PhD Defenses - UMD Physics

John Silk - July 27, 2023

Scott Hancock - July 27, 2023

Dissertation Title: Spatiotemporal optical vortices
Date and Time: Thursday, July 27, 9:00 AM
Location: ERF 1207
Dissertation Committee Chair: Howard Milchberg

Committee:

Julius Goldhar
Phillip Sprangle
Eric Rosenthal
Timothy Koeth

Abstract:

Light beams carrying orbital angular momentum (OAM) have become a mainstay of optical science and technology. In these beams, well-known examples of which are the Laguerre-Gaussian and Bessel-Gaussian beams, the OAM vector points parallel or anti-parallel to propagation, and is associated with a phase winding in the plane transverse to the propagation direction, where integer is the winding order or the “topological charge”. Such beams can be monochromatic.

Recently, our group discovered a new type of OAM structure that naturally emerges from nonlinear self-focusing, which we dubbed the spatio-temporal optical vortex (STOV). Here, the phase winding exists in a spatiotemporal plane, with the OAM pointing transverse to propagation. In this dissertation, we extend the generation of STOV-carrying pulses to the linear regime, demonstrating their generation using a pulse shaper and measuring their free-space propagation using a new ultrafast single-shot space- and time-resolving diagnostic, TG-SSSI (transient-grating single-shot supercontinuum spectral interferometry). We then demonstrate that transverse OAM is a property of photons by experimentally confirming the conservation of transverse OAM in second harmonic generation. Because the field of STOVs is so new, a first principles theory for their transverse OAM was lacking. We developed such a theory for transverse OAM that predicts half integer values of OAM and the existence of a STOV polariton in dispersive media. The surprise of half-integer OAM values launched a debate in the OAM community, which has been resolved in favor of our theory by our most recent experiments. These explore how phase and amplitude perturbations can impart spatiotemporal torques to light. We find that transverse OAM can be imparted to light pulses only for (1) sufficiently fast transient phase perturbations or (2) energy removal from a pulse already possessing transverse OAM.

Jaron Shrock - July 26, 2023

Pradeep Niroula - July 25, 2023

Dissertation Title: Phase Transitions in Random Quantum Circuits
Date and Time: Tuesday, July 25, 10:00 am
Location: Atlantic 3100A and Zoom
Dissertation Committee Chair: Maissam Barkeshli

Committee:

Michael Gullans (co-chair, co-advisor)
Alexey Gorshkov (co-chair, co-advisor)
Christopher Monroe
Christopher Jarzynski (Dean’s Representative)

Abstract:

Random Circuits has emerged as an invaluable tool in the quantum computing toolkit. On one hand, the task of sampling outputs from a random circuit has established itself as a leading contender to experimentally demonstrate the intrinsic superiority of quantum computers using near-term, noisy platforms. On the other hand, random circuits have also been used to deduce far-reaching conclusions about the theoretical foundations of quantum information and communication.

One intriguing aspect of random circuits is exemplified by the measurement-induced entanglement phase transition that occurs in monitored quantum circuits, where unitary gates compete with projective measurements to determine the entanglement structure of the resulting quantum state. When the measurements are sparse, the circuit is unaffected and entanglement grows ballistically; when the measurements are too frequent, the unitary dynamics is arrested or frozen. The two phases are separated by a sharp-phase transition. In this work, we discuss an experiment probing such phases using a trapped-ion quantum computer.

While entanglement is an important resource in quantum communication, it does not capture the non-classicality needed to achieve universal quantum computation. A new family of measures, termed magic, is used to quantify the extent to which a quantum state is non-classical or can enable universal quantum computation. In this dissertation, we also discuss a newly uncovered phase transition in magic, using quantum circuits that implement a random stabilizer code. This `magic' phase transition is intimately related to the error-correction threshold. In this work, we present numerical and analytic characterizations of the magic transition.

Finally, we use a statistical mechanical map from random circuits acting on qubits to Ising models to suggest thresholds in error mitigation whenever the underlying noise of a quantum device is imperfectly characterized. We show the existence of an error-mitigation threshold for random circuits in dimensions larger than one.

Yizhou Huang - July 25, 2023

Dissertation Title: Improvements and Studies of Planar Transmon Qubits
Date and Time: Tuesday, July 25, 10:30am
Location: PSC 2136
Dissertation Committee Chair: Professor Christopher J. Lobb

Committee:

Dr. Benjamin S. Palmer
Professor Frederick C. Wellstood
Professor Steven Anlage
Professor Thomas Murphy, Dean’s Representative

Abstract:

This dissertation describes three main projects focused on characterizing and improving superconducting transmon qubits operating nominally at temperatures of 20 mK. The first topic I discuss is characterization of ground state fidelity of a passively cooled 3D transmon qubit using two techniques. The first technique was counting the number of false counts when performing single-shot read-out measurements of the weak resonator signal using a nearly quantum limited traveling wave parametric amplifier. Over about a million shots, only 772 counts were found with the system in the excited state, corresponding to a residual excited state population of Pe = 0.083%. The second technique used was performing detailed Rabi oscillations between the first and second excited state levels of the 3D transmon qubit. By fitting the measured data, the residual excited state population was shown to be Pe = 0.088% +- 0.018%. These state of the art low values for the infidelity of the ground state suggest that the effective temperature of the transmon qubit with fundamental transition frequency of 3.6 GHz is T < 25 mK.

The second topic I discuss is improvements in the coherence of our planar transmon qubits such that I was able to measure energy relaxation times and coherence times up to tens of microseconds. There were two main improvements that I achieved during the course of this research. First, I identified a significant loss mechanism in the way that the planar transmon qubits were packaged. This was identified by measuring the internal quality factors (Qi) for a series of thin-film Al quarter-wave resonators with fundamental resonant frequencies varying between 4.9 and 5.8 GHz. By utilizing resonators with different widths and gaps, I sampled different electromagnetic energy volumes for the resonators affecting Qi. When the backside of the sapphire substrate of the resonator device was adhered to a Cu package with a conducting silver glue, a monotonic decrease in the maximum achievable Qi was found as the electromagnetic sampling volume is increased. My simulations and modeling show that the observed dissipation is a result of induced currents in large surface resistance regions underneath the substrate. By placing a hole underneath the substrate and using superconducting material for the package, I was able to decrease the Ohmic losses and increase the maximum Qi by an order of magnitude for the larger size resonators. The second improvement I made to achieve improvements to our planar transmon qubits was developing a new fabrication process to improve the quality of the interface between the substrate and the superconducting shunting capacitor of the transmon. For this new process, the large features (> 1 um) of the thin superconducting film are subtractively defined by etching the film. The small Al/AlOx/Al junction is added in a second step by defining the junction in electron-beam lithography, an ion mill step, and standard double-angle evaporation.

