## Sarthak Subhankar - June 12, 2024

Dissertation Titles: **Engineering optical lattices for ultracold atoms with spatial features and periodicity below the diffraction limit **

**&**

**Dual-species optical tweezer arrays for Rubidium and Ytterbium for Rydberg-interaction-mediated quantum simulations**

Date and Time:** Wednesday, June 12, 11:00 am**

Location:

**PSC 2136 and Zoom**

Dissertation Committee Chair:

**Prof. Steven Rolston (co-advisor)**

Committee:

Prof. Trey Porto (co-chair/co-advisor)

Prof. Ian Spielman

Prof. Nathan Schine

Prof. Ronald Walsworth

Abstract:

This dissertation is based on two independent projects.

Ultracold atoms trapped in optical lattices have proven to be a versatile, highly controllable, and pristine platform for studying quantum many-body physics. However, the characteristic single-particle energy scale in these systems is set by the recoil energy *E _{R} *=

*h*

^{2}

*/*(8

*md*

^{2}). Here,

*m*is the mass of the atom, and

*d*, the spatial period of the optical lattice, is limited by diffraction to

*λ/*2, where

*λ*is the wavelength of light used to create the optical lattice. Although the temperatures in these systems can be exceedingly low, the energy scales relevant for investigating many-body physics phenomena, such as superexchange or magnetic dipole interactions, can be lower yet. This limitation can be overcome by raising the relevant energy scales of the system (

*E*=

_{R}^{eff}*h*

^{2}

*/*(8

*md*

_{eff}

^{2})) by engineering optical lattices with spatial periodicities below the diffraction limit (

*d*

_{eff}

*< λ/*2).

To realize this subwavelength-spaced lattice, we first generated a Kronig-Penney-like optical lattice using the nonlinear optical response of three-level atoms in spatially varying dark states. This conservative Kronig-Penney-like optical potential has strongly subwavelength barriers that can be less than 10 nm (*≡ **λ/*50) wide and are spaced *λ/*2 apart, where *λ *is the wavelength of light used to generate the optical lattice. Using the same nonlinear optical response, we developed a microscopy technique that allowed the probability density of atoms in optical lattices to be measured with a subwavelength resolution of *λ/*50. We theoretically investigated the feasibility of stroboscopically pulsing spatially shifted 1D Kronig-Penney-like optical lattices to create lattices with subwavelength spacings. We applied the lattice pulsing techniques developed in this theoretical investigation to realize a *λ/*4-spaced optical lattice. We used the subwavelength resolution microscopy technique to confirm the existence of this *λ/*4-spaced optical lattice by measuring the probability density of the atoms in the ground band of the *λ/*4-spaced optical lattice.

Single neutral atoms trapped in optical tweezer arrays with Rydberg interaction-mediated entangling gate operations have recently emerged as a promising platform for quantum computation and quantum simulation. These systems were first realized using atoms of a single species, with alkali atoms being the first to be trapped in optical tweezers, followed by alkaline-earth (like) atoms, and magnetic lanthanides. Recently, dual-species (alkali-alkali) optical tweezer arrays were also realized. Dual-species Rydberg arrays are a promising candidate for large-scale quantum computation due to their capability for multi-qubit gate operations and crosstalk-free measurements for mid-circuit readouts. However, a dual-species optical tweezer array of an alkali atom and an alkaline-earth (like) atom, which combines the beneficial properties of both types of atoms, has yet to be realized. In this half of the thesis, I present details on the design and construction of an apparatus for dual-species Rydberg tweezer arrays of Rb (alkali) and Yb (alkaline-earth like).

## Yongchan Yoo - June 11, 2024

Dissertation Title:** Tensor Network Approaches in Non-equilibrium Quantum Many-body Dynamics**

Date and Time:** Tuesday, June 11, 2:00 pm**

Location:** PSC 3150 **

Dissertation Committee Chair:

**Brian Swingle, Jay Sau**

Committee:

Maissam Barkeshli

Michael Gullans

Christopher Jarzynski

Abstract:

Understanding the dynamics of non-equilibrium quantum systems has long been a challenge across a wide range of physical phenomena. Recent advancements in various quantum technologies have propelled theoretical investigations of non-equilibrium quantum many-body dynamics. A key part of the theory is the development of computational methods, leading to significant results and a promising direction for further development. Nevertheless, simulating these complex systems using classical algorithms remains notoriously difficult, primarily due to the challenge of identifying physically principled and efficient approximations capable of reducing their computational complexity.

This thesis presents a collection of theoretical and numerical studies focusing on non-equilibrium dynamics of quantum many-body systems, with a particular emphasis on the steady-state transport of conserved quantities in various one-dimensional spin systems. In these systems, transport phenomena can be effectively captured by utilizing boundary-driven open quantum setups. Our investigations focus on various aspects of the non-equilibrium dynamics induced by novel boundary-driven systems. The aim is to comprehend a range of steady-state phenomena and to push the boundaries of simulation capacity using current state-of-the-art technologies.

In the first part, we delve into the non-equilibrium steady-state (NESS) phases of an interacting Aubry-André-Harper model, where a quasiperiodic potential gives rise to intriguing emergent collective phenomena. The peculiar spin transport and quantum correlation structure observed suggest the existence of multiple dynamical phases lying between the extensively studied thermal and many-body-localized phases.

Moving on to the second part, we explore the impact of operator weight dissipation on the scaling behavior of transport in various spin models. Our findings indicate that the effect of dissipation on transport is influenced by its impact on the system's conserved quantities. When dissipation preserves these symmetries, it maintains the scaling of the system's transport properties; however, when it breaks these conserved quantities, it leads the system towards diffusive scaling of transport.

In the third part, we investigate energy transport within the non-integrable regime of the Z3 chiral clock model, employing Lindblad operators with adjustable size and temperature. Through scaling analysis, we extract the transport coefficients of the model at relatively high temperatures, both above its gapless and gapped low-temperature phases. Furthermore, we calculate the temperature dependence of the energy diffusion constant across various model parameters, including the regime where the model exhibits quantum critical behavior at low temperatures.

