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.
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.