stochastic processes in physical systems, the deterministic origins of their emergence, and the subtleties associated with controlling them. First, we study time-dependent billiards in the quivering limit - a limit where a billiard system is indistinguishable from a stochastic system, and where the simplified stochastic system allows us to view issues associated with deterministic time-dependent billiards in a new light and address some long-standing problems. Then, we embark on an exploration of the deterministic microscopic Hamiltonian foundations of non-equilibrium thermodynamics, and we find that important results from mesoscopic stochastic thermodynamics have simple microscopic origins which would not be apparent without the benefit of both the micro and meso perspectives. Finally, we study the problem of stabilizing a stochastic Brownian particle with feedback control, and we find that in order to avoid paradoxes involving the first law of thermodynamics, we need a model for the fine details of the thermal driving noise. The underlying theme of this thesis is the argument that the deterministic microscopic perspective and stochastic mesoscopic perspective are both important and useful, and when used together, we can more deeply and satisfyingly understand the physics occurring over either scale.
Xu Jiang - December 2, 2016
Dissertation Title: QUANTITATIVE STUDY OF LONGITUDINAL RELAXATION (T1) CONTRAST MECHANISMS IN BRAIN MRI
Date and Time: Friday, December 2, 3:00 pm
Location: PSC 3150
Committee Chair: Prof. Steven M. Anlage
Committee:
Dr. Wolfgang Losert
Dr. Rajarshi Roy
Dr. Peter van Gelderen
Dr. Jeff H. Duyn
Dr. Yang Tao
Abstract:
Longitudinal relaxation (T1) contrast in MRI is important for studying brain morphology and is widely used in clinical applications. Although MRI only detects signal from water hydrogen (1H) protons (WPs), T1 contrast is known to be influenced by other species of 1H protons, including those in macromolecules (MPs), such as lipids and proteins, through magnetization transfer (MT) between WPs and MPs. This complicates the use and quantification of T1 contrast to study the underlying tissue composition and the physiology of brain.
MT contributes to T1 contrast to an extent that is generally dependent on MT kinetics, as well as the concentration and NMR spectral properties of MPs. However, the MP spectral properties and MT kinetics are both difficult to measure directly, as the signal from MPs is generally invisible to MRI. Therefore, to investigate MT kinetics and further quantify T1 contrast, we first developed a reliable way to indirectly measure the MP fraction and its exchange rate with WPs, with minimal dependence on the spectral properties of MPs. For this purpose, we used brief, high power radiofrequency (RF) NMR excitation pulses to almost completely saturate the magnetization of MPs. Based on this, both MT kinetics and the contribution of MPs to T1 contrast through MT were studied. The thus obtained knowledge allowed us to subsequently infer the spectral properties of MPs by applying low power, frequency-selective off-resonance RF pulses and measuring the offset-frequency dependent effect of MPs on the WP MRI signal. A two pool exchange model was used in both cases to account for direct effects of the RF pulse on WP magnetization.
Consistent with earlier work using MRI and post-mortem analysis of brain tissue, our novel measurement approach found that MPs constitute an up to 27% fraction of the total 1H protons in human brain white matter, and their spectrum follows a super-Lorentzian line with a T2 of 9.6±0.6 μs and a resonance frequency centered at -2.58±0.05 ppm. T1 contrast was found to be dominated by MP fraction, but also negatively correlated with iron concentration in iron rich regions of brain.
Ismail Volkan Inlek - November 7, 2016
Dissertation Title: Multi-Species Trapped Atomic Ion Modules for Quantum Networks
Date and Time: Monday, November 7, 3:00 pm
Location: PSC 2136
Committee Chair: Prof. Christopher Monroe
Committee:
Dr. Alexey Gorshkov
Dr. Mohammad Hafezi
Dr. Alan Migdall
Dr. Edo Waks
Abstract:
Trapped atomic ions are among leading platforms in quantum information processing with their long coherence times and high fidelity quantum operations. Scaling up to larger numbers of qubits is a remaining major challenge. A network of trapped ion modules offers a promising solution by keeping a manageable number of qubits within a module while photonic interfaces connect separate modules together to increase the number of controlled memory qubits. Since the generation of entanglement between qubits in different modules is probabilistic, an excessive number of connection trials might result in decoherence on the memory qubits through absorption of stray photons. This crosstalk issue could be circumvented by introducing a different atomic species as photonic qubits. Compared to a system that only utilizes single species of atoms, there are also additional advantages in a multi-species apparatus where attractive features of each atom can be employed for certain tasks.