Finally, the last studies I discuss are studies of the recovery time after injecting quasipacticles in several transmon qubits. For these studies, quasiparticles across the transmon junction were created by applying a large-amplitude microwave pulse resonant with the readout resonator. Immediately after generating the quasiparticles, a significant decrease of the energy relaxation time and dephasing time of the transmon qubit is measured. By performing T1 and T2 measurements over the course of the several milliseconds I tracked the recovery T1 and T2, which was used as a metric of the quasiparticle density at the junction. By fitting the recovery data with a numerical model involving differential equations, I extracted the quasiparticle trapping rates around 1 ms^-1 and recombination rates around 1 / 25 ns^-1 at the sites of a few transmon qubits. I measured transmons that were either galvanically connected or isolated, fabricated with superconducting aluminum (gap 200uV) or tantalum (gap 600uV) for the shunting capacitor. Finally, with a larger quasiparticle injection power and by measuring two transmons on the same chip, I observed a phenomenon that's consistent with “phonon-assisted quasiparticle poisoning”. I quantitatively modelled such data, in addition to discussing how this presents challenges to further improving the coherence times of transmon qubits.

Sohitri Ghosh - July 13, 2023

Dissertation Title: Quantum Enhanced Impulse Measurements and Their Applications in Searches for Dark Matter
Date and Time: Thursday, July 13, 10:15 am
Location: PSC 3150
Dissertation Committee Chair: Peter Shawhan

Committee:

Jacob M. Taylor, Co-Chair
Daniel Carney
Ronald L. Walsworth
Christopher J. Lobb

Abstract:

Optomechanical systems have enabled a variety of novel sensors that transduce an external force on a mechanical sensor to an optical signal which can be read out through different measurement techniques. Based on recent advances in these sensing technologies, we suggest that heavy dark matter candidates around the Planck mass range could be detected solely through their gravitational interaction. After our understanding of the possibility of direct gravitational detection of dark matter, a coalition of researchers came together to form a lasting collaboration — Windchime — to explore the potential implementation and impact of this approach. With this ultimate goal in mind, the Windchime collaboration is developing the necessary techniques, systems, and experimental apparatus using arrays of optomechanical sensors that operate in the regime of high-bandwidth force detection, i.e., impulse metrology. One of the key challenges in achieving more sensitive measurements is to mitigate the noise which arises due to the fundamental uncertainty principle while trying to precisely measure the variable of interest. Today’s state-of-the-art sensors can be limited by this added noise due to the act of measurement itself. One of the techniques to go beyond this limit include squeezing of the light used for measurement. The other technique is using backaction evading measurements by estimating quantum non-demolition operators — typically the momentum of a mechanical resonator well above its resonance frequency.

In our work, we have explored various backaction evading techniques based on this principle. In the first part of the thesis, we present the impulse metrology task in the context of gravitational detection of dark matter. In the next part, we present a practical way to achieve backaction evading measurements in the optical domain. In the subsequent part, we analyze the theoretical limits to noise reduction while combining different quantum enhanced readout techniques for these mechanical sensors. In the last part of the thesis, we explore the possibility of a microwave domain readout for maximizing the energy efficiency and noise reduction while having a scalable system for our dark matter detection purpose.

Saurav Das - July 7, 2023

Dissertation Title: Exploring Beyond Standard Model Physics with Cosmological and Terrestrial Probes
Date and Time: Friday, July 7, 12:00 pm
Location: PSC 1136
Dissertation Committee Chair: Prof. Anson Hook

Committee:

Prof. Kaustubh Agashe
Prof. Zackaria Chacko
Prof. Kara Hoffman
Prof. Richard Wentworth

Abstract:

The standard model for particle physics has been extremely successful as a description of nature. Despite this success, there remain many unsolved puzzles both observationally and theoretically. In this thesis we explore a few ideas in search of beyond the standard model physics, especially we focus on the Higgs mass, magnetic monopole and vector dark matter.

In the first part of the thesis, we show that the Goldstone bosons of discrete symmetry can be parametrically lighter than otherwise expected. While non-linear realizations of continuous symmetries feature derivative interactions and have no potential, non-linear realizations of discrete symmetries feature non-derivative interactions and have a highly suppressed potential. These Goldstone bosons of discrete symmetries have a non-zero potential, but the potential generated from quantum corrections is inherently very highly suppressed. We explore various discrete symmetries and to what extent the potential is suppressed for each of them.

In the second part, we showed that in the early universe, evaporating black holes heat up the surrounding plasma and create a temperature profile around the black hole that can be more important than the black hole itself. As an example, we demonstrate how the hot plasma surrounding evaporating black holes can efficiently produce monopoles via the Kibble-Zurek mechanism. In the case where black holes reheat the universe, reheat temperatures above $\sim 500$ GeV can already lead to monopoles overclosing the universe.

In the last part of the thesis, we showed that vector Dark Matter (VDM) that couples to lepton flavor ($L_e$, $L_{\mu}$, $L_{\tau}$) acts similarly to a chemical potential for the neutrino flavor eigenstates and modifies neutrino oscillations. VDM imparts unique signatures such as time and directional dependence with longer baselines giving better sensitivity. We use the non-observation of such a signal at Super-Kamiokande to rule out the existence of VDM in a region of parameter space several orders of magnitude beyond other constraints and show the projected reach of future experiments such as DUNE.

Arushi Bodas - July 5, 2023

Dissertation Title: Hunting inflationary fossils in primordial inhomogeneities
Date and Time: Wednesday, July 5, 2:00 pm
Location: PSC 3150 and Zoom
Dissertation Committee Chair: Professor Raman Sundrum (advisor)

Committee:

Professor Zackaria Chacko
Professor Anson Hook
Professor Peter Shawhan
Professor Richard Wentworth (Dean’s representative)

Abstract:

Cosmological observables such as the Cosmic Microwave Background (CMB) allow us to probe the early universe at extremely high energies far beyond the reach of any particle collider on Earth. In the inflationary paradigm, small perturbations in the energy distribution across space can be directly linked to the quantum fluctuations of an ''inflaton'' field that drives inflation. Using these perturbations, it is therefore possible to learn about physics at energies as high as 10¹³ GeV. In this thesis, we exploit this powerful connection and explore novel mechanisms to hunt for previously unexplored inflationary dynamics.

