Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item INTEGRATION OF ATOMIC EMITTERS IN PHOTONIC PLATFORMS FOR CLASSICAL AND QUANTUM INFORMATION APPLICATIONS(2024) Zhao, Yuqi; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Integrated photonics provide a powerful toolbox for a wide range of classical and nonclassical applications. In addition to their scalability and significantly lower power consumption, integrated photonic structures enable new design knobs and functionalities that are inaccessible in their bulk counterparts.Solid-state atomic emitters, such as rare-earth ions (REIs) and quantum dots, serve as excellent sources for scalable quantum memories and exhibit strong nonlinear resonant absorption. Integrating atomic emitters with photonic devices enhances light-matter interactions, unlocking new opportunities for advanced optoelectronic systems in both classical and quantum regimes. This thesis tackles two main challenges utilizing the integration of photonic devices and atomic emitters: (1) developing scalable quantum network components, and (2) creating low-power nonlinear components for classical on-chip optical signal processing. Specifically, we focus on a platform of rare-earth ion doped thin-film lithium niobate (TFLN), leveraging the ions’ stable optical transitions with thin-film lithium niobate’s rich toolbox of high-performance photonics. We first demonstrate an integrated atomic frequency comb (AFC) memory in this platform, an essential component for quantum networks. This memory exhibits a broad storage bandwidth exceeding 100 MHz and optical storage time as long as 250 ns. As the first demonstrated integrated AFC memory, it features a significantly enhanced optical confinement compared to the previously demonstrated REI memories based on ion-diffused waveguides, leading to a three orders of magnitude reduction in optical power required for a coherent control. Next, we develop reconfigurable narrowband spectral filters using ring resonators in the REI:TFLN platform. These on-chip optical filters, with linewidths in the MHz and kHz range and extinction ratios of 13 dB – 20 dB, are crucial for reducing background noise in quantum frequency conversion. By spectral hole burning at 100 mK temperature in a critical-coupled resonance mode, we achieve bandpass filters with a linewidth of as narrow as 681 kHz. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental component for optical signal processing and optical memories on-a-chip. We also demonstrate picowatt-threshold power nonlinearity in TFLN, utilizing the strong resonant nonlinear absorption induced by three-level REIs and enhanced by TFLN ring resonators. This work presents three distinct nonlinear transmission functions by adjusting the ring’s coupling strength. The lifetime of the nonlinear transmission is measured to be ~3 ms, determined by the ion’s third-level lifetime. Finally, we propose a novel nonlinear device design based on a different material system and mechanism - an ultrathin optical limiter with low threshold intensity (0.45 kW/cm2), utilizing a 500 nm-thick GaAs zone plate embedded with InAs quantum dots. The optical limiting performance, enabled by the zone plate’s nonlinear focusing behavior, is investigated using FDTD simulations. We also explore the effects of the zone plate’s thickness and radius on its optical limiting performance.Item QUANTUM APPLICATIONS, PARALLEL OPERATIONS, AND NOISE CHARACTERIZATION ON A TRAPPED ION QUANTUM COMPUTER(2024) Zhu, Yingyue; Linke, Norbert M.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum computing holds vast potential for solving classically hard problems ranging from optimization to simulations critical in material science research and drug discovery. While large-scale fault-tolerant quantum computers capable of these tasks are yet to come, small and noisy prototypes have been demonstrated on several candidate platforms. Among these, trapped-ion qubits have been at the forefront of quantum computing hardware because of their long coherence times, high-fidelity quantum gates, and all-to-all connectivity. This dissertation investigates new methods for efficient quantum computing at the interface of quantum information theory and trapped-ion experiments, and advances both the control of physical trapped-ion hardware and the characterization of their decoherence processes. We present a number of proof-of-principle experiments for early quantum applications on a trapped-ion quantum computer (TIQC). First, we experimentally show that the results of the Quantum Approximate Optimization Algorithm (QAOA)---a method to solve graph combinatorial optimization problems by applying multiple rounds of variational circuits---improve with deeper circuits for multiple graph-theoretic problems on several arbitrary graphs. We also demonstrate a modified version of QAOA that allows sampling of all optimal solutions with predetermined weights. Additionally, we