In this thesis, I present experimental demonstrations of necessary ingredients of a multi-species module for quantum networking. In these experiments, barium ions are intended to be used as photonic communication qubits with visible photon emission lines that are more convenient for current fiber optics and detector technologies while ytterbium ions are used for storing and processing quantum information where long coherence times available in hyperfine clock states make them suitable memory qubits. The key experiments include demonstration of atom-photon entanglement using the barium qubit and utilizing the Coulomb interaction between ytterbium and barium with spin-dependent forces for transfer of information from communication to memory qubits.
David Green - November 2, 2016
Dissertation Title: Measurement of the cosmic-ray proton spectrum from 54 GeV to 9.5 TeV with the Fermi Large Area Telescope
Date and Time: Wednesday, November 2, 9:00 am
Location: PSC 3150
Committee Chair: Prof. Kara Hoffman
Committee:
Dr. Elizabeth A. Hays
Dr. Jordan Goodman
Dr. Julie McEnery
Dr. M. Coleman Miller
Abstract:
Cosmic rays are a near-isotropic continuous flux of energetic particles from extraterrestrial origin. First discovered in 1912, cosmic rays span over 10 decades of energy and originate from Galactic and extragalactic sources. The Fermi Gamma-ray Space Telescope observations have recently confirmed supernova remnants (SNR) as a source class for Galactic cosmic-ray protons. Additionally, recent measurements made by AMS-02 of the cosmic-ray proton spectrum to 1.8 TeV in kinetic energy have shown an unexpected spectral break at 415 ± 117 GeV with a primary spectral index of −2.794±0.006 and a secondary spectral index of −2.702±0.047. The Fermi Large Area Telescope (LAT), one of two instruments on Fermi, has an ideal energy range for confirming a spectral break and extending a space-based cosmic-ray proton spectrum measurement to overlap with higher energy balloon-borne measurements.
In this thesis, I present the measurement of the cosmic-ray proton spectrum from 54 GeV to 9.5 TeV with the Fermi-LAT. Using the LAT's anti-coincidence detector and tracker as two independent measures of charge, I estimated a residual contamination in our proton data set of less that 5% primarily from cosmic-ray electrons and positrons. The LAT calorimeter provides an energy estimation of the electromagnetic fraction of an induced cosmic-ray proton shower. I use the charge and energy measurements to build instrument response functions, such as acceptance and response for the LAT, and measure cosmic-ray proton flux. I estimate the systematic uncertainties associated with the acceptance and the energy measurement. Using a broken power-law spectrum, I find a primary spectral index of −2.80 ± 0.03, a secondary spectral index of −2.60 ± 0.04, and an energy break of 467 ± 144 GeV. I discuss possible astrophysical and cosmic-ray physics interpretations for the observed spectral break.