During inflation, particles with masses larger than the inflationary Hubble scale H_inf< 10¹³ GeV are produced due to an accelerating spacetime. If coupled to the inflaton, these particles could imprint distinct oscillatory features in higher moments of the density perturbations. In Chapter 2, we explore a mechanism that can boost spin-0 particle production by mining the kinetic energy of the inflaton. This leads to an enhancement of the oscillatory features, which can extend the observability of heavy particles by two orders of magnitude beyond H_inf.

In the final part of the thesis, spanning Chapters 3 and 4, we explore the viability of gravitational wave backgrounds (GWB) as data sources for new inflationary physics. It was recently shown that a GWB from a first-order phase transition must exhibit fluctuations, much like the CMB. Despite the close analogy, it is possible for fluctuations of the GWB to differ significantly in their detailed pattern from those of the CMB, which would imply the existence of a second light field during inflation in addition to the inflaton. Such a GWB could thus unlock a wealth of new information about multi-field inflation. In Chapter 3, we elaborate on this point with an example. We show that there may exist signals that cannot be extracted using standard cosmological probes such as the CMB and galaxy surveys, but can in principle be detected within GWB with upcoming and proposed gravitational wave experiments. Lastly, in Chapter 4, we focus on the detectability of GWB itself. We discuss a cosmological mechanism that can enhance the strength of the gravitational wave signal from phase transitions, thereby increasing their detection prospects significantly.

Ron Belyansky - June 26, 2023

Dissertation Title: Quantum Simulation and Dynamics with Synthetic Quantum Matter
Date and Time: Monday, June 26, 2:30 pm
Location: ATL 3100A
Dissertation Committee Chair: Professor Zohreh Davoudi

Committee:

Professor Alexey Gorshkov, Advisor, Co-chair
Professor Andrew Childs, Dean’s Representative
Professor Alicia Kollar
Professor Nathan Schine

Abstract:

Significant advancements in controlling and manipulating individual quantum degrees of freedom have paved the way for the development of programmable strongly-interacting quantum many-body systems. Quantum simulation emerges as one of the most promising applications of these systems, offering insights into complex natural phenomena that would otherwise be difficult to explore. Motivated by these advancements, this dissertation delves into several analog quantum simulation proposals spanning different fields, including high-energy and condensed matter physics, employing various synthetic quantum systems. A primary objective is the theoretical investigation of the dynamical phenomena that can be effectively studied using these simulation approaches.

The first part of the dissertation focuses on quantum simulation utilizing superconducting circuits. We demonstrate that this platform can natively realize several intriguing models including the massive Schwinger model (quantum electrodynamics (QED) in 1+1 dimensions) and various strongly interacting quantum impurity models. By studying high-energy scattering of quark and meson states within the Schwinger model, we reveal a wealth of rich phenomenology encompassing inelastic particle production, hadron disintegration as well as dynamical string formation and breaking. Furthermore, we demonstrate how the presence of a single impurity (artificial atom) can profoundly modify the properties of light-matter interactions in a waveguide, leading to anomalous transport of a single photon, strong photon decay, and the emergence of atom-photon bound states.

The second part of the dissertation focuses on quantum simulation with atomic, molecular,

and optical (AMO) systems. Leveraging the tunable and long-range interactions available in platforms such as cavity-QED and trapped ions, we explore exotic regimes of quantum information dynamics. On the one hand, we demonstrate that the combination of simple and uniform all-to-all interactions together with chaotic short-range interactions can induce fast scrambling, a central feature associated with quantum black holes. On the other hand, we investigate how short-range yet non-local Rydberg interactions can strongly suppress atom tunneling in an optical lattice, resulting in frozen dynamics and Hilbert-space fragmentation. Finally, we propose a method of sympathetic cooling of neutral atoms using state-insensitive Rydberg interactions, potentially enabling longer quantum simulations and computations with this platform.

Wen-Chen Lin - June 12, 2023

Dissertation Title: High Magnetic Field Studies of Unconventional Superconductor UTe2: Unveiling the Phase Diagram and Unconventional Hall Effect
Date and Time: Monday, June 12, 2:00 pm
Location: Toll 2219
Dissertation Committee Chair: Prof. Johnpierre Paglione

Committee:

Prof. Nicholas Butch
Prof. Steven Anlage
Prof. Min Ouyang
Prof. Efrain Rodriguez

Abstract:

The recent discovery of superconductivity in UTe2 has drawn strong attention owing to a fascinating list of properties -- including the absence of magnetic order at ambient pressure, extremely high upper critical fields – that have led to proposals of spin-triplet pairing, and multiple superconducting phases.

The first part of this dissertation unveils the comprehensive (H, T, P) phase diagram of UTe2 under magnetic fields reaching 41 T along the crystallographic b-axis, combined with applied pressures of up to 18 kbar. Utilizing magnetoresistance and tunnel diode oscillator measurements, we investigated the pressure-induced evolutions of multiple phases. Our findings indicate that the superconductivity in UTe2 is encapsulated within two orthogonal magnetic phases, specifically, a field-induced polarized phase and a pressure-induced magnetic ground state. This establishes the boundaries of triplet superconductivity in this system.

In the latter part of the thesis, we delve into an unconventional Hall effect in UTe2 that is induced by an in-plane magnetic field. An extensive study was conducted on the field and angle dependence of the Hall response under in-plane fields, confirming its field-odd symmetry nature, which aligns with the recently identified in-plane Hall effect. We examined the field dependence of the Hall response up to 33 T, and observed a remarkable enhancement, indicating a significant linkage to the well-identified metamagnetic transition. In addition, we will discuss the symmetry restrictions on the in-plane Hall response, and evidence of symmetry lowering in our system.

