Physics Theses and Dissertations

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    (2023) Shrock, Jaron E; Milchberg, Howard M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Plasma based electron accelerators offer a promising path to overcoming the significant technological and economic challenges facing the evolution to higher energies by radiofrequency (RF) accelerator technology. In particular, laser-driven wakefield acceleration (LWFA) in plasmas can produce accelerating gradients 1000 times larger than linear RF accelerators, enabling the production of GeV-scale electron bunches in just a few centimeters of acceleration. Efficient LWFA of electrons to this energy scale requires the use of optical guiding to maintain drive laser intensity over much longer distances than the characteristic diffraction length of the pulse. In this dissertation, I will present the first successful implementations of optically generated plasma waveguides in multi-GeV laser wakefield acceleration. I will focus on three primary topics: (1) experimental considerations for generating and diagnosing meter-scale plasma waveguides and the wakefield acceleration process, (2) the experimental demonstration of electron bunches accelerated up to 5 GeV in an all-optical LWFA, and (3) development of a model of drive pulse evolution and electron injection in agreement with a broad range of our experimental results, including the demonstration of localized electron injection through modification of the waveguide properties.
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    (2023) Silk, John James; Hall, Carter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Searches for Weakly Interacting Massive Particles (WIMPs) carried out by liquid xenon time projection chambers (TPCs) require a careful accounting of all background sources. Because WIMPs are a leading dark matter candidate, their possible existence is of great interest to particle physicists, astrophysicists, and cosmologists. The LUX-ZEPLIN (LZ) detector has completed an initial science run finding no evidence for WIMP scattering events. The data excludes scattering cross sections above 6.5x10$^{-48}$ cm$^{2}$ for a WIMP mass of 30 GeV/c$^{2}$. Background contributions from the beta decay of dispersed \isotope[85]{Kr} were reduced prior to the initial science run using charcoal chromatography to remove trace krypton. Over 10 tonnes of xenon were processed, and a custom mass spectrometry system measured a final mass averaged krypton concentration of 123 $\pm$ 22 parts-per-quadrillion (ppq) $\frac{gram}{gram}$ $\frac{ \isotope[nat]{Kr} }{\isotope[nat]{Xe}}$. A delayed coincidence $\beta$ - $\gamma$ search was also conducted to identify rare decays from \isotope[85]{Kr} in the LZ WIMP search data. The 11.0 $\pm$ 4.0 identified events are equivalent to a concentration of 183 $\pm$ 67 ppq. The total background contribution from \isotope[85]{Kr} to the WIMP search region of interest is 30 $ \pm $ 11 electron recoil events.
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    Spatiotemporal Optical Vortices
    (2023) Hancock, Scott; Milchberg, Howard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    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 (LG_pm ) and Bessel-Gaussian (BG_m ) beams, the OAM vector points parallel or anti-parallel to propagation, and is associated with a phase winding 2πm in the plane transverse to the propagation direction, where integer m 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 4f 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.
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    (2023) Huang, Yizhou; Lobb, Christopher J; Palmer, Benjamin S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation describes three main projects focused on characterizing and improving superconducting transmon qubits operating nominally at temperature 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 P_e = 0.083%. The second technique used was performing Rabi oscillations between the first and second excited state levels of the transmon qubit. By fitting the data, the residual excited state population was shown to be P_e = 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 was 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 this research. First, I identified a significant loss mechanism associated with the way that the planar transmon qubits were packaged. I did this 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 that affected 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 was increased. Simulations and modeling showed that the observed dissipation was 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, I discuss my study of the coherence recovery time after injecting quasiparticles in several transmon qubits. Quasiparticles in 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 of the transmon qubit was observed. By performing relaxation time T1 and coherence time T2 measurements over the course of several milliseconds, I tracked the recovery T1 and T2, which I then used as metrics of the quasiparticle density at the junction. I fitted the recovery data with a numerical model involving differential equations and extracted the quasiparticle trapping rates around 1/ms and recombination rates around 1 / 25ns at the sites of a few transmon qubits. I measured transmons that were either galvanically connected to ground or isolated, fabricated with aluminum (with superconducting gap ~ 200 ueV) or tantalum (with ~ 600 ueV ) 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 was consistent with phonon-assisted quasiparticle poisoning. I discuss my quantitative modelling of such data, and how this effect presents challenges to further improving the coherence times of transmon qubits.
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    (2023) Cheng, Gong; Swingle, Brian; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This work is focused on two main topics in quantum information theory: the scramblingof quantum information and the preservation of quantum information in many-body system. In terms of information scrambling, the main focus of this work is on the Out-of-time-order corre- lator (OTOC), which is used to probe the dynamics of quantum information as it spreads from localized degrees of freedom to those that are distributed throughout the system. On the other hand, the aim of the study of 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 stronglyinteracting 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 whereconformal symmetry emerges. The second case deals with the scrambling dynamics with con- servation law constraints in holographic quantum field theory. The result also clarifies how con- served 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.
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    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.