Alex Jeffers - October 31, 2016
Dissertation Title: 3D MAGNETIC IMAGING USING SQUIDS AND SPIN-VALVE SENSORS
Date and Time: Monday, October 31, 1:00 pm
Location: PHY 0360 (CNAM Conference Room)
Committee Chair: Prof. Frederick Wellstood
Committee:
Dr. Richard Greene
Dr. Christopher Lobb
Dr. Antonio Orozco
Dr. Ichiro Takeuchi
Abstract:
We have used 2 µm by 4 µm thin-film Cu-Mn-Ir spin-valve sensors and high Tc YBa 2 Cu 3 O 7-x dc SQUIDs to take magnetic images of test samples with current paths that meander between 1 and 5 metallization layers separated by 1 µm to 10 µm vertically. I describe the development and performance of a 3D magnetic inverse for reconstructing current paths from a magnetic image. I present results from the inverse technique that demonstrates the reconstruction of the 3D current paths from magnetic images of samples. This technique not only maps active current paths in the sample but also extracts key parameters such as the layer-to-layer separations. When imaging with 2 µm by 4 µm spin valve sensors I typically applied currents of 1mA at 95 kHz and achieved system noise of about 200 nT for a 3 ms averaging time per pixel. This enabled a vertical resolution of 1 µm and a lateral resolution of 1 µm in the top layers and 3 µm in the bottom layer. For our roughly 30 µm square SQUID sensors, I typically applied currents of 1mA at 5.3 kHz, and achieved system noise of about 200 pT for a 3 ms averaging time per pixel. The higher sensitivity compared to the spin-valve sensor allowed me to resolve more deeply buried current paths.
Shantanu Debnath - September 28, 2016
Dissertation Title: A Programmable Five Qubit Quantum Computer Using Trapped Atomic Ions
Date and Time: Wednesday, September 28, 1:00 pm
Location: PSC 3150
Committee Chair: Prof. Christopher Monroe
Committee:
Dr. Steve Rolston
Dr. Eite Tiesinga
Dr. Trey Porto
Dr. Andrew Childs
Abstract:
Quantum computers can solve certain problems much more efficiently than conventional classical methods. Driven by this motivation, small scale demonstrations of quantum algorithms have been implemented across several physical platforms where each system have been adapted to run a limited number of instances of a single algorithm. Here, we present the experimental realization of a fully re-configurable quantum computer based on five trapped Yb+ ions that offers the flexibility to be programmed by the user in order to run any quantum algorithm. The computer follows an architecture where high level sequences of standard logic gates are decomposed into fundamental single- and two-qubit quantum gates that are native to the hardware consisting of a linear chain of trapped ions. Each qubit is resolved in space to implement optical addressing for the manipulation and measurement at the single qubit level. By using an array of Raman laser beams that individually address the qubits, a complete set of single-qubit and fully connected two-qubit gates can be implemented where the connectivity between qubits, being defined by the optical fields, can be reconfigured in the software thereby allowing arbitrary gate sequences to be executed. This makes the system a general purpose quantum processor where we implement several algorithms such as the Deutsch-Jozsa and Bernstein-Vazirani algorithm. We further implement a fully coherent five qubit quantum Fourier transform and apply it to solve the quantum period finding and the quantum phase estimation problem. This architecture is also shown to be scalable where the system size can be increased by simply hosting more ions inside a single processor where the number of experimental controls scale favorably.
Xunnong Xu - September 26, 2016
Dissertation Title: Quantum Optics with Optomechanical Systems in the Linear and Nonlinear Regime: With Applications in Force Sensing and Environmental Engineering
Date and Time: Monday, September 26, 2:00 PM
Location: CSS 2115
Committee Chair: Prof. Jay D. Sau
Committee:
Dr. Jacob Taylor
Dr. Mohammad Hafezi
Dr. Victor Yakovenko
Dr. Edo Waks
Abstract:
Optomechanical system, a hybrid system where mechanical and optical degrees of freedom are mutually coupled, is a new platform for studying quantum optics. Many interesting effects arise from linearized optomechanical interaction, such as the dynamical modification of the properties of the mechanical resonator and the modulation of the amplitude and phase of the light coming out of the cavity. When the single-photon optomechanical coupling is comparable to the optical and mechanical loss, it is also possible to study optomechanically induced nonlinear phenomena such as photon-blockade, Kerr nonlinearity, etc. In this talk, we study quantum optics with optomechanical systems both in the linear and nonlinear regime, with emphasis on its applications in force sensing and environmental engineering.