Joseph Murray - June 6, 2023

Tamoghna Barik - May 25, 2023

Dissertation Title: Tunneling properties in Fe-based superconductor, FeTe_.55Se_.45 (FTS)
Date and Time: Thursday, May 25, 10:30 AM
Location: ATL 4402
Dissertation Committee Chair: Prof. Jay D Sau

Committee:

Prof. Sankar Das Sarma
Prof. Victor Yakovenko
Prof. Maissam Barkeshli
Prof. Ichiro Takeuchi (Dean’s representative)

Abstract:

The Fe-based chalcogenide compound, FeTe_1-xSe_x (FTS), has recently attracted attention as a potential candidate for a readily available platform for topological superconducting (TSC) phase on its surface. The co-existence of the strong topological insulating (TI) phase and cylindrical Fermi sheets provide the two necessary ingredients for the TSC phase on its surface.

The strong TI phase in FTS depends on the relative Te/Se composition. The widely studied FTS sample, FeTe_.55Se_.45, interestingly, lies close to the topological-trivial phase boundary which is estimated to be close to x = .5 by a recent experiment. Additionally, a typical sample of FTS suffers from fluctuations in Te/Se composition at multiple length scales due to its alloy nature. Thus, in a topological FTS sample such fluctuations can give rise to regions where the phase is driven out of its topological nature. Such trivial domains would be scattered throughout the sample. After carefully exploring the effects of such topological domain disordered phase in an effective model of FTS, we conclude that its resultant signatures can be distinguished in a scanning tunneling spectroscopy (STS) measurement of the topological surface states (TSS) density.

Another possibility of an exotic phase arises on the TSC surface when a linear defect forms a Josephson junction (JJ) characterized by a phase shift of 𝜋 for the SC order parameter (OP). Such a 𝜋-JJ induces exotic helical Majorana modes - signatures of which has been observed by a recent tunneling experiment on the surface of FTS in the density of states (DOS) within the SC gap at a crystalline domain wall (DW) associated with half-unit-cell-shift (HUCS). Observation of such signatures naturally poses question about the origin of such 𝜋-JJ which is yet to be fully understood. We propose a mechanism that stabilizes a 𝜋-junction at the HUCS DW when the intrinsic superconducting pairing is of s_± character which is the case for bulk FTS. We argue that if the DW induced inter-pocket transmission between the Γ and M pockets in FTS is strong enough, the resultant enhancement of the two OPs of opposite sign stabilizes the 𝜋-junction.

Naren Manjunath - May 24, 2023

Dissertation Title: Classification and characterization of crystalline topological invariants in quantum many-body states
Date and Time: Wednesday, May 24, 10:30 am
Location: ATL 4402 (CMTC conference room)
Dissertation Committee Chair: Professor Maissam Barkeshli (advisor)

Committee:

Professor Jay Deep Sau
Professor Sankar Das Sarma
Professor Johnpierre Paglione
Professor Mohammad Hafezi (Dean’s Representative)

Abstract:

The theory of topological phases of matter, now a major direction in condensed matter physics, is framed around two complementary problems. The first is to mathematically classify topological states of matter given various symmetries, in terms of suitable topological invariants. The second is to characterize a given state by numerically or experimentally extracting the topological invariants associated with it. Although remarkable progress has been made for topological states with internal symmetries, major open questions remain in the case of many-body systems with crystalline symmetries, especially those with a nonzero Chern number and a magnetic field.

The first part of this thesis develops a theory of crystalline topological response in (2+1) dimensions based on the idea of crystalline gauge fields and their effective actions, which we derive using topological quantum field theory. We use this to obtain a complete classification of topological states with U(1) charge conservation, discrete magnetic translation, and point group rotation symmetries, finding several new invariants. We separately consider symmetry-enriched topological states of bosons, which admit anyonic excitations with fractional statistics, and invertible fermionic states, which do not.

The second part of this thesis focuses on numerically extracting these invariants from many-body invertible states. First, we study two quantized invariants, the discrete shift, and a charge polarization which is quantized by rotational symmetries. We show how to extract these invariants in multiple different ways, which include the fractional charge bound to lattice defects, as well as the angular and linear momentum of magnetic flux. Thereafter, we obtain a complete characterization of the theoretically predicted invariants, by studying the expectation value of the ground state under partial rotation operators.

An immediate application of these ideas is to fully characterize the celebrated Hofstadter model of spinless free fermions on a square lattice. Although the Chern number and filling were first computed in this model in 1982, our theory predicts seven nontrivial invariants, including four new invariants which depend on the crystalline symmetry. We compute these numerically and obtain several additional colorings of Hofstadter's butterfly.

Saurabh Vasant Kadam - May 23, 2023

Dissertation Title: Theoretical developments in lattice gauge theory for applications in double-beta decay processes and quantum simulation
Date and Time: Tuesday, May 23, 12:00 pm
Location: PSC 2136
Dissertation Committee Chair: Zohreh Davoudi

Committee:

Paulo Bedaque
Zackaria Chacko
Manuel Franco Sevilla
Konstantina Trivisa, Dean’s Representative

Abstract:

Nuclear processes have played, and continue to play, a crucial role in unraveling the fundamental laws of nature. They are governed by the interactions between hadrons, and in order to draw reliable conclusions from their observations, it is necessary to have accurate theoretical predictions of hadronic systems. The strong interactions between hadrons are described by quantum chromodynamics (QCD), a non-Abelian gauge theory with symmetry group SU(3). QCD predictions require non-perturbative methods for calculating observables, and as of now, lattice QCD (LQCD) is the only reliable and systematically improvable first-principles technique for obtaining quantitative results. LQCD numerically evaluates QCD by formulating it on a Euclidean space-time grid with a finite volume, and requires formal prescriptions to match numerical results with physical observables.

This thesis provides such prescriptions for a class of rare nuclear processes called double beta decays, using the finite volume effects in LQCD framework. Double beta decay can occur via two different modes: two-neutrino double beta decay or neutrinoless double beta decay. The former is a rare Standard Model transition that has been observed, while the latter is a hypothetical process whose observation can profoundly impact our understating of Particle Physics. The significance and challenges associated with accurately predicting decay rates for both modes are emphasized in this thesis, and matching relations are provided to obtain the decay rate in the two-nucleon sector. These relations map the hadronic decay amplitudes to quantities that are accessible via LQCD calculations, namely the nuclear matrix elements and two-nucleon energy spectra in a finite volume. Finally, the matching relations are employed to examine the impact of uncertainties in the future LQCD calculations. In particular, the precision of LQCD results that allow constraining the low energy constants that parameterize the hadronic amplitudes of two-nucleon double beta decays is determined.