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    Tight-binding simulations of random alloy and strong spin-orbit effects in InAs/GaBiAs quantum dot molecules
    (2023) Lin, Arthur; Bryant, Garnett W; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Self-assembled \ce{InAs} quantum dots (QDs), which have long hole-spin coherence times and are amenable to optical control schemes, have long been explored as building blocks for qubit architectures. One such design consists of vertically stacking two QDs to create a quantum dot molecule (QDM). The two dots can be resonantly tuned to form "molecule-like" coupled hole states with the hybridization of hole states otherwise localized in each respective dot. Furthermore, spin-mixing of the hybridized states in dots offset along their stacking direction enables qubit rotation to be driven optically, allowing for an all-optical qubit control scheme. Increasing the magnitude of this spin-mixing is important for optical quantum control protocols. We introduce the incorporation of dilute \ce{GaBi_xAs_{1-x}} alloys in the barrier region between the two dots, as \ce{GaBiAs} is expected to provide an increase in spin-mixing of the molecular states over \ce{GaAs}. Using an atomistic tight-binding model, we compute the properties of \ce{GaBi_xAs_{1-x}} and the modification of hole states that arise when the alloy is used in the barrier of an \ce{InAs} QDM. We show that an atomistic treatment is necessary to correctly capture non-traditional alloy effects of \ce{GaBiAs}. Additionally, an atomistic model allows for the study of configurational variances and clustering effects of the alloy. We find that in \ce{InAs} QDMs with a \ce{GaBiAs} inter-dot barrier, hole states are well confined to the dots up to an alloy concentration of 7\%. By independently studying the alloy-induced strain and electronic scattering off \ce{Bi} and As orbitals, we conclude that an initial increase in QDM hole state energy at low Bi concentration is caused by the alloy-induced strain. Additionally, a comparison between the fully alloyed barrier and a partially alloyed barrier shows that fully alloying the barrier applies an asymmetric strain between the top and bottom dot. By lowering the energetic barrier between the two dots, \ce{GaBiAs} is able to promote the tunnel coupling of hole states in QDMs. We obtain a three fold increase of hole tunnel coupling strength in the presence of a 7\% alloy. Additionally, we show how an asymmetric strain between the two dots caused by the alloy results in a shift in the field strength needed to bring the dots to resonance. We explore different geometries of QDMs to optimize the tunnel coupling enhancement the alloy can provide, as well as present evidence that the change in tunnel coupling may affect the heavy-hole and light-hole components of the ground state in a QDM. The strong spin-orbit coupling strength of \ce{GaBiAs} allows for the enhancement of spin-mixing in QDMs. A strong magnetic field can be applied directly in the TB Hamiltonian. In order to fit the TB results to a simple phenomenological Hamiltonian, we found it necessary to include second order magnetic field terms in the phenomenological Hamiltonian as a diamagnetic correction to the hole state energies. Fitting to the corrected phenomenological model, we obtain a three-fold enhancement for the spin-mixing strength of offset dots at 7\% \ce{Bi}. Additionally, at higher alloy concentrations, a combination of enhanced spin-mixing and increase resonance change in g-factor results in intra-dot spin-mixing between Zeeman split states of the lower energy dot. A perturbative analysis of the magnetic field shows that both the spin-mixing and resonance g-factor change are effects of the Peierls contribution, or the component of the magnetic field applied to the effective spatial angular momentum of the wavefunction. When spin-orbit coupling is removed from the system, there is no longer a preferred alignment between the spin of the system and the Peierls effective angular momentum, thus removing any magnetic field effects of the Peierls contribution. The analysis of spin-orbit effects can be extended to single dots with in-plane magnetic and electric fields. This thesis concludes with some preliminary results utilizing electric fields, in conjunction with spin-locking effects provided by spin-orbit coupling, to manipulate the spin polarization in single dots. TB calculations with a magnetic field are performed to show the preferred alignment of the effective angular momentum, given by the geometry of the dot, also spatially locks the spin-polarization of hole states. An electric field can then be applied to bias the charge density to either side of the dot, using the spatial texture of the spin to obtain a spin polarized in $z$ while both the magnetic and electric field is in the $xy$-plane. The same perturbative analysis with the QDMs can be applied to show sufficient spin-orbit coupling is needed to generate such an effect. We propose the utilization of spin texture and electric fields as a novel method for rotating the spin in QDs.