implement the real-time evolution of a one-dimensional scattering process and demonstrate a more efficient and accurate method to extract the phase shift, forming a tentative first step toward the goal of lattice quantum chromodynamics (QCD) simulation. Furthermore, we demonstrate two Bell-type nonlocal games that can be used to prove quantum computational advantage as well as offer a set of practical and scalable benchmarks for quantum computers in the pre-fault-tolerant regime. Our experimental results indicate that the performance of quantum strategies for the non-local games exceeds basic classical bounds, and is on the cusp of demonstrating quantum advantage against more complicated classical strategies. We propose and demonstrate a high-fidelity and resource-efficient scheme for driving simultaneous entangling gates on different sets of orthogonal motional modes of a trapped-ion chain. We show the advantage of parallel operation with a simple digital quantum simulation where parallel implementation improves the overall fidelity significantly. We test and improve the performance of an ancilla-assisted protocol for learning Pauli noise in Clifford gates on a TIQC. With N ancilla, Pauli noise in an N-qubit Clifford gate can be learned with a sample size linear to N. We also design and demonstrate a way to improve the protocol's performance by reducing ancilla noise in post-processing.Item Ultracold Gases in a Two-Frequency Breathing Lattice(2024) Dewan, Aftaab; Rolston, Steven L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Driven systems have been of particular interest in the field of ultracold atomic gases. Theprecise control and relative purity allows for construction of many novel Hamiltonians. One such system is the ‘breathing’ lattice, where both the frequency and amplitude is modulated in time, much like an accordion. We present the results of a phenomenological investigation of a proposed experiment, one where we apply a two-frequency breathing lattice to an atomic system. The results are surprising, as they indicate the possibility of a phase-dependent transition between nearest-neighbour and beyond nearest-neighbour interactions.Item EXPLORING QUANTUM MANY-BODY SYSTEMS IN PROGRAMMABLE TRAPPED ION QUANTUM SIMULATORS(2024) De, Arinjoy; Monroe, Christopher R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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 is foreseeable to achieve `quantum advantage' over classical computers, to enable 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 evolution of many-body Hamiltonians -- 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 the systems. In this dissertation, I present my research work, emphasizing both the scalability and controllability aspects of \ion based trapped-ion platforms, 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. We 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, we detail the implementation of a scheme to generate phase-stable spin-spin interactions that are robust to vibration noise. In the second part, we use a trapped-ion quantum simulator operating at room temperature, to 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 using a quantum spin simulator. We employ an integrated trapped-ion apparatus with $13$ spins that utilizes the individual controllability of laser beams to program a uniform spin-spin interaction profile across the chain, alongside 3-dimensional 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 monitor non-equilibrium charge evolution with spatio-temporal resolution that elucidates the dynamical string breaking. 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.Item How Non-Hermitian Superfluids are Special? Theory and Experiments(2024) Tao, Junheng; Spielman, Ian Bairstow; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultracold atoms emerge as a promising advanced platform for researching the principles of quantum mechanics. Its development of scientific understanding and technology enriches the toolbox for quantum simulations and quantum computations. In this dissertation work, we describe the methods we applied to build our new high-resolution 87Rb Bose-Einstein condensate (BEC) machine integrated with versatile quantum control and measurement tools. Then we describe the applications of these tools to the research of novel superfluidity and non-Hermitian physics. Superfluids and normal fluids were often studied in the context of Landau’s two-fluid model, where the normal fluid stemmed from thermally excited atoms in a superfluid background. But can there be normal fluids in the ground state of a pure BEC, at near zero temperature? Our work addressed the understanding of this scenario, and then measured the anisotropic superfluid density in a density-modulated BEC, where the result matched the prediction of the Leggett formula proposed for supersolids. We further considered and measured this BEC in rotation and found a non-classical moment