## Arinjoy De - June 11, 2024

Dissertation Title:** Exploring Quantum Many-body Systems in Programmable Trapped Ion Quantum Simulators**

Date and Time:** Tuesday, June 11, 11:00 am**

Location:** Zoom **

Dissertation Committee Chair:** Professor Christopher R. Monroe**

Committee:

Professor Alexey V. Gorshkov

Professor Zohreh Davoudi

Professor Norbert M. Linke

Professor Christopher Jarzynski

Abstract:

Quantum simulation is perhaps the most natural application of a quantum computer, where a precisely controllable quantum system is designed to emulate a more complex or less accessible quantum system. Significant research efforts over the last decade have advanced quantum technology to the point where it can achieve `quantum advantage' over classical computers, enabling the exploration of complex phenomena in condensed-matter physics, high-energy physics, atomic physics, quantum chemistry, and cosmology. While the realization of a universal fault-tolerant quantum computer remains a future goal, analog quantum simulators--featuring continuous unitary evolutions with 50 to 100 qubits--have been developed across several experimental platforms. A key challenge in this field is balancing the control of these systems with the need to scale them up to address more complex problems. Trapped-ion platforms, with exceptionally high levels of control enabled by laser-cooled and electromagnetically confined ions, and all-to-all entangling capabilities through precise control over their collective motional modes, have emerged as a strong candidate for quantum simulation and provide a promising avenue for scaling up.

In this dissertation, I present my research work, emphasizing both the scalability and controllability aspects of a ^{171}Yb^{+} based trapped-ion platform, with an underlying theme of analog quantum simulation. The initial part of my research involves utilizing a trapped ion apparatus operating within a cryogenic vacuum environment, suitable for scaling up to hundreds of ions. I address various challenges associated with this approach, particularly the impact of mechanical vibrations originating from the cryostat, which can induce phase errors during coherent operations. Subsequently, I detail the implementation of a scheme to generate phase-stable spin-spin interactions that are robust to vibration noise.

In the second part, using a trapped-ion quantum simulator operating at room temperature, we investigate the non-equilibrium dynamics of critical fluctuations following a quantum quench to the critical point. Employing systems with up to 50 spins, we show that the amplitude and timescale of post-quench fluctuations scale with system size, exhibiting distinct universal critical exponents. While a generic quench can lead to thermal critical behavior, a second quench from one critical state to another (i.e., double quench) results in unique critical behavior not seen in equilibrium. Our results highlight the potential of quantum simulators to explore universal scaling beyond the equilibrium paradigm.

In the final part of the thesis, we investigate an analog of the paradigmatic string-breaking phenomena of Quantum Chromodynamics using a quantum spin simulator. For this purpose, we employ an integrated trapped ion apparatus with 13 spins. This setup utilizes the individual controllability of laser beams to program a uniform spin-spin interaction profile across the chain, alongside 3D control of the local magnetic fields. We introduce two static probe charges, realized through local longitudinal magnetic fields, that create string tension. By implementing quantum quenches across the string-breaking point, we elucidate the patterns of dynamical string breaking, monitoring non-equilibrium charge evolution with spatio-temporal resolution. Furthermore, by initializing the charges away from the string boundary, we generate isolated charges and observe localization effects that arise from the interplay between confinement and lattice effects.

## Su-Kuan Chu - June 10, 2024

Dissertation Title:** Quantum Circuits for Chiral Topological Order**

Date and Time:** Monday, June 10, 1:00-3:00 pm**

Location:

**ATL 3100A and Zoom**

Dissertation Committee Chair:

**Mohammad Hafezi**

Committee:

Alexey Gorshkov

Maissam Barkeshli

Ian Spielman

Andrew Childs

Abstract:

Quantum simulation stands as an important application of quantum computing, offering insights into quantum many-body systems that are beyond the reach of classical computational methods. For many quantum simulation applications, accurate initial state preparation is typically the first step for subsequent computational processes. This dissertation specifically focuses on state preparation procedures for quantum states with chiral topological order, states that are notable for their robust edge modes and topological properties. These states are interesting due to their profound connections to the behavior of electrons and spins in real-world solid-state materials. In this dissertation, we explore a type of state preparation procedure known as entanglement renormalization circuits. This class of quantum circuits is characterized by its hierarchical arrangement of quantum gates (or quantum operations in general), which systematically organize and prepare the entanglement of the target states across various length scales.

In the first part of the dissertation, we present an entanglement renormalization circuit for a non-interacting chiral topological system. The non-interacting chiral topological system we consider is a continuous Chern insulator model, which can serve as a toy model for the integer quantum Hall effect. The entanglement renormalization circuit for the continuous Chern insulator is the continuous multiscale entanglement renormalization ansatz (cMERA). The cMERA circuit, adapted for field theories, provides a natural framework for quantum systems that are continuous in momentum space. One of the key findings of this work is that we find a scale-invariant cMERA for which the continuous Chern insulator is a fixed-point wavefunction, a property that is believed to be impossible within the traditional lattice multiscale entanglement renormalization ansatz (MERA) framework. Furthermore, we provide an experimental proposal to realize the cMERA circuit using cold atoms.

In the second part of this dissertation, we shift our focus to entanglement renormalization circuits for interacting chiral topologically ordered states. We analytically derive a class of exactly solvable chiral spin liquids, classified under Kitaev's 16-fold way. Some of these chiral spin liquids share universal properties with certain fractional quantum Hall states. We then construct entanglement renormalization circuits for these chiral spin liquids by combining traditional MERA circuits with time-dependent quasi-local Hamiltonians. We refer to this class of circuits as the multiscale entanglement renormalization ansatz with quasi-local evolution (MERAQLE).

## Michael Winer - June 10, 2024

Dissertation Title:** Spectral Statistics, Hydrodynamics, and Quantum Chaos**

Date and Time:** Monday, June 10, 10:00 am**

Location: **PSC 2136**

Dissertation Committee Chair:** Brian Swingle, Victor Galitski**

Committee:

Maissam Barkeshli

Jay Sau

Jonathan Rosenberg (Dean’s representative)

Abstract:

One of the central problems in many-body physics, both classical and quantum, is the relations between different notions of chaos. Ergodicity, mixing, operator growth, the eigenstate thermalization hypothesis, and spectral chaos are defined in terms of completely different objects in different contexts, don't necessarily co-occur, but still seem to be manifestations of closely related phenomena.