Lattice QCD, albeit being a very successful framework, has several limitations when general finite-density and real-time quantities are concerned. Hamiltonian simulation of QCD is another non-perturbative method of solving QCD that, by its nature, does not suffer from those limitations. With the advent of novel computational tools, like tensor network methods and quantum simulation, Hamiltonian simulation of lattice gauge theories (LGTs) has become a reality. However, different Hamiltonian formulations of the same LGT can lead to different computational-resource requirements with their respective system sizes. Thus, a search for efficient formulations of Hamiltonian LGT is a necessary step towards employing this method to calculate a range of QCD observables. Toward that goal, a loop-string-hadron (LSH) formulation of an SU(3) LGT coupled to dynamical matter in 1+1 dimensions is developed in this thesis. Development of this framework is motivated by recent studies of the LSH formulation of an SU(2) LGT that is shown to be advantageous over other formulations, and can be extended to higher-dimensional theories and ultimately QCD.

En-Jui Kuo - May 16, 2023

Dissertation Title:  Quantum Simulation of Bosonic Systems and Applications of Machine Learning
Date and Time: Tuesday, May 16, 11:00 am
Location:   PSC 3150
Dissertation Committee Chair:   Mohammad Hafezi

Committee:

Alexey Gorshkov
Maissam Barkeshli
Victor Albert
Amin Gholampour

Abstract:

First, we introduce the notion of "generalized bosons," whose exchange statistics resemble those of bosons, but the local bosonic commutator $[a_i,a_i^{\dagger}]=1$ is replaced by an arbitrary single-mode operator that is diagonal in the generalized Fock basis. Examples of generalized bosons include boson pairs and spins. We consider the analogue of the boson sampling task for these particles and observe that its output probabilities are still given by permanents, so the results regarding the difficulty of sampling carry over directly. Finally, we propose implementations of generalized boson sampling in circuit-QED and ion-trap platforms.

In the rest of the thesis, we move on to different topics. Firstly, we incorporate machine learning techniques in quantum information. We use machine learning to classify rational two-dimensional conformal field theories (CFTs). We first use the energy spectra of these minimal models to train a supervised learning algorithm. In contrast to conventional methods that are typically qualitative and involve system size scaling, our method quantifies the similarity of the spectrum of a system at a fixed size to candidate CFTs. Such an approach allows us to correctly predict the nature and value of critical points of several strongly correlated spin models using only their energy spectra. Our results are also relevant for the ground-state entanglement Hamiltonian of certain topological phases of matter described by CFTs. Remarkably, we achieve high prediction accuracy by only using the lowest few Rényi entropies as the input. Finally, using autoencoders, an unsupervised learning algorithm, we find a hidden variable that has a direct correlation with the central charge and discuss prospects for using machine learning to investigate other conformal field theories, including higher-dimensional ones.

Next, we demonstrate how machine learning techniques, especially unsupervised learning algorithms, can be used to study Symmetry-Protected Topological (SPT) phases of matter. SPT phases are short-range entangled phases of matter with a non-local order parameter that are preserved under a local symmetry group. Here, we use an unsupervised learning algorithm, namely diffusion maps, to differentiate between symmetry-broken phases and topologically ordered phases and between non-trivial topological phases in different classes. Specifically, we show that phase transitions associated with these phases can be detected in various bosonic and fermionic models in one dimension, including the interacting SSH model, the AKLT model and its variants, and weakly interacting fermionic models. Our approach provides a cost-effective computational method for detecting topological phase transitions associated with SPT systems, which can also be applied to experimental data obtained from quantum simulators.

Batoul Banihashemi - May 16, 2023

Dissertation Title: Thermodynamics of quantum gravitational ensembles
Date and Time: Tuesday, May 16, 10:30 am
Location: PSC 1136
Dissertation Committee Chair: Theodore Jacobson

Committee:

Alessandra Buonanno
Christopher Jarzynski, Dean’s Representative
Raman Sundrum
Brian Swingle

Abstract:

The discovery of black hole thermodynamics and its extension to cosmological horizons demonstrated a deep connection between thermodynamics and the nature of spacetime as a quantum system. It is then of great importance to properly understand the statistical mechanics of gravitational systems with horizon from first principles. While employing a partition function and the gravitational “Euclidean path integral” produces the expected physical result for entropy, a number of fundamental questions about the underlying analysis persist. This dissertation sharpens and resolves some puzzles regarding statistical mechanics of gravitational ensembles and the gravitational path integral, with a focus on cosmological horizon and de Sitter space.

The main questions addressed in this dissertation are: how is the entropy of de Sitter space derived in absence of any boundary on which the statistical ensemble can be properly defined? What is the correct interpretation of the first law of de Sitter horizon, according to which the horizon area shrinks upon adding matter in de Sitter static patch? And finally, how can entropy of horizon-bounded systems be derived from a Hamiltonian approach and phase space path integral, without the trickery of the gravitational Euclidean path integral? The first two questions are answered by introducing an artificial boundary in the system on which a gravitational ensemble can be properly defined. Once the ensemble is defined, the semiclassical approximation of the statistical partition function yields the entropy, and the interpretation of the de Sitter first law becomes clear by identifying the system energy as the quasilocal energy defined on the boundary. To tackle the last question, the real-time phase space path integral is utilised in the Hamiltonian formulation which maintains connection to the Hilbert space of the system, and it is found that the horizon entropy is derived from a nearly Lorentzian stationary point.

Gong Cheng - May 16, 2023

Dissertation Title: Quantum information scrambling and protection in many-body systems
Date and Time: Tuesday, May 16, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Prof. Maissam Barkeshli

Committee:

Prof. Brian Swingle (Co-Chair)
Prof. Paulo Bedaque
Prof. Victor Albert
Prof. Xiaodi Wu (Dean’s representative)

Abstract:

This work focuses on two topics in quantum information theory: the scrambling of quantum information and the preservation of quantum information in large degrees of freedom. The primary object I investigate in this topic is the Out-of-time-order correlator (OTOC), which probes the dynamics of quantum information as it spreads from localized degrees of freedom to those that are distributed throughout the system. Meanwhile, the goal of studying quantum information protection is to construct a system that can preserve quantum information for a sufficiently long time when coupled to a finite-temperature environment.

The many-body systems analyzed in this work belong or are related to a class of strongly interacting systems known as holographic quantum models. The standard examples in this class are believed to be equivalent to gravitational theory in spacetime that is one-dimensional higher than that the quantum model lives in. Therefore, the results may also provide insights into topics in quantum gravity.