of inertia that sometimes turns negative. We distinguished the roles of superfluid and normal fluid flows, and linked some features to the dipolar and spin-orbit coupled supersolids. As a second direction, we describe our capability to create non-Hermiticity with Raman lasers, digital-micromirror device (DMD), and microwave, and present our work in engineering the real space non-Hermitian skin effect with a spin-orbit coupled BEC. By use of a spin-dependent dissipative channel, we realized an imaginary gauge potential which led to nonreciprocal transport in the flat box trap. We studied the system dynamics by quenching the dissipation, and further prepared stationary edge states. We link our discoveries to a non-Hermitian topological class characterized by a quantized winding number. Finally, we discuss the exciting promises of using these tools to study many-body physics open quantum systems.Item Photon-Mediated Interactions in Lattices of Coplanar Waveguide Resonators(2024) Amouzegar, Maya; Kollár, Alicia; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item Analog-Digital Quantum Simulations with Trapped Ions(2023) Collins, Katherine Sky; Monroe, Christopher; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since its inception in the early 1920s, the theory of quantum mechanics has provided a framework to describe the physics of nature; or at least our interpretations about systems in nature. However, even though quantum theory works, the unsettling question of “why?’ still remains. The field of quantum information science and technology (QIST) has brought together a collection of disciplines forming a united multidisciplinary collaborative effort towards realizing a large-scale quantum processor as an attempt to understand quantum mechanics better. It has been established in the field that the most efficient architectural design of this quantum processor would be composed of numerous individual quantum computers, quantum simulators, quantum networks, quantum memories, and quantum sensors that are “wired” together creating just the hardware layer in the full stack of the machine. Realizing a module-based quantum processor on such a macroscopic scale is an ongoing and challenging endeavor in itself. However, existing noisy intermediate-scale quantum (NISQ) devices across all the quantum applications above are still worth building, running, and studying. NISQ quantum computers can still provide quantum advantages over classical computation for given algorithms, and quantum simulators can still probe complex many-body dynamics that remain improbable to consider even on the best supercomputer. One such system is the trapped-ion quantum simulator at the center of this dissertation. Using 171Yb+ ions, we expand our “analog” quantum simulation toolbox by incorporating “digital” quantum computing techniques in each of the three experiments presented in this work. In the first experiment, we perform a quantum approximate optimization algorithm (QAOA) to estimate the ground-state energy of a transverse-field antiferromagnetic Ising Hamiltonian with long-range interactions. For the second project, we develop and demonstrate dynamically decoupled (DD) quantum simulation sequences in which the coherence in observed dynamics evolving under the unitary operator of the target Hamiltonian is extended while the known noise is suppressed. Finally, in the third project, we implement an experimental protocol to measure the spectral form factor (SFF) and its generalization, the partial spectral form factor (PSFF), in both an ergodic many-body quantum system and in a many-body localized (MBL) model. As a result, a quantum simulator can be utilized to test universal random matrix theory (RMT) predictions, and simultaneously, probe subsystem eigenstate thermalization hypothesis (ETH) predictions of a quantum many-body system of interest.Item Development of the Cold Atom Vacuum Standard(2023) Scherschligt, Julia; Porto, Trey; Rolston, Steven; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We describe the inception, design, development, and initial results of The Cold Atom Vacuum Standard (CAVS). It has been known for many years that vacuum level limits the lifetime of a cold atom cloud; we invert this to create a vacuum pressure measurement tool based on the trapped cloud lifetime. The difference between a standard and a sensor is of great concern to metrologists: a primary standard defines a unit, and a sensor transduces it. To have a device capable of both functions is to have a calibration-free measurement tool, which is of interest to many stakeholders in academia, industry, and defense. We describe all aspects of construction of the CAVS, including a lengthy investigation of vacuum technology. We ultimately demonstrate that the device is traceable to pressure through the fundamental physics of collision cross sections, thereby elevating it to status as not just a sensor, but a standard.Item An effective Mexican-hat band for ultracold atoms in a time-modulated optical lattice(2022) Bracamontes, Carlos Alberto; Porto, James V.