In this dissertation, we study the relation between two notions of chaos: thermalization and spectral chaos. We define a quantity called the Total Return Probability (TRP) which measures how a system forgets its initial state after time $T$, and show that it is closely connected to the Spectral Form Factor (SFF), a measure of chaos deriving from the energy level spectrum of a quantum system.

The main thrust of this work concerns hydrodynamic systems- systems where locality prevents charge or energy from spreading quickly, this putting a throttle on thermalization. We show that the detailed spacings of energy levels closely capture the dynamics of these locally conserved charges.

We also study spin glasses, a phase of matter where the obstacle to thermalization comes not from locality but from the presence of too many neighbors. Changing one region requires changing nearby regions which requires changing nearby-to-nearby regions, until only catastrophic realignments of the whole system can fully explore phase space. In spin glasses we find our clearest analytic link between thermalization and spectral statistics. We analytically calculate the spectral form factor in the limit of large system size and show it is equal to the TRP.

Finally, in the conclusion, we discuss some ideas for the future of both the SFF and the TRP.

## Amit Vikram Anand - June 7, 2024

Dissertation Title:** Constructing an ergodic theory of quantum information dynamics**

Date and Time:** Friday, June 7, 4:00 pm**

Location:** PSC 2136**

Dissertation Committee Chair:** Victor Galitski**

Committee:

Paulo Bedaque

Alexey Gorshkov

Christopher Jarzynski

Nicole Yunger Halpern

Abstract:

The ergodic theory of classical dynamical systems, originating in Boltzmann's ergodic hypothesis, provides an idealized description of how the flow of information within energy surfaces of a classical phase space justifies the use of equilibrium statistical mechanics. While it is an extremely successful mathematical theory that establishes rigorous foundations for classical chaos and thermalization, its basic assumptions do not directly generalize to quantum mechanics. Consequently, previous approaches to quantum ergodicity have generally been limited to model-specific studies of thermalization, or well-motivated but imprecise general conjectures.

In this Dissertation, we develop a general theoretical framework for understanding how the energy levels of a quantum system drive the flow of quantum information and constrain the applicability of statistical mechanics, guided by two prominent conjectures. The first of these, the Quantum Chaos Conjecture (QCC), aims to characterize which quantum systems may thermalize, by postulating a connection between ergodicity or chaos and the statistical properties of random matrices. The second, the Fast Scrambling Conjecture (FSC), is concerned with how fast a quantum system may thermalize, and posits a maximum speed of thermalization in a sufficiently “local” many-body system.

This Dissertation is divided into three main parts. In the first part, Theory of Quantum Dynamics and the Energy Spectrum, we tackle these conjectures for a general isolated quantum system through results that may be understood as new formulations of the energy-time uncertainty principle. For QCC, we introduce precise quantum dynamical concepts of ergodicity and quantitatively establish their connections to the statistics of energy levels, deriving random matrix statistics as a special consequence of these dynamical notions. We subsequently build on one of these connections to derive an energy-time uncertainty principle that accounts for the full structure of the spectrum, introducing sufficient sensitivity for many-body systems. The resulting quantum speed limit allows us to prove a precise formulation of FSC from the mathematical properties of the energy spectrum. In doing so, we generalize QCC beyond the statistics of random matrices alone, and FSC beyond requirements of locality, establishing precise versions of these statements for the most general quantum mechanical Hamiltonian.

In the second part, Quantum Systems Beyond the Chaotic-Integrable Dichotomy, we demonstrate the need for the aforementioned precise formulations of these conjectures, by showing that looser formulations can be readily violated in “maximally” chaotic or integrable systems that would be most expected to satisfy them. Finally, in the third part, Experimental Probes of Many-Body Quantum Ergodicity, we develop tools to experimentally probe the structure of energy levels associated with ergodic dynamics, and demonstrate a generalization of these probes to open systems in an experiment with trapped ions.

## Jacob Bringewatt - May 23, 2024

Dissertation Title:** Harnessing Quantum Systems for Sensing, Simulation, and Optimization**

Date and Time:** Thursday, May 23 at 2:00 pm**

Location:** ATL 3100A**

Dissertation Committee Chair:** Zohreh Davoudi**

Committee:

Alexey Gorshkov

Andrew Childs (Dean’s Representative)

Yi-Kai Liu

Ronald Walsworth

Abstract:

Quantum information science offers a remarkable promise: by thinking practically about how quantum systems can be put to work to solve computational and information processing tasks, we gain novel insights into the foundations of quantum theory and computer science. Or, conversely, by (re)considering the fundamental physical building blocks of computers and sensors, we enable new technologies, with major impacts for computational and experimental physics.

In this dissertation, we explore these ideas through the lens of three different types of quantum hardware, each with a particular application primarily in mind: (1) networks of quantum sensors for measuring global properties of local field(s); (2) analog quantum computers for solving combinatorial optimization problems; and (3) digital quantum computers for simulating lattice (gauge) theories.

For the setting of quantum sensor networks, we derive the fundamental performance limits for the sensing task of measuring global properties of local field(s) in a variety of physical settings (qubit sensors, Mach-Zehnder interferometers, quadrature displacements) and present explicit protocols that achieve these limits. In the process, we reveal the geometric structure of the fundamental bounds and the associated algebraic structure of the corresponding protocols. We also find limits on the resources (e.g. entanglement or number of control operations) required by such protocols.

For analog quantum computers, we focus on the possible origins of quantum advantage for solving combinatorial optimization problems with an emphasis on investigating the power of adiabatic quantum computation with so-called stoquastic Hamiltonians. Such Hamiltonians do not exhibit a sign problem when classically simulated via quantum Monte Carlo algorithms, suggesting deep connections between the sign problem, the locality of interactions, and the origins of quantum advantage. We explore these connections in detail.

Finally, for digital quantum computers, we consider the optimization of two tasks relevant for simulating lattice (gauge) theories. First, we investigate how to map fermionic systems to qubit systems in a hardware-aware manner that consequently enables an improved parallelization of Trotter-based time evolution algorithms on the qubitized Hamiltonian. Second, we investigate how to take advantage of known symmetries in lattice gauge theories to construct more efficient randomized measurement protocols for extracting purities and entanglement entropies from simulated states. We demonstrate how these protocols can be used to detect a phase transition between a trivial and a topologically ordered phase in $Z_2$ lattice gauge theory. Detecting this transition via these randomized methods would not otherwise be possible without relearning the symmetries.