The first part of the thesis explores the scrambling dynamics close to a critical point where conformal symmetry emerges. The second case deals with the scrambling dynamics with conservation law constraints in holographic quantum field theory. The result also clarifies how conserved charges influence the dynamics in the bulk dual.

The third part of the thesis presents a matrix model with a large matrix rank N that belongs to the class of approximate quantum error correction codes. We investigate its thermal stability by coupling it to a thermal bath and demonstrate that it behaves as a self-correcting quantum memory at finite temperature. The coherent memory time scales polynomially with the system size N.

Troy Sewell - May 15, 2023

Dissertation Title:  Variational Algorithms and Resources for Near-Term Quantum Simulation
Date and Time:  Monday, May 15, 3:00 pm
Location:   PSC 3204
Dissertation Committee Chair: Jay Sau

Committee:

Stephen Jordan, Advisor / Co-chair
Brian Swingle
Maissam Barkeshli
Andrew Childs, Dean’s Representative

Abstract:

The difficulty of efficiently simulating quantum many-body systems was one of the first motivations for developing quantum computers and may also be one of the first applications to find practical computational advantage on real quantum hardware. With the relatively recent advent of publicly available quantum technologies, we have now entered the era of noisy intermediate-scale quantum (NISQ) computing. The capabilities of these technologies are evolving rapidly, and with them the computational affordances to which we have access. The time is now ripe to test the capabilities of existing quantum hardware and leverage them to the best of our ability toward achieving a practical quantum computational advantage.

In this dissertation, we address these aims by benchmarking a class of variational multi-scale quantum circuits for state preparation which are locally robust against noise. We demonstrate the advantages of these multi-scale circuits compared to a purely local circuit ansatz using the critical transverse-field Ising model to optimize circuit parameters, numerically test the noise resilience of observables and customized error mitigation techniques using a local gate noise model, and demonstrate the robustness of local subregion preparation on an existing ion-trap quantum computer. We then show how the ground state optimized circuit can be simply extended to an ansatz for thermal state preparation using the separation of energy scales afforded by the multi-scale circuit structure.

Additionally, we evaluate the quantum resources needed for some quantum simulation tasks. We estimate the gate complexity of the site-by-site algorithm for fault-tolerant ground state preparation, which we extend to the case of degenerate Hamiltonians. Using matrix product states we evaluate the non-stabilizer quantum resources needed to represent thermalized subregions of a chaotic Potts model, which we use to address the feasibility for classical simulation of quantum hydrodynamics.

Stefano Antonini - May 15, 2023

Dissertation Title: Holographic Cosmological Models and the AdS/CFT Correspondence
Date and Time:  Monday, May 15, 10:00 AM
Location:   PSC 3150
Dissertation Committee Chair:  Theodore Jacobson

Committee:

Zackaria Chacko
Christopher Jarzynski (Dean’s Representative)
Raman Sundrum
Brian Swingle (Academic Advisor)

Abstract:

The formulation of a quantum theory of gravity is a central open problem in theoretical physics. In recent years, the development of holography---and in particular the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence---provided a new framework to investigate quantum gravity and led to consistent advancement. However, how to describe cosmology within holography remains an unanswered question whose solution could determine whether holography is able to capture physics in our universe.

This dissertation describes a new proposal for embedding cosmological physics in the holographic paradigm. This is articulated in two different but related approaches, both involving time-symmetric Big Bang-Big Crunch cosmologies with negative cosmological constant Λ .

In the first approach, the cosmological universe is given by a four-dimensional end-of-the-world brane moving in a five-dimensional AdS black hole spacetime. The proposed holographic dual description is given by a boundary conformal field theory. Under specific conditions, gravity is localized on the brane and effectively four-dimensional: an observer living on the brane is unaware of the existence of the extra dimension. In this dissertation, I show how these conditions can be met in an AdS-Reissner-Nordström background while retaining a holographic dual description.

The second approach focuses on spatially flat Λ<0 cosmologies which analytically continue to Euclidean wormholes connecting two asymptotic AdS boundaries. The proposed dual theory is given by two holographic 3D CFTs coupled by non-holographic 4D degrees of freedom on a strip. A different analytic continuation of the Euclidean wormhole leads to a Lorentzian traversable wormhole. After discussing the general features of these holographic cosmologies, I describe how the traversable wormhole can be reconstructed from the dual theory and how the existence of the former constrains the latter. Finally, I show that these Λ<0 cosmologies can undergo phases of accelerated expansion and match observational data for the scale factor evolution.

Sanket Doshi - May 10, 2023

Dissertation Title:  Dark Matter and Neutrino Masses from a Composite Hidden Sector
Date and Time:  Wednesday, May 10, 1:30pm
Location:   PSC 2204
Dissertation Committee Chair: Dr. Zackaria Chacko

Committee:

Dr. Raman Sundrum
Dr. Kaustubh Agashe
Dr. Massimo Ricotti (Dean’s Representative)
Dr. Christopher Palmer

Abstract:

Despite the remarkable success of the Standard Model in explaining the interactions of the elementary particles, there is now indisputable evidence that it is incomplete. Although the Standard Model predicts that neutrinos are massless, over the last few decades experiments have established that the masses of the neutrinos, although very small, are nonvanishing. Furthermore, cosmological and astrophysical observations have established that about 80% of the matter in the universe is composed of some form of non-luminous dark matter, but there is no particle in the Standard Model that can play this role. Any explanation of the origin of neutrino masses and the nature of dark matter therefore requires physics beyond the SM. In this thesis, we present a novel class of models that can explain both the origin of neutrino masses and the observed abundance of dark matter.

In these models, the particle that constitutes dark matter arises as the composite state of a strongly coupled hidden sector that couples to the Standard Model through the neutrino portal. A discrete symmetry ensures that the dark matter particle is stable and does not decay. The hidden sector is in thermal equilibrium with the Standard Model in the early universe. The abundance of dark matter is set by its annihilation into final states containing neutrinos. The neutrino portal coupling also gives rise to small Majorana masses for the neutrinos through the inverse seesaw mechanism, with the role of the singlet neutrinos being played by composite states. The Standard Model neutrinos mix with the singlet neutrinos, and so the Standard Model neutrinos are partially composite in this framework. The dynamics of the hidden sector is taken to be approximately conformal in the ultraviolet, and a relevant deformation leads to breaking of the conformal symmetry in the infrared. Since the hidden sector is uncharged under the Standard Model gauge groups, the compositeness scale can lie below the weak scale, leading to striking experimental signals.