We first propose a mirror-in-the-middle system and show that when driving near optomechanical instability, the optomechanical interaction will generate squeezed states of the output light, which can be used to detect weak forces far below the standard quantum limit. Subsequently, we find that this particular driving scheme can also lead to enhanced optomechanical nonlinearity in a certain regime and by measuring the output field appropriately. We study the photon-blockade effect and discuss the conditions for maximum photon antibunching. We then focus on thermal noise reduction for mechanical resonators, by designing a system of two coupled resonators whose damping is primarily clamping loss. We show that optomechanical coupling to the clamping region leads to a reduction in the temperature and linewidth of the mechanical mode with increasing optical power. We also consider the Brillouin scattering induced optomechanical interaction in ring wave-guide resonators where phonon scattering via impurities is present. We find that it is possible to realize chiral transport behavior of phonons by modifying the phonon environment with optomechanics. We study a simple few-mode theory and it can explain experimental data well. Finally, we study a continuum multi-mode theory and calculate the phonon Green's function using a diagrammatic perturbative expansion, showing that a decrease in the phonon diffusion constant is possible with increasing optical pump power.
Kale Johnson - September 19, 2016
Dissertation Title: Experiments with Trapped Ions and Ultrafast Laser Pulses
Date and Time: Monday, September 19, 8:30 am
Location: PSC 2136
Committee Chair: Prof. Christopher Monroe
Committee:
Dr. William Phillips
Dr. James Williams
Dr. Alexey Gorshkov
Dr. Christopher Davis
Abstract:
Laser-cooled, trapped atomic ions have been one of the most compelling systems for the physical realization of a quantum computer. By applying qubit state dependent forces to the ions, their collective motional modes can be used as a bus to realize entangling quantum gates. Ultrafast state-dependent kicks can provide a universal set of quantum logic operations, in conjunction with ultrafast single qubit rotations, which uses only ultrafast laser pulses. This may present a clearer route to scaling a trapped ion processor. In addition to the role that spin-dependent kicks (SDKs) play in quantum computation, their utility in fundamental quantum mechanics research is also apparent. In this talk, we present a set of experiments which demonstrate some of the principle properties of SDKs including ion motion independence, high speed operations, and multi-qubit entanglement operations with speed that is not fundamentally limited by the trap oscillation frequency. These ultrafast atomic qubit manipulation tools demonstrate inherent advantages over conventional techniques, offering a fundamentally distinct regime of control and speed not previously achievable. Additionally, we use an aberration correction method to produce a diffraction limited spot from an ion in a high numerical aperture system.This allows for the highest position sensitivity of an isolated atom to date.
Wrick Sengupta - August 30, 2016
Dissertation Title: Sub-Alfvenic reduced equations in a tokamak
Date and Time: Tuesday, August 30, 1:00 pm
Location: AVW 3460, Conference Room
Committee Chair: Prof. Adil Hassam
Committee:
Dr. James Drake
Dr. Jason TenBarge
Dr. William Dorland
Dr. Thomas Antonsen
Abstract:
Magnetized fusion experiments generally perform under conditions where ideal magnetohydrodynamic (MHD) modes are stable. It is therefore desirable to develop a reduced formalism which would order out Alfvenic frequencies. This is challenging because the sub-Alvenic phenomena are sensitive to magnetic geometries. In this work an attempt is made to develop a formalism to study plasma phenomena on time scales much longer than the Alfvenic time scales. In Part I, a reduced set of MHD equations is derived, applicable to large aspect ratio tokamaks. A major advantage is that the resulting system is 2D in space, and the system incorporates self-consistent dynamic Shafranov shifts. A limitation is that the system is valid only in radial domains where the tokamak safety factor q, is close to a rational. Various limits of our equations, including axisymmetric and subsonic limits, are considered. In the tokamak core, the system is well suited as a model to study the sawtooth discharge in the presence of Mercier modes. In Part II, we begin a reduced description of sub Alfvenic phenomena for collisionless kinetic MHD. We study the role of trapped particles dynamics in a collisionless axisymmetric toroidal systems.