## Hanyu Liu - May 22, 2024

Dissertation Title:** Understanding and Controlling Nanoscale Chirality: Materials Synthesis, Characterizations, Modeling and Applications**

Date and Time:** Wednesday, May 22, 10:30 am**

Location:** PHYS 2219 **

Dissertation Committee Chair:** Min Ouyang**

Committee:

Gregory Jenkins

Thomas Murphy

Taylor Woehl

Lourdes Salamanca-Riba (Dean’s Representative)

Abstract:

Chirality, the property of objects possessing non-superimposable mirror images, initially identified and explored in organic and biological molecules, has gained growing interests in the realm of inorganic nanomaterials due to its foreseeable applications in the fields such as Enantiochemistry, Nanophotonics, Spintronics.

In the first segment of this dissertation, we demonstrate a bottom-up synthetic strategy to induce chirality in plasmonic nanoparticles and hybrid plasmonic-semiconductor nanostructures. Subsequently, we detail a simplified analytical coupled-oscillators model to facilitate the understanding of plasmonic-chiral coupling and predict various chiroptical responses based on different coupling strengths, validated through finite element method simulations. Furthermore, advancements in characterizing nanoscale chirality with high spatial resolution at the single nanoparticle level are explored using a novel polarization-dependent optical atomic force microscopy technique, overcoming resolution limits in far field measurements. Finally, we demonstrate the employment of nanoscale chirality to induce spin polarization and enable unique nanoscale chiral Floquet engineering.

## Adam Ehrenberg - May 1, 2024

Dissertation Title:** Quantum Advantage in Sensing and Simulation**

Date and Time:** Wednesday, May 1, 11:00 am**

Location:** Atlantic 3332 and Zoom **

Dissertation Committee Chair:** Steven Rolston**

Committee:

Alexey Gorshkov (co-chair)

Andrew Childs (Dean’s Representative)

Nathan Schine

Michael Gullans

Abstract:

Since the discovery of Shor’s factoring algorithm, there has been a sustained interest in finding more such examples of *quantum advantage*, that is, tasks where a quantum device can outperform its classical counterpart. While the universal, programmable quantum computers that can run Shor’s algorithm represent one direction in which to search for quantum advantage, they are certainly not the only one. In this dissertation, we study the theory of quantum advantage along two alternative avenues: sensing and simulation.

Sensing refers to the task of measuring some unknown quantity with the smallest possible error. In many cases, when the sensing apparatus is a quantum device, this ultimate achievable precision, as well as specific protocols achieving this precision, are unknown. In this dissertation, we help close this gap for both qubit-based and photonic quantum sensors for the specific task of measuring a linear function of unknown parameters. We use quantum Fisher information and the quantum Cramér-Rao bound to derive limits on their ultimate precision. We further develop an algebraic framework that allows us to derive protocols saturating these bounds and also better understand the quantum resources, such as entanglement, that are necessary to implement these protocols. In doing so, we help clarify how quantum resources like entanglement lead to more precise sensing.

We also study a specific form of simulation called Gaussian Boson Sampling, which is a member of a broad framework of random sampling tasks that have become a popular method for demonstrating quantum advantage. While many of the theoretical underpinnings of these random sampling tasks, including Gaussian Boson Sampling, are well understood, many questions remain. Anticoncentration, which is strongly related to the moments of the output distribution, is a particularly relevant property when it comes to formally proving the existence of quantum advantage. We develop a graph-theoretic framework to calculate these moments, and we show that there is a transition in the strength of anticoncentration as a function of how many of the photonic modes were initially squeezed. We therefore demonstrate a transition in the evidence for the so-called approximate average-case hardness of Gaussian Boson Sampling, hence clarifying in what regimes we have the strongest evidence for quantum advantage.## Rodrigo Andrade e Silva - April 26, 2024

Dissertation Title:** Quantization of Causal Diamonds in (2+1)-Dimensional Gravity**

Date and Time:** Friday, April 26, 2:00 pm**

Location:** PSC 3150**

Dissertation Committee Chair: **Ted Jacobson**

Committee:

Maissam Barkeshli

Steven Carlip

William Goldman (Dean’s Representative)

Raman Sundrum

Abstract:

## Dhruvit Patel - April 12, 2024

Dissertation Title: **Machine Learning for Predicting Non-Stationary Dynamical Systems, and Global Subseasonal-To-Seasonal Weather Phenomenon**

Date and Time:** Friday, April 12, 3:00 PM**

Location:

**ERF1207 (IREAP Large Conference Room)**

Dissertation Committee Chair:

**Prof. Edward Ott**Committee:

Prof. Brian Hunt

Prof. Michelle Girvan

Prof. Rajarshi Roy

Prof. Thomas Antonsen

Abstract:

In this thesis, we are interested in modeling (1) the long-term statistical behavior of non-stationary dynamical systems, and (2) global weather patterns on the Subseasonal-to-Seasonal (S2S) time scale (2 weeks - 6 months). The first part of this thesis is primarily concerned with the situation where we have available to us measured time series data of the past states of the system of interest, and in some cases, a (perhaps inaccurate) scientific-knowledge-based model of the system. The central problem here lies in predicting a future behavior of the system that may be fundamentally different than that observed in the measured time series of its past. We develop machine learning -based methods for accomplishing this task and test it in various challenging scenarios (e.g., predicting future abrupt changes in dynamics mediated by bifurcations encountered that were not included in the training data). We also investigate the effects of dynamical noise in the training data on the predictability of such systems. For the second part of this thesis, we modify a machine learning -based global climate model to predict various weather phenomenon on the S2S time scale. The model is a hybrid between a purely data-driven machine learning component and an atmospheric general circulation model.

