## 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)_{2}Te_{3}. 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 ^{87}Rb 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_{ }*f*_{0} of 6.15 GHz, quality factors *Q* of 1.5 x 10^{5} to 6.5 x 10^{5} 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 10^{3}. 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 *f*_{0} 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 ^{87}Rb 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 *S*_{21} through the resonator. I measured shifts in *f*_{0} 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 (^{138}Ba^{+}) and a memory qubit (^{171}Yb^{+}). 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 ^{171}Yb^{+} 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

**Monday, March 22, 12:00 pm**

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

**The Role of Transcription Factor Dynamics in Gene Expression: Does Time Matter?**

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.