Joshua Wood - August 12, 2016
Dissertation Title: An All-Sky Search for Bursts of Very High Energy Gamma Rays with HAWC
Date and Time: Friday, August 12, 10:00 am
Location: PSC 1136
Committee Chair: Prof. Jordan Goodman
Committee:
Dr. Gregory Sullivan
Dr. Peter Shawhan
Dr. Julie McEnery
Dr. Christopher Reynolds
Abstract:
This dissertation reports on a new measurement for prompt gamma-ray burst (GRB) emission in the largely unexplored very-high energy (VHE) regime. GRBs are the most luminous events in the known universe. They consist of intense gamma-ray flashes coming from cosmological distances and are believed to be produced by two separate progenitor populations, collapsing massive stars and merging compact object binaries, that result in a newly formed accreting black hole. Yet many open questions remain about the physical processes behind their prompt gamma-ray emission despite the observational and theoretical advances made in the nearly 50 years since their discovery.
One major challenge has been to explain the large variety of observed GRB spectra using the most widely accepted emission model of a jetted, relativistic fireball interacting with itself and surrounding interstellar material to form internal and external shocks. While we are certain that synchrotron radiation and inverse-Compton interactions from electrons accelerated at shock fronts accounts for a large portion of the emission, it may be suppressed at the highest energies due to absorption in the emission region. Observations of the highest energy photons therefore provide direct measurements of the characteristics of the emission region.
These observations are extremely difficult to perform with satellite-based experiments due to their compact areas given the relatively low fluxes expected from VHE emission in GRB events. Thus far, only a single cutoff has been measured in the 8 year operation of the Fermi satellite. This highlights the need for ground-based observations of prompt GRB emission as the current generation of ground-based detectors have effective areas for ~100 GeV gamma-rays that are 100x the size of the Fermi satellite or larger. Yet ground-based observations of GRB events, which occur randomly throughout the sky, by Imaging Air Chereknov Telescopes have thus far been impeded by the small field-of-view and low duty cycle associated with this class of instruments.
A new ground-based wide-field extensive air shower array known as the High Altitude Water Cherenkov (HAWC) Observatory promises a new window to monitoring the ~100 GeV gamma-ray sky with the potential for detecting such a cutoff in GRBs. It represents a roughly 15 times sensitivity gain over the previous generation of wide-field gamma-ray air shower instruments and is able to detect the Crab nebula at high significance (>5 sigma) with each daily transit. Its wide field-of-view (~2 sr) and >95% uptime make it an ideal instrument for discovering gamma-ray burst (GRB) emission at ~100 GeV with an expectation for observing ~1 GRB per year based on existing measurements of GRB emission.
An all-sky, self-triggered search for VHE emission produced by GRBs with HAWC has been developed. We present the results of this search on three characteristic GRB emission timescales, 0.2 seconds, 1 second, and 10 seconds, in the first year of the fully-populated HAWC detector which is the most sensitive dataset to date. No significant detections were found, allowing us to place upper limits on the rate of GRBs containing appreciable emission in the ~100 GeV band. These constraints exclude previously unexamined parameter space.
Jacob Smith - August 11, 2016
Dissertation Title: Quantum Thermalization and Localization in a Trapped Ion Quantum Simulator
Date and Time: Thursday, August 11, 1:00 pm
Location: CSS 2115
Committee Chair: Prof. Christopher Monroe
Committee:
Dr. Jacob Taylor
Dr. Vladimir Manucharyan
Dr. Charles Clark
Dr. Edo Waks
Abstract:
When a system thermalizes it loses all memory of its initial conditions. Even within a closed quantum system, subsystems usually thermalize using the rest of the system as a heat bath. Exceptions to quantum thermalization have been observed, but typically require inherent symmetries or noninteracting particles in the presence of static disorder. The prediction of many-body localization (MBL), in which disordered quantum systems can fail to thermalize despite strong interactions and high excitation energy, was therefore surprising and has attracted considerable theoretical attention. We experimentally generate MBL states by applying an Ising Hamiltonian with long-range interactions and programmably random disorder to ten spins initialized far from equilibrium. Using experimental and numerical methods we observe the essential signatures of MBL: initial state memory retention, Poissonian distributed energy level spacings, and evidence of long-time entanglement growth. Our platform can be scaled to more spins, where detailed modeling of MBL becomes impossible.