We begin by formulating a purely machine learning approach that utilizes the measured time series of the past states of the target non-stationary system, as well as knowledge of the time-dependence of the non-stationarity inducing time-dependent system parameter. We demonstrate that this method can enable the prediction of the future behavior of the non-stationary system even in situations where the future behavior is qualitatively and quantitatively different from the behavior in the training data. For situations where the training data contains dynamical noise, we develop a scheme to enable the trained machine learning model to predict trajectories which mimic the effects of dynamical noise on typical trajectories of the target system. We test our methods on the discrete time logistic map, the continuous time Lorenz system, and the spatiotemporal Kuramoto-Sivashinsky system, and for a variety of non-stationary scenarios.

Next, we study the ability of our approach to not only extrapolate to previously unseen dynamics, but also to regions previously unexplored by the training data of the target system's state space. We find that while machine learning models can exhibit some capabilities to extrapolate in state space, they fail quickly as the amount of extrapolation required increases (as expected of any purely data-driven extrapolation method). We explore ways in which such failures can be mitigated. For instance, we show that a hybrid model which combines machine learning with a knowledge-based component can provide substantial improvements in extrapolation. We test our methods on the Ikeda map, the Lorenz system, and the Kuramoto-Sivashinsky system, under challenging scenarios (e.g., predicting future hysteretic transitions in dynamical behavior).

Finally, we modify a machine learning -based hybrid global climate model to forecast global weather patterns on the S2S time scale. Predicting on this time scale is crucial for many domains (e.g., land, water, and energy management), yet it remains a difficult period to obtain useful predictions for. We demonstrate that our model has useful skill in predicting a number of phenomenon including global precipitation anomalies, the El Nino Southern Oscillation and its related teleconnections, and various equatorial waves.

## Edgar Perez- April 5, 2024

Dissertation Title: **High Performance Nanophotonic Cavities and Interconnects for Optical Parametric Oscillators and Quantum Emitters**

Date and Time:** Friday, April 5, 10:00 am**

Location:** PSC 2136**

Dissertation Committee Chair:** Mohammad Hafezi and Kartik Srinivasan**

Committee:

Yanne Chemo

Efrain Rodriguez

Edo Waks

Xiyuan Lu

Abstract:

Integrated photonic devices like photonic crystals, microring resonators, and quantum emitters produce useful states of light, like solitons or single photons, through carefully engineered light-matter interactions. However, practical devices demand advanced integration techniques to meet the needs of cutting-edge technologies. High performance nanophotonic cavities and interconnects present opportunities to solve outstanding issues in the integration of nanophotonic devices. In this dissertation I develop three core tools required for the comprehensive integration of quantum emitters: wavelength-flexible excitation sources with enough pump power to drive down stream systems, photonic interconnects to spatially link the excitation sources to emitters, and cavities that can Purcell enhance quantum emitters without sacrificing other performance metrics.

To create wavelength-flexible excitation sources, high performance χ(3) microring Optical Parametric Oscillation (OPO) is realized in silicon nitride. Microring OPOs are nonlinear frequency conversion devices that can extend the range of a high quality on-chip (or off-chip) laser source to new wavelengths. However, parasitic effects normally limit the output power and conversion efficiency of χ(3) microring OPOs. This issue is resolved by using a microring geometry with very strongly normal dispersion, that uses multiple spatial mode families to satisfy the phase and frequency matching conditions. Our OPO achieves world-class performance with a conversion efficiency of up to 29% and an on-chip output power of over 18 mW.

To create photonic interconnects, Direct Laser Writing (DLW) is used to fabricate 3-dimensional (3D) nanophotonic devices that can couple light into and out of photonic chips. In particular, polymer microlenses of 20 µm diameter are fabricated on the facet of photonic chips that increase the tolerance of the chips to misaligned input fibers by a factor of approximately 4. DLW is also used to fabricate Polymer Nanowires (PNWs) with diameters smaller than 1 µm that can directly couple photons from quantum emitters into Gaussian-like optical modes. Comparing the same quantum emitter system before and after the fabrication of a PNW, a (3±0.7)× increase in the fiber-coupled collection efficiency is measured in the system with the PNW.

To refine the design of quantum emitter cavities, a toy model is used to understand the underlying mechanisms that shape the emission profiles of Circular Bragg Gratings (CBGs). Insights from the toy model are used to guide the Bayesian optimization of high performance CBG cavities suitable for coupling to single mode fibers. I also demonstrate cavity designs with Qs on the order of 105 that can be used in future experiments in cavity quantum electrodynamics or nonlinear optics. Finally, I show that these cavities can be optimized for extraction to a cladded PNW while producing a Purcell enhancement factor of 100 with efficient extraction into the fundamental PNW mode.

The tools developed in this dissertation can be used to integrate individual quantum emitter systems or to build more complex systems, like quantum networks, that require the integration of multiple quantum emitters with multiple photonic devices.

## Shiyi Sheng - April 3, 2024

Dissertation Title: **Quantum Computing and Machine Learning Approaches to Many-Body Physics**

Date and Time:** Wednesday, April 3, 10:00 AM**

Location:** PSC 3150**

Dissertation Committee Chair:** Professor Paulo F. Bedaque**

Committee:

Professor Thomas Cohen

Professor Zackaria Chacko

Professor Manuel Franco Sevilla

Professor Mohammad Hafezi

Abstract:

Lattice field theory provides a framework for which to explore properties of quantum field theories non-perturbatively. However for certain lattice calculations, for example when considering real-time dynamics or fermionic systems at finite density, sign problems occur which render those calculations intractable. One approach to solving the sign problem is to avoid it altogether by instead considering a simulation of the field theory on a quantum computer. For bosonic field theories, a procedure of qubitizing the bosonic fields is a necessary first step. The infinite-dimensional Hilbert space of the bosonic fields must be properly truncated as to encode those fields on a finite-dimensional Hilbert space spanned by the qubits on the quantum computer.

This thesis first discusses various strategies of making such a truncation. Ideally, the truncation yields a discrete spin system that contains a critical point in the same universality class as the untruncated field theory. That way, the physics of the original field theory is reproduced in the continuum limit of the truncated theory without needing to take a second limit of removing the truncation. Simulations of different models arising from various truncation strategies of the (1+1)-dimensional O(3) nonlinear sigma model are performed and different qubitizations for SU(2) gauge fields are considered and proposed. Due to a lack of an efficient method for solving many-body systems in more than one dimension, numerical simulations of these SU(2) qubitizations are unavailable.