We employ the AdS/CFT correspondence to construct a holographic dual of this scenario. This takes the form of a Randall Sundrum model with two branes. Within this framework we explore the signals of these models at various current and future experiments. These include searches for lepton flavor violation in and conversion experiments, direct and indirect detection of dark matter and searches at colliders and beam dumps. We determine the current bounds on this scenario and show that future experiments can significantly expand the reach.

John Collini - April 28, 2023

Dissertation Title: HIGH PRESSURE DRIVEN EVOLUTION OF CHARGE AND
STRUCTURAL ORDER IN NEMATIC SUPERCONDUCTOR, Ba1−xSrxNi2As2
Date and Time: Friday, April 28, 1:30 pm
Location: Toll 0360. QMC Conference Room
Dissertation Committee Chair: Johnpierre Paglione

Committee:

Richard Green
Nicholas Butch
Ichiro Takeuchi
Jeffrey Lynn

Abstract:

The desire for a complete understanding of high temperature unconventional superconductivity
has illustrated a necessity for the study of non-magnetic sources of superconducting
enchantment, such as nematically driven fluctuations and charge order fluctuations. BaNi2As2,
a non-magnetic counterpart to high Tc superconductor BaFe2As2, shows a six-fold superconducting
enhancement neighboring charge and nematic orders, positioning it as an excellent candidate
for studying the interactions between charge order, nematic order, and enhanced superconductivity.
In this thesis, I will present X-ray diffraction and electrical transport evidence for the
development of complex charge order within the system as functions of isovalent chemical
substitution via Ba1−xSrxNi2As2 and applied hydrostatic pressure. The discovery of three
separate charge order will be detailed: an incommensurate charge order at Q = 0.28
and two commensurate charge orders at Q = 0.33 and Q = 0.5. X-ray diffraction
measurements of the Q = 0.28 charge order will be used to show a strong correlation
between it and a previously established nematic order for Ba1−xSrxNi2As2. Applied
pressure of BaNi2As2 up to 10.4 GPa will detail the development of all three charge orders and be used to show a correlation between pressure and isovalvent substitution in BaNi2As2. The critical substitution of 71% Sr and the critical pressure of 9 ± 0.5 GPa will be directly compared by X-ray measurements of their lattice parameters, revealing a collapsed tetragonal phase. This phase is shown to be analogous to the collapsed
tetragonal phase of the Fe-pnictide superconductors, likely playing a key role seen at
the critical substitution and pressure of BaNi2As2

Rahul Gaur - April 14, 2023

Dissertation Title: Optimization of high-beta fusion devices against linear instabilities
Date and Time: Friday, April 14, 9:45 AM
Location: ERF 1207 Large conference room, Energy Research Facility
Dissertation Committee Chair: Prof. William Dorland

Committee:

Dr. Ian Abel
Dr. Matt Landreman
Prof. Alexander Philippov
Prof. Thomas Antonsen (Dean’s representative)

Abstract:

Magnetic confinement fusion is a technique in which a strong magnetic field is used to contain a hot plasma, which enables nuclear fusion. Regarding overall energy efficiency, the two most promising magnetic confinement concepts are tokamaks (axisymmetric devices) and stellarators (nonaxisymmetric devices). The power P produced by a magnetically confined nuclear fusion device is proportional to , where V is the device's volume, β is the plasma pressure - magnetic pressure ratio, and B is the magnetic field strength. Most tokamaks and stellarators currently in operation are low-β devices. Broadly speaking, there are three ways to increase P, one may increase the operating β (as in Spherical Tokamaks), the magnetic field (as in the SPARC and EAST), or the volume of the device (as in ITER and W7X). The cost of these devices is proportional to V, making large devices extremely expensive. Similarly, a large magnetic field (>10T) requires superconducting magnets that, even after the recent innovations in HTS (High-Temperature Superconductors), are expensive to operate.

In addition, more research needs to be done on the effect of neutron flux on the lifecycle of HTS magnets. High-β devices are an attractive idea for producing fusion energy efficiently. However, a high β generally also implies a large gradient in plasma pressure that can be a source of numerous magnetohydrodynamic (MHD) and kinetic instabilities. If fusion devices could be optimized against such instabilities, high-β operation would become attractive compared to high-field or large-volume reactors. Therefore, this thesis examines the stability of the high-β tokamak and stellarator equilibrium equilibria.

We will start by investigating the stability of high-β tokamaks and stellarator equilibria

against the infinite-n, ideal ballooning mode, an important pressure-driven magnetohydrodynamic (MHD) instability. We optimize these equilibria for stability against the ideal-ballooning mode. To achieve this, we formulate a gradient-based adjoint technique and demonstrate its speed and effectiveness by stabilizing these equilibria. We also formulate how this technique can be easily extended to low-n ideal-MHD modes in both tokamaks and stellarators.

After demonstrating the process of stabilizing against ideal MHD unstable modes, we analyze the kinetic stability of high-β tokamak and stellarator equilibria by numerically solving the δf gyrokinetic model – a simplified version of the Vlasov-Maxwell model. These kinetic instabilities are driven by temperature and density gradients. To better understand these instabilities, we scan multiple values of the plasma β, temperature and density gradients, and plasma boundary shapes, discovering useful relationships between equilibrium-dependent quantities and growth rates of these instabilities.

From the microstability study, we find that electromagnetic effects are important for high-β devices. Hence, we use the numerical tools and knowledge derived from the previous chapters to write an optimization framework that searches for axisymmetric equilibria that are stable to electromagnetic kinetic instabilities. Due to the similarity between axisymmetry and quasisymmetry – a hidden symmetry in stellarators – we extend the microstability optimizer to search for high-β quasisymmetric stellarator equilibria.