Rachel Lee - July 27, 2016
Dissertation Title: Guided Migration and Collective Behavior: Cell Dynamics During Cancer Progression
Date and Time: Wednesday, July 27, 12:30 pm
Location: PSC 1136
Committee Chair: Prof. Wolfgang Losert
Committee:
Dr. Michelle Girvan
Dr. Carole A. Parent
Dr. Jose Helim Aranda-Espinoza
Dr. John Fourkas
Abstract:
Daniel Barker - July 25, 2016
Dissertation Title: Degenerate Gases of Strontium for Studies of Quantum Magnetism
Date and Time: Monday, July 25, 12:30 pm
Location: PSC 2136
Committee Chair: Prof. Steven Rolston
Committee:
Dr. Gretchen Campbell
Dr. Eite Tiesinga
Dr. Luis Orozco
Dr. John Fourkas
Abstract:
Aaron Lee - July 19, 2016
Dissertation Title: Engineering a Quantum Many-body Hamiltonian with Trapped Ions
Date and Time: Tuesday, July 19, 1:00 pm
Location: PSC 2136
Committee Chair: Prof. Christopher Monroe
Committee:
Dr. Mohammad Hafezi
Dr. Ian Spielman
Dr. James Williams
Dr. Andrew Childs
Abstract:
While fault-tolerant quantum computation might still be years away, analog quantum simulators offer a way to leverage current quantum technologies to study classically intractable quantum systems. Cutting edge quantum simulators such as those utilizing ultracold atoms are beginning to study physics which surpass what is classically tractable. As the system sizes of these quantum simulators increase, there are also concurrent gains in the complexity and types of Hamiltonians which can be simulated. In this work, I describe advances toward the realization of an adaptable, tunable quantum simulator capable of surpassing classical computation. We simulate long-ranged Ising and XY spin models which can have global arbitrary transverse and longitudinal fields in addition to individual transverse fields using a linear chain of up to 24 Yb+ 171 ions confined in a linear rf Paul trap. Each qubit is encoded in the ground state hyperfine levels of an ion. Spin-spin interactions are engineered by the application of spin-dependent forces from laser fields, coupling spin to motion. Each spin can be read independently using state-dependent fluorescence. The results here add yet more tools to an ever growing quantum simulation toolbox. One of many challenges has been the coherent manipulation of individual qubits. By using a surprisingly large fourth-order Stark shifts in a clock-state qubit, we demonstrate an ability to individually manipulate spins and apply independent Hamiltonian terms, greatly increasing the range of quantum simulations which can be implemented. As quantum systems grow beyond the capability of classical numerics, a constant question is how to verify a quantum simulation. Here, I present measurements which may provide useful metrics for large system sizes and demonstrate them in a system of up to 24 ions during a classically intractable simulation. The observed values are consistent with extremely large entangled states, as much as ~95% of the system entangled. Finally, we use many of these techniques in order to generate a spin Hamiltonian which fails to thermalize during experimental time scales due to a meta-stable state which is often called prethermal. The observed prethermal state is a new form of prethermalization which arises due to long-range interactions and open boundary conditions, even in the thermodynamic limit. This prethermalization is observed in a system of up to 22 spins. We expect that system sizes can be extended up to 30 spins with only minor upgrades to the current apparatus. These results emphasize that as the technology improves, the techniques and tools developed here can potentially be used to perform simulations which will surpass the capability of even the most sophisticated classical techniques, enabling the study of a whole new regime of quantum many-body physics.