The second half of the thesis explores the use of machine learning techniques in providing effective ways to solve quantum many-body problems. Neural network structures, such as feed-forward networks and restricted Boltzmann machines are universal approximators for continuous and discrete functions respectively. Therefore, they can be used as flexible wavefunction ansatze. Gradient descent algorithms can be applied to variationally search the general functional space spanned by neural-network-based ansatze for ground states of interacting, many-body systems. An ansatz is constructed explicitly for a system of indistinguishable bosons in one dimension and tested by comparing numerical results with analytic solutions of several exactly-solvable models. An extension of these neural-network ansatze to systems of identical bosons and fermions and discrete spin systems in higher dimensions would allow for concrete simulations of systems ranging from nuclei and qubitization models.

## Hong Nhung Nguyen - April 3, 2024

Dissertation Title: **Quantum Simulation of High-Energy Physics with Trapped Ions**

Date and Time:** Wednesday, April 3, 1:00 pm**

Location:** PSC 2136 and Zoom **

Dissertation Committee Chair:** Professor Alicia Kollár**

Committee:

Professor Norbert Linke, Advisor and Co-Chair

Professor Zohreh Davoudi

Professor Ian Spielman

Professor Xiaodi Wu

Abstract:

Trapped ions stand out as a leading platform for quantum computing due to their long coherence times, high-fidelity quantum gates, and the ability to precisely control individual qubits, enabling scalable and precise quantum computations. This dissertation reports advances in quantum computing with trapped ions, focusing on robust and high-fidelity entanglement generation, logical qubit encoding, and applications in quantum simulations of high-energy physics.

In particular, we report the implementation of a novel pulse optimization scheme for

achieving high-fidelity entangling gates in our setup. The scheme enables a balanced trade-

off between robustness to experimental drift, laser power, and gate duration, without the

need for expensive optimization. We also demonstrate the implementation of the Shor code with different code distances on our trapped-ion quantum computer, highlighting the fault-tolerant preparation of a logical qubit with high fidelity and showcasing the potential for reliable quantum computing.

Finally, we detail an experimental quantum simulation of the Schwinger model, a quantum electrodynamics theory in 1+1 dimensions, using two, four, and six qubits, demonstrating non-perturbative effects such as pair creation over extended periods of time. We study the gate requirement for two formulations of the model using a quantum simulation algorithm, considering the trade-offs between Hamiltonian term ordering, the number of time steps, and experimental errors. We employ a symmetry-protection protocol with random unitaries and a symmetry based post-selection technique to minimize errors. This work emphasizes the importance of the integrated approach between theory, algorithms, and experiments for efficient simulation of complex physical systems like lattice gauge theories.

## Elijah Willox - April 2, 2024

Dissertation Title: **The Search for Coincident Gamma-Ray Emission From Fast Radio Bursts with the HAWC Observatory**

Date and Time:** Tuesday, April 2, 3:00 PM**

Location:** PSC 3150**

Dissertation Committee Chair:** Jordan Goodman**

Committee:

Andrew Smith

Gregory Sullivan

Brian Clark

M. Coleman Miller

Abstract:

In 2007 a new class of radio transients was discovered, coming from outside of our galaxy with high fluence emitted in the radio band on millisecond timescales, emitting within an order of magnitude of the power of a gamma-ray burst or supernova. These fast radio bursts (FRBs) have since become the target of many searches across radio observatories and multiwavelength follow-up campaigns, but their origin remains unknown. In order to understand more about these fascinating events, a complete multiwavelength understanding in necessary to get a complete picture of what is creating such powerful bursts. The High Altitude Water Cherenkov (HAWC) observatory is a very-high-energy gamma-ray detector covering the range of 100 GeV to 300 TeV that is well suited to the detection of transient phenomena due its high live-time and wide field of view, and in particular for a follow-up search on FRBs to determine possible very high energy gamma-ray coincidences. The search for gamma-ray signals from FRBs consists of two searches: first is a persistent source search to identify if FRB emission ever comes from TeV gamma-ray emitting galaxies, and a transient search centered on the reported burst time and location. The results of the FRB search within the HAWC data sets the most constraining limits on the widest population ever searched in the VHE band.

## Maya Amouzegar - March 29, 2024

Dissertation Title: **Photon-Mediated Interactions in Lattices of Coplanar Waveguide Resonators**

Date and Time:** Friday, March 29, 1:30 PM**

Location:** PSC 3150**

Dissertation Committee Chair:** Professor Alicia Kollár**

Committee:

Professor Mohammad Hafezi (Dean’s Representative)

Professor Steven Rolston

Professor Trey Porto

Professor Sarah Eno

Abstract:

Circuit quantum electrodynamics (circuit QED) has become one of the main platforms for quantum simulation and computation. One of its notable advantages is its ability to facilitate the study of new regimes of light-matter interactions. This is achieved due to the native strong coupling between superconducting qubits and microwave resonators, and the ability to lithographically define a large variety of resonant microwave structures, for example, photonic crystals. Such geometries allow the implementation of novel forms of photon-mediated qubit-qubit interaction, cross-Kerr qubit-mediated interactions, and studies of many-body physics. In this dissertation, I will show how coplanar waveguide (CPW) lattices can be used to create engineered photon-mediated interactions between superconducting qubits. I will discuss the design and fabrication of a quasi one-dimensional lattice of CPW resonators with unconventional bands, such as gapped and ungapped flat bands. I will then present experimental data characterizing photon-mediated interactions between tunable transmon qubits and qubit-mediated non-linear photon-photon interactions in the said lattice. Our results indicate the realization of unconventional photon-photon interactions and qubit-qubit interactions, therefore, demonstrating the utility of this platform for probing novel interactions between qubits and photons. In future design iterations, one can extend the study of these interactions to two-dimensional flat and hyperbolic lattices.