Rui Zhang - April 11, 2023

Dissertation Title: Lattice Quantum Chromodynamics (QCD) Calculations of Parton Physics with Leading Power Accuracy in Large Momentum Expansion
Date and Time: Tuesday, April 11, 2:00 pm
Location: PSC 3150
Dissertation Committee Chair: Xiangdong Ji

Committee:

Thomas Cohen
Drew Baden
Zohreh Davoudi
Alice Migenery

Abstract:

Parton distributions describing how momenta of quarks and gluons are distributed inside a hadron moving at the speed of light, are important inputs to the standard model prediction of collider physics. Their non-perturbative nature does not allow traditional perturbative calculations from quantum field theory. Besides a global fitting to experimental data, it is also possible to calculate parton physics from lattice-QCD, a first principle non-perturbative Monte Carlo simulation of the strong interaction on super computers. Among the different strategies to extract information for parton physics, the large momentum effective theory, based on a large momentum expansion of non-local Euclidean correlation functions, allows us to directly calculate the momentum fraction x-dependence. When matching the lattice-QCD calculations to the physical parton physics in the large momentum expansion, there are unavoidable power corrections in the expansion parameter Lambda/Pz, which is determined by the QCD characteristic non-perturbative scale Lambda~300 MeV and the hadron momentum Pz, and the leading term appears as order Lambda/2xPz due to the linear divergent self-energy of Wilson line in the Euclidean lattice correlators. For current lattice calculations of Pz~2 to 3 GeV, this correction can be as large as 30% at small x, dominating the uncertainties in the calculation. Achieving power accuracy in linear order of Lambda/Pz is thus crucial for a high precision calculation of the parton physics from lattice.

In this dissertation, I summarize our work to eliminate this linear correction by consistently define the renormalization for the linear divergence in lattice data and the resummation scheme of the factorially growing infrared-renormalon series in the perturbative matching. We show that the method significantly reduces the linear uncertainty by a factor of 3 to 5 and improves the convergence of the perturbation theory. We then apply the strategy to the calculation of pion distribution amplitude, which describes the pion light-cone wave function in a quark-antiquark pair. The method improves the short distance behavior of the renormalized lattice correlations, which is now consistent with the prediction of the short distance operator product expansion, showing a reasonable value for the moments of pion distribution amplitude. We also develop the first strategy to resum the large logarithms in the matching to physical pion distribution amplitude when the momentum of quark or antiquark in the pion are small, that could improve the accuracy of the prediction near the endpoint regions. After extracting the x-dependence from the large momentum expansion in mid-x region, we complete the endpoint regions by fitting to the short distance correlations. Then a complete x-dependence is obtained for the pion distribution amplitude, which suggests a broad distribution compared to previous lattice calculations or model predictions.

Yuxun Guo - April 6, 2023

DinhDuy Vu - March 29, 2023

Kaustubh Wagh- March 27, 2023

Dissertation Title: REGULATING GENE EXPRESSION: THE ROLE OF TRANSCRIPTION FACTOR DYNAMICS
Date and Time: Monday, March 27, 12:30 PM
Location: Conference Room, 1116, Institute for Physical Science and Technology (IPST) Building
Dissertation Committee Chair: Prof. Arpita Upadhyaya

Committee:

Dr. Gordon L. Hager
Dr. Michelle Girvan
Dr. Wolfgang Losert
Dr. Helim Aranda-Espinoza

Abstract:

The genetic information encoded within our DNA is converted into RNA in a process called transcription. This is a tightly regulated process where multiple proteins act in concert to activate appropriate gene expression programs. Transcription factors (TFs) are key players in this process, with TF binding being the first step in the assembly of the transcriptional machinery. TFs are sequence-specific DNA binding proteins that bind specific motifs within chromatin. How TFs navigate the complex nuclear microenvironment to rapidly find their target sites remains poorly understood. Technological advances over the past 20 years have enabled us to follow single TF molecules within live cells as they interact with chromatin. Most TFs have been shown to exhibit power law distributed residence times, which arise from the broad distribution of binding affinities within the nucleus. This blurs the line between specific and non-specific binding and renders it impossible to distinguish between different binding modes based on residence times alone.

In this dissertation, I combine single molecule tracking (SMT) with statistical algorithms to identify two distinct low-mobility states for chromatin (histone H2B) and bound transcriptional regulators within the nucleus. On our timescales, the TF mobility states represent the mobility of the piece of chromatin that they are bound to. Ligand activation results in a dramatic increase in the proportion of steroid receptors in the lowest mobility state. Mutational analysis revealed that only chromatin interactions in the lowest mobility state require an intact DNA-binding domain as well as oligomerization domains. Importantly, these states are not spatially separated as previously believed but in fact, individual H2B and chromatin-bound TF molecules can dynamically switch between them. Single molecules presenting different mobilities exhibit different residence time distributions, suggesting that the mobility of a TF is intimately coupled with their temporal dynamics. This provides a way to identify different binding modes that cannot be detected by measuring residence times alone. Together, these results identify two unique and distinct low-mobility states of chromatin that appear to represent common pathways for transcription activation in mammalian cells.

Next, I demonstrate how SMT can complement genome wide assays to paint a complete picture of gene regulation by TFs using two case studies: corticosteroid signaling and endocrine therapy resistance in breast cancer. Finally, I conclude with a roadmap for future work on examining the role of mechanical cues within the cellular microenvironment (such as stiffness and topography) in regulating TF dynamics and gene expression.

Yipeng Sun- March 1, 2023

Zishuo Yang - February 20, 2023

PhD Defenses 2023

John Silk - July 27, 2023

Scott Hancock - July 27, 2023

Jaron Shrock - July 26, 2023

Pradeep Niroula - July 25, 2023

Yizhou Huang - July 25, 2023

Sohitri Ghosh - July 13, 2023

Saurav Das - July 7, 2023

Arushi Bodas - July 5, 2023

Ron Belyansky - June 26, 2023

Wen-Chen Lin - June 12, 2023

Joseph Murray - June 6, 2023

Tamoghna Barik - May 25, 2023

Naren Manjunath - May 24, 2023

Saurabh Vasant Kadam - May 23, 2023

En-Jui Kuo - May 16, 2023

Batoul Banihashemi - May 16, 2023

Gong Cheng - May 16, 2023

Troy Sewell - May 15, 2023

Stefano Antonini - May 15, 2023

Sanket Doshi - May 10, 2023

John Collini - April 28, 2023

Rahul Gaur - April 14, 2023

Rui Zhang - April 11, 2023

Yuxun Guo - April 6, 2023

DinhDuy Vu - March 29, 2023

Kaustubh Wagh- March 27, 2023

Yipeng Sun- March 1, 2023

Zishuo Yang - February 20, 2023

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