Ryan Maunu - July 14, 2016
Dissertation Title: A Search for Muon Neutrinos in Coincidence with Gamma-Ray Bursts in the Southern Hemisphere Sky Using the IceCube Neutrino Observatory
Date and Time: Thursday, July 14, 3:00 pm
Location: PSC 2136
Committee Chair: Prof. Kara Hoffman
Committee:
Dr. Gregory Sullivan
Dr. Peter Shawhan
Dr. Julie McEnery
Dr. Richard Mushotzky
Abstract:
The origin of observed ultra-high energy cosmic rays (UHECRs, energies in excess of 10^19 eV) remains unknown, as extragalactic magnetic fields deflect these charged particles from their true origin. Interactions of these UHECRs at their source would invariably produce high energy neutrinos. As these neutrinos are chargeless and nearly massless, their propagation through the universe is unimpeded and their detection can be correlated with the origin of UHECRs.
Gamma-ray bursts (GRBs) are one of the few possible origins for UHECRs, observed as short, immensely bright outbursts of gamma-rays at cosmological distances. The energy density of GRBs in the universe is capable of explaining the measured UHECR flux, making them promising UHECR sources. Interactions between UHECRs and the prompt gamma-ray emission of a GRB would produce neutrinos that would be detected in coincidence with the GRB's gamma-ray emission.
The IceCube Neutrino Observatory can be used to search for these neutrinos in coincidence with GRBs, detecting neutrinos through the Cherenkov radiation emitted by secondary charged particles produced in neutrino interactions in the South Pole glacial ice. Restricting these searches to be in coincidence with GRB gamma-ray emission, analyses can be performed with very little atmospheric background. Previous searches have focused on detecting muon tracks from muon neutrino interactions from the Northern Hemisphere, where the Earth shields IceCube's primary background of atmospheric muons, or spherical cascade events from neutrinos of all flavors from the entire sky, with no compelling neutrino signal found.
Neutrino searches from GRBs with IceCube have been extended to a search for muon tracks in the Southern Hemisphere in coincidence with 664 GRBs over five years of IceCube data in this dissertation. Though this region of the sky contains IceCube's primary background of atmospheric muons, it is also where IceCube is most sensitive to neutrinos at the very highest energies as Earth absorption in the Northern Hemisphere becomes relevant. As previous neutrino searches have strongly constrained neutrino production in GRBs, a new per-GRB analysis is introduced for the first time to discover neutrinos in coincidence with possibly rare neutrino-bright GRBs. A stacked analysis is also performed to discover a weak neutrino signal distributed over many GRBs.
Results of this search are found to be consistent with atmospheric muon backgrounds. Combining this result with previously published muon neutrino track searches in the Northern Hemisphere, cascade event searches over the entire sky, and an extension of the Northern Hemisphere track search in three additional years of IceCube data that is consistent with atmospheric backgrounds, the most stringent limits yet can be placed on prompt neutrino production in GRBs, which increasingly disfavor GRBs as primary sources of UHECRs in current GRB models.
Anton de la Fuente - July 12, 2016
Dissertation Title: Non-Perturbative Methods in Quantum Field Theory and Quantum Gravity
Date and Time: Tuesday, July 12, 3:00 pm
Location: PSC 3150
Committee Chair: Prof. Raman Sundrum
Committee:
Dr. Rabindra Mohapatra
Dr. Thomas Cohen
Dr. Zackaria Chacko
Dr. Jonathan Rosenberg
Abstract:
A conformal field theory is so constrained by symmetry that all its correlation functions are completely determined by its set of 2- and 3-point functions, which are in turn determined by a discrete set of numbers, known as the conformal data of the CFT. In order for the conformal data to consistently determine all higher-point functions, they must satisfy a highly nontrivial set of consistency conditions, known as the conformal bootstrap equations. It has recently been discovered that numerical methods can efficiently identify large regions in the space of conformal data that are inconsistent with the bootstrap equations. Using these methods, we have ruled out a large region in the space of conformal data except for a tiny "island" around the 2D Ising CFT. This gives a different perspective on the concept of universality in the study of critical phenomena.