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ring-shaped energy bands, where a continuum of degenerate minima lie along a closed loop in momentum space, are of interest in ultracold fermionic and bosonic gases since the associated singularity in the density of states is expected to stabilize unconventional phases of matter. These moatlike dispersions are also linked to enhanced properties in solid-state materials. This thesis describes the realization and characterization of a Mexican-hat band generated with an amplitude modulated double-well optical lattice, where the effective static Hamiltonian giving rise to the moat band can be understood using a Floquet analysis. Since our experimental approach allowed for the coherent preparation of Bose condensed (BEC) clouds in this hybridized ring-shaped band, we also examined the stability of BEC dressed states in the presence of the moatlike dispersion, which we modeled using a linear stability analysis of the mean-field solutions to the driven Gross-Pitaevskii equation. Our observations are in fair agreement with the theoretical prediction that a single-momentum BEC at the minimum of a moatlike band (which is a competing bosonic ground state in several interesting phase diagrams associated with ring-shaped dispersions) lies at the edge of an instability region and should, hence, be unstable in any realistic scenario. Motivated by the necessity to understand and mitigate dissipative mechanisms that curtail the applicability of Floquet engineering in bosonic optical lattices, this thesis also discusses a framework to model drive-induced instabilities in condensates subject to time-modulated lattice potentials. A linear stability analysis, similar to the one employed to model the BEC stability in the presence of the effective moat band, leads to parametric instabilities. Unlike the moat-induced instability, which is inherent to the Floquet generated effective static band, these instabilities are coupled via the modulation, and depend on the details of the modulation parameters. The predictions of this model are contrasted with the results from an experimental investigation of the condensate depletion in shaken optical lattices, as a function of the modulation parameters, from which we assess the validity and limitations of the theory.Item Topology from Quantum Dynamics of Ultracold Atoms(2023) Reid, Graham Hair; Rolston, Steven L; Spielman, Ian B; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultracold atoms are a versatile platform for studying quantum physics in the lab. Usingcarefully chosen external fields, these systems can be engineered to obey a wide range of effective Hamiltonians, making them an ideal system for quantum simulation experiments studying exotic forms of matter. In this work, we describe experiments using 87Rb Bose–Einstein condensates (BECs) to study exotic topological matter based on out-of-equilibrium effects. The topological states are prepared through the quantum dynamics of the ultracold atom system subjected to a highly tunable lattice potential described by the bipartite Rice–Mele (RM) model, created by combining dressing from a radiofrequency (RF) magnetic field and laser fields driving Raman transitions. We describe a form of crystal momentum-resolved quantum state tomography, which functions by diabatically changing the lattice parameters, used to reconstruct the full pseudospin quantum state. This allows us to calculate topological invariants characterizing the system. We apply these techniques to study out-of-equilibrium states of our lattice system, described by various combinations of sublattice, time-reversal and particle-hole symmetry. Afterquenching between lattice configurations, we observe the resulting time-evolution and follow the Zak phase and winding number. Depending on the symmetry configuration, the Zak phase may evolve continuously. In contrast, the winding number may jump between integer values when sublattice symmetry is transiently present in the time-evolving state. We observe a scenario where the winding number changes by ±2, yielding values that are not present in the native RM Hamiltonian. Finally, we describe a modulation protocol in which the configuration of the bipartite latticeis periodically switched, resulting in the Floquet eigenstates of the system having pseudospin-momentum locked linear dispersion, analogous to massless particles described by the Dirac equation. We modulate our lattice configuration to experimentally realize the Floquet system and quantify the drift velocity associated with the bands at zero crystal momentum. The linear dispersion of Floquet bands derives from nontrivial topology defined over the micromotion of the system, which we measure using our pseudospin quantum state tomography, in very good agreement with theory.