## Eli Mizrachi - March 29, 2024

Dissertation Title: **Studies of Ionization Backgrounds in Noble Liquid Detectors For Dark Matter Searches**

Date and Time:** Friday, March 29, 1:00 pm**

Location:** PSC 2136**

Dissertation Committee Chair:** Professor Carter Hall **

Committee:

Research Professor Anwar Bhatti

Assistant Professor Brian Clark

Professor Sarah Penniston-Dorland

Dr. Jinkge Xu

Abstract:

Dark matter is believed to make up almost 85% of the total mass of the universe, yet its identity remains unclear. Weakly Interacting Massive Particles (WIMPs) have historically been a favored dark matter candidate, and dual-phase noble liquid time projection chambers (TPCs) have set the strongest interaction limits to date on WIMPs with a mass greater than several GeV. However, because no definitive interactions have been observed, the parameter space for conventional WIMPs is highly constrained. This has sparked greater interest in new sub-GeV dark matter models. At this mass scale, dark matter interactions with xenon or argon target media may still produce detectable signals at or near the single electron limit. However, these signals are currently obscured by delayed ionization backgrounds (“electron-trains”) which persist for seconds after an ionization event occurs. Electron-trains have been observed in many different experiments and exhibit similar characteristics, but their cause is only partially understood.

This work examines the nature of electron-trains in various contexts, as well as possible strategies to mitigate them. First, a characterization of electron-trains in the LZ experiment is presented, including new evidence of a dependence on detector conditions. The characterization also informed the development of an electron-train veto for LZ’s first WIMP search, which set world-leading limits on the spin-independent and spin-dependent WIMP-nucleon cross-sections for medium and high-mass WIMPs.

Next, to complement the analysis of LZ data, hardware upgrades were performed in XeNu, a small xenon TPC at Lawrence Livermore National Lab. These included replacing plastics with low-outgassing metal and machinable ceramic components, as well as a replacement of XeNu’s photomultiplier tube array with silicon photomultipliers. The resulting reduction in the intensity of electron-trains and better position resolution from the respective upgrades will improve future studies of low energy interactions and phenomena. Concurrent with this work, XeNu was used to perform a nuclear recoil calibration and a search for the Migdal effect, the latter of which can substantially enhance an experiment's low-mass dark matter sensitivity.

Finally, the development of CoHerent Ionization Limits in Liquid Argon and Xenon (CHILLAX), is reported. CHILLAX is a new xenon-doped, dual-phase argon test stand that has the potential to have a higher sensitivity to low-mass dark matter interactions and lower backgrounds than current liquid xenon TPCs. The system is designed to handle high (percent level) xenon concentrations in liquid argon that can enable a range of ionization signal production and collection benefits. CHILLAX demonstrated the feasibility of such concepts by achieving a world record xenon doping concentration with stable operation.

## Robert Dalka - March 11, 2024

Dissertation Title: **Developing Methods and Theories for Modeling Student Leadership Development and Student’s Experiences of Academic Support**

Date and Time:** Monday, March 11, 1:00 pm**

Location:** Atlantic Building 3332 and Zoom**

Dissertation Committee Chair:** Dr. Chandra Turpen **

Committee:

Dr. Justyna P. Zwolak

Dr. Diana Sachmpazidi

Dr. Andrew Elby

Dr. Michelle Girvan

Dr. Christopher Palmer

Abstract:

This dissertation brings together two research strands that study: (a) the ways in which physics and STEM students contribute to growing capacity for institutional change within collaborative teams and (b) the support structures of graduate programs through an innovative methodology grounded in network science.

The first research strand is explored within two different team environments, one of a student-centric interinstitutional team and a second of departmental change teams. Across both of these contexts, I identify how by engaging in an interaction-based agency, students are able to jointly define their own roles and the projects they pursue. In comparing across these contexts, we identify how students navigate different leadership structures and how this can support or limit student contributions in these teams. A central contribution of this work is a model for cultivating capacity for change through shared leadership and relational agency. This model captures how capacity can be built in different domains tied to the activity systems of the work. I show how this model can help practitioners and facilitators better partner with students as well as how researchers can use this model to capture how students contribute to the work of the team.

The second research strand focuses on developing and applying an innovative methodology for network analysis of Likert-style surveys. This methodology generates a meaningful network based on survey item response similarity. I show how researchers can use modular analysis of the network to identify larger themes built from the connections of particular aspects. Additionally, I apply this methodology to provide a unique interpretation of responses to the Aspects of Student Experience Scale instrument for well-defined demographic groups to show how thematic clusters identified in the full data set re-emerge or change for different groups of respondents. These results are important for practitioners who seek to make targeted changes to their physics graduate programs in hopes of seeing particular benefits for particular groups.

This dissertation opens up lines for future work within both strands. The model for building capacity for change draws attention to the mediating processes that emerge on a team and in students’ interactions with others. This model can be developed further to include additional constructs and leadership structures, as well as explore the relevance to other academic contexts. For quantitative researchers, the network analysis for Likert-style surveys methodology is widely applicable and provides a new way to investigate the wide range of phenomena assessed by Likert-style surveys.

## Evan Dowling - March 5, 2024

Dissertation Title: **Feedback experiments using entangled photons for polarization control in future quantum networks**

Date and Time:** Tuesday, March 5, 9:00 AM**

Location:** ERF (IREAP) 1207**

Dissertation Committee Chair:** Professor Thomas E. Murphy **

Committee:

Professor Rajarshi Roy, Co-Advisor

Professor Julius Goldhar

Professor Yanne Chembo

Professor Wendell T. Hill

Abstract:

Control of the measurement frames that project on polarization entangled photons is an important experimental task for near term fiber-based quantum networks. Because of the changing birefringence in optical fiber arising from temperature fluctuations or external vibrations, the polarization projection direction at the end of a fiber channel is unpredictable and varies with time. This polarization drift can cause errors in quantum information protocols, like quantum key distribution, that rely on the alignment of measurement bases between users sharing a quantum state. Polarization control within fiber is typically accomplished using feedback measurements from classical power alignment signals, multiplexed in time or wavelength with the quantum signal that coexist in the same fiber. This thesis explores ways to use only measurements on the entangled photons for polarization control and perform entanglement measures without multiplexing alignment signals. This approach is experimentally less complex and can reduce the noise within the quantum channel arising from the alignment signals. In the first part of this dissertation, we study how to use distributed measurements on polarization entangled photons for polarization drift correction in a 7.1 km deployed fiber between the University of Maryland and the Laboratory of Telecommunication Sciences for two individuals sharing a near maximally entangled Bell state, $\hat \rho = |\Psi^-\rangle\langle\Psi^-|$. In the second part of the dissertation, we examine how to use feedback measurements to maximize the violation of a Bell's inequality used as an entanglement measure. Both polarization drift correction and the maximization of a Bell's inequality violation use iterative optimization algorithms to actuate upstream polarization controllers. In the Bell's inequality investigation, three numerical methods: Bayesian optimization, Nelder-Mead simplex optimization, and stochastic gradient descent are implemented and compared against each other. For complete polarization control and Bell's inequality violation experiments, we developed a polarization and time multiplexed detection system that reduced the number of photon detectors needed and mitigates the demand on the coincidence counting electronics for real-time feedback and control.