Matthew Adams - July 11, 2016
Dissertation Title: Magnetic and Acoustic Investigations of Turbulent Spherical Couette Flow
Date and Time: Monday, July 11, 4:00 pm
Location: ERF 1207
Committee Chair: Prof. Daniel Lathrop
Committee:
Dr. Vedran Lekic
Dr. James Duncan
Dr. James Drake
Dr. Thomas Antonsen
Abstract:
Generation of magnetic field via flows of conducting fluid, the so-called dynamo effect, is a widespread phenomenon in the universe. The experiments described here are aimed at helping to understand more about the interaction of turbulent flows with magnetic fields, and how this can lead to the spontaneous generation of large-scale magnetic fields. Specifically, the motivation for the experiments is the geodynamo of Earth's outer core, responsible for the Earth's magnetic field. The experiments are designed to be geometrically similar to Earth's core. They consist of an outer spherical shell and an inner sphere concentric with it; the working fluid lies in between them, analogous to the Earth's liquid outer core lying in between the mantle and the solid inner core. The two spheres share a rotation axis, and can be rotated differentially to drive a turbulent shear flow in the fluid. The two experiments are 60cm and 3m in diameter, respectively. For hydromagnetic investigations, liquid sodium is used as the working fluid, and an external magnetic field is applied to the experiment. Liquid sodium brings specific experimental challenges, including its opacity, making determination of the fluid velocities within the experimental volume difficult. We investigate the possibility of using frequency splittings of acoustic modes of the fluid volume to infer azimuthal velocities of the flow. This is analogous to techniques used in the field of helioseismology to determine flow patterns in the solar interior. We used gas (air or nitrogen) as the working fluid in the initial investigations due to the ease of instrumentation, and measured splittings for a variety of different inner and outer rotation rates, in the 60cm radius experiment. Working with our colleagues Vedran Lekic and Anthony Mautino, we use these splittings to put constraints on the azimuthal flow profile. Ongoing hydromagnetic investigations of the 3m experiment also give information about possible flow patterns, and preliminary investigations of acoustics in liquid sodium were carried out.
Christopher Verhaaren - June 8, 2016
Dissertation Title: Using the Higgs to Probe Naturalness
Date and Time: Wednesday, June 8, 3:00 pm
Location: PSC 3150
Committee Chair: Prof. Zackaria Chacko
Committee:
Dr. Raman Sundrum
Dr. Sarah Eno
Dr. Kaustubh Agashe
Dr. Alice Mignerey
Abstract:
The mass of the Higgs boson is extremely sensitive to quantum corrections from high mass states, making it 'unnaturally' light in the standard model. This 'hierarchy problem' can be solved by symmetries, which reveal themselves through new particles related, by the symmetry, to standard model fields. The Large Hadron Collider (LHC) can potentially discover these new particles. However, the particle most sensitive to the physics that affects the Higgs mass is the Higgs itself. We show that the Higgs is sometimes the best probe of the hierarchy problem at the LHC and future colliders.
If the top partners carry the color charge of the strong nuclear force, the production of Higgs pairs is affected. However, we show that on general grounds this effect is tightly correlated with single Higgs production, which the LHC is much more sensitive to. The current LHC data then implies that the most we can expect are modest enhancements in di-Higgs production. We verify this result in the context of a simplified supersymmetric model. If the top partners do not carry color charge, their direct production is greatly reduced. Nevertheless, we show that such scenarios can be revealed through Higgs dynamics. We find that many color neutral frameworks leave observable traces in Higgs couplings, which, in some cases, may be the only way to probe these theories at the LHC. Some realizations of the color neutral framework also lead to exotic decays of the Higgs with displaced vertices. These decays are so striking that the projected sensitivity for these searches, at hadron colliders, is comparable to that of searches for colored top partners. Taken together, these three case studies show the efficacy of the Higgs as a probe of naturalness.