## Mingshu Zhao - March 1, 2024

Dissertation Title: **Turbulence and superfluidity in atomic Bose-Einstein condensates**

Date and Time:** Friday, March 1, 12:00 PM**

Location:** PSC 3150**

Dissertation Committee Chair:** Daniel Lathrop**

Committee:

Ian Spielman

Nathan Schine

Thomas Antonsen

Johan Larsson (Dean’s Rep)

Abstract:

In this dissertation, I investigate superfluid properties of atomic Bose-Einstein condensates (BECs) including laminar flow experiments that probe the superfluid density and turbulent flow experiments that explore connections to Kolmogorov theory. In this presentations, I focus on a novel technique to measure the BECs velocity field and apply it to turbulence. While turbulence in classical fluids has been extensively studied, there are many open questions in atomic superfluids, particularly regarding the existence of an inertial scale and the applicability of Kolmogorov scaling laws. I developed a velocimetry method, similar to particle image velocimetry using spinor impurities as tracers to measure the velocity field in a spatially resolved way. This enables the first observation of velocity structure functions (VSFs) in BECs, turbulent or otherwise. The observed VSFs reveal that superfluid turbulence in BECs conforms to Kolmogorov theory, including the so-called intermittency evident in both higher-order VSFs and the distribution of velocity increments.

## Henry Elder - January 17, 2024

Dissertation Title: **Nonlinear Propagation of Orbital Angular Momentum Light in Turbulence and Fiber**

Date and Time:** Wednesday, January 17, 3:00 PM**

Location:** AVW 2460**

Dissertation Committee Chair:** Phillip Sprangle**

Committee:

Thomas Murphy

Howard Milchberg

Thomas Antonsen

Wendel Hill (Dean’s Rep)

Abstract:

Light that carries orbital angular momentum (OAM), also referred to as optical vortices or twisted light, is characterized by a helical or twisted wavefront ∝exp[*imφ*]. In contrast to spin angular momentum (SAM), where photons are limited to two states, OAM allows for, in principle, an infinite set of spatially orthogonal states. OAM-carrying light has found applications ranging from quantum key distribution in free space and guided-wave communication systems, particle trapping and optical tweezers, nanoscopy, and remote sensing. Understanding how OAM light propagates through complex environments, and how to efficiently generate particular OAM states, is critical for any such application.

In the first part of this dissertation, we describe how OAM light propagates through a turbulent atmosphere. We build analytic models which describe (1) the OAM mode mixing caused by turbulence, (2) the evolution of short, high-power OAM pulses undergoing the effects of self-phase modulation (SPM) and group velocity dispersion (GVD), and (3) the evolution of high-power Gaussian pulses including SPM, GVD, and turbulence. The models are compared to experimental data and nonlinear, turbulent pulse propagation simulation programs, which we have made freely available. We also explore how self-focusing can minimize certain deleterious effects of turbulence for OAM light.

The second part of this dissertation considers nonlinear effects of OAM light propagating in azimuthally symmetric waveguides. Such waveguides have so-called spin-orbit (SO) modes, which are quantized based on their total angular momentum (TAM). We develop a generalized theory of four wave mixing-based parametric amplification of SO modes and show that these processes conserve TAM, but under certain circumstances can be taken to conserve SAM and OAM independently. Our theory is validated against a nonlinear multimode beam propagation simulation program which we developed and, again, have made freely available.

## Huan-Kuang Wu - January 8, 2024

Dissertation Title: **Aspects of Unconventional Transport and Quasiparticle in Condensed Matter Systems**

Date and Time:** Monday, January 8, 1:00pm**

Location:** ATL 4402**

Dissertation Committee Chair:** Jay Sau**

Committee:

Steven Anlage

Christopher Jarzynski

Johnpierre Paglione

Victor Yakovenko

Abstract:

The Boltzmann transport equation (BTE) is a successful framework for describing transport in condensed matter systems. The assumption of BTE is the localized energy excitations, or quasiparticles, and it is applicable to length scales above the mean free path of the quasiparticles. In this dissertation defense, I will cover two example systems where the assumptions of BTE fail. One is a new proposed system for Majorana zero mode whose topological phase is controlled by the phases of three s-wave superconductors. When the time reversal symmetry is restored, it can be tuned to a class DIII topological superconductor, which exhibits helical Majorana edge states. The second system I will talk about is the Josephson junction chain in the insulating regime, whose high-frequency plasmon remains coherent while the low energy excitations are Anderson-localized due to charge disorder.

## Sara Nabili - January 4, 2024

Dissertation Title: **Search for boosted semi-visible jets in all hadronic final states with the CMS experiment at CERN**

Date and Time:** Thursday, January 4, 10:30 am**

Location:** PSC 3150**

Dissertation Committee Chair:** Sarah Eno**

Committee:

Thomas Cohen

Christopher Palmer

Peter Shawhan

Mohamad Al-Sheikhly

Abstract:

This dissertation represents the search for the dark sector beyond the standard model using the Compact Muon Solenoid experiment simulated Monte Carlo data of the Large Hadron Collider at CERN. The search is focused on the strongly coupled Hidden Valley models that couple with the standard model via a leptophobic (fully hadronic) Z′ mediator. The final state of resonant production consists of one large jet composed of both visible and invisible particles. This search focuses on the lower mediator mass range (mZ′ ≤ 550 GeV) with the boosted topology that recoils against the initial state radiation (ISR) jet such that its decay products are contained within a single large-diameter “semi-visible” jet. The main parameters of our model are the mediator mass, the mass of the dark mesons, and the fraction of invisible stable particles. In the event of no discovery, the exclusion limits for mediator mass of 275 to 550 GeV are expected.