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    Unveiling secrets of brain function with generative modeling: Motion perception in primates & Cortical network organization in mice
    (2023) Vafaii, Hadi; Pessoa, Luiz; Butts, Daniel A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This Dissertation is comprised of two main projects, addressing questions in neuroscience through applications of generative modeling. Project #1 (Chapter 4) is concerned with how neurons in the brain encode, or represent, features of the external world. A key challenge here is building artificial systems that represent the world similarly to biological neurons. In Chapter 4, I address this by combining Helmholtz's “Perception as Unconscious Inference”---paralleled by modern generative models like variational autoencoders (VAE)---with the hierarchical structure of the visual cortex. This combination results in the development of a hierarchical VAE model, which I subsequently test for its ability to mimic neurons from the primate visual cortex in response to motion stimuli. Results show that the hierarchical VAE perceives motion similar to the primate brain. I also evaluate the model's capability to identify causal factors of retinal motion inputs, such as object motion. I find that hierarchical latent structure enhances the linear decodability of data generative factors and does so in a disentangled and sparse manner. A comparison with alternative models indicates the critical role of both hierarchy and probabilistic inference. Collectively, these results suggest that hierarchical inference underlines the brain's understanding of the world, and hierarchical VAEs can effectively model this understanding. Project #2 (Chapter 5) is about how spontaneous fluctuations in the brain are spatiotemporally structured and reflect brain states such as resting. The correlation structure of spontaneous brain activity has been used to identify large-scale functional brain networks, in both humans and rodents. The majority of studies in this domain use functional MRI (fMRI), and assume a disjoint network structure, meaning that each brain region belongs to one and only one community. In Chapter 5, I apply a generative algorithm to a simultaneous fMRI and wide-field calcium imaging dataset and demonstrate that the mouse cortex can be decomposed into overlapping communities. Examining the overlap extent shows that around half of the mouse cortical regions belong to multiple communities. Comparative analyses reveal that calcium-derived network structure reproduces many aspects of fMRI-derived network structure. Still, there are important differences as well, suggesting that the inferred network topologies are ultimately different across imaging modalities. In conclusion, wide-field calcium imaging unveils overlapping functional organization in the mouse cortex, reflecting several but not all properties observed in fMRI signals.
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    (2023) Li, Kungang; Lobb, Christopher; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this thesis, I discuss the fabrication and measurement of Al/AlOx/Al transmons that have electrodes with different superconducting gaps. With gap-engineering, the tunneling of single quasiparticle from the low-gap side to the high-gap side can be suppressed, hence increasing the relaxation time T1. The best gap-engineered device showed T1 exceeding 300 μs. Large T1 fluctuations in my devices were also observed. I proposed a mechanism for exploring the T1 fluctuation data and discuss the possible underlying cause of the T1 fluctuations. I first discuss the theory of the loss in gap-engineered transmons, with a focus on the loss from non-equilibrium quasiparticles. The model yields the quasiparticle-induced loss in transmons and its dependence on temperature. I also discuss how multiple Andreev reflection (MAR) effects might alter these conclusions, leading to a further reduction in T1. I then describe the design, fabrication and basic characterization of the transmon chip SKD102, which features two transmons – one with thin-film electrodes of pure Al and another that had one electrode made from oxygen-doped Al. I next examined T1 vs temperature and how the T1 fluctuations depended on temperature. I compare my results to a simple model and find reasonable agreement in transmons on chip SKD102, KL103 and KL109, which had different electrode and layer configurations. Finally, I analyze T1 fluctuations in different devices and as a function of temperature and propose a model to explain this behavior. Over the different devices, the T1 fluctuation magnitude roughly scaled as T13/2. With increasing temperature, T1 decreases due to a higher density of thermally generated quasiparticles. In contrast, for an individual device measured from 20mK to 250 mK, the fluctuation magnitude appears to be proportional to T1. I present a model of quasiparticle dissipation channels that reproduces both of these observed scaling relationships.
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    Combining Physics-based Modeling, Machine Learning, and Data Assimilation for Forecasting Large, Complex, Spatiotemporally Chaotic Systems
    (2023) Wikner, Alexander Paul; Ott, Edward; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    We consider the challenging problem of forecasting high-dimensional, spatiotemporally chaotic systems. We are primarily interested in the problem of forecasting the dynamics of the earth's atmosphere and oceans, where one seeks forecasts that (a) accurately reproduce the true system trajectory in the short-term, as desired in weather forecasting, and that (b) correctly capture the long-term ergodic properties of the true system, as desired in climate modeling. We aim to leverage two types of information in making our forecasts: incomplete scientific knowledge in the form of an imperfect forecast model, and past observations of the true system state that may be sparse and/or noisy. In this thesis, we ask if machine learning (ML) and data assimilation (DA) can be used to combine observational information with a physical knowledge-based forecast model to produce accurate short-term forecasts and consistent long-term climate dynamics. We first describe and demonstrate a technique called Combined Hybrid-Parallel Prediction (CHyPP) that combines a global knowledge-based model with a parallel ML architecture consisting of many reservoir computers and trained using complete observations of the system's past evolution. Using the Kuramoto-Sivashinsky equation as our test model, we demonstrate that this technique produces more accurate short-term forecasts than either the knowledge-based or the ML component model acting alone and is scalable to large spatial domains. We further demonstrate using the multi-scale Lorenz Model 3 that CHyPP can incorporate the effect of unresolved short-scale dynamics (subgrid-scale closure). We next demonstrate how DA, in the form of the Ensemble Transform Kalman Filter (ETKF), can be used to extend the Hybrid ML approach to the case where our system observations are sparse and noisy. Using a novel iterative scheme, we show that DA can be used to obtain training data for successive generations of hybrid ML models, improving the forecast accuracy and the estimate of the full system state over that obtained using the imperfect knowledge-based model. Finally, we explore the commonly used technique of adding observational noise to the ML model input during training to improve long-term stability and climate replication. We develop a novel training technique, Linearized Multi-Noise Training (LMNT), that approximates the effect of this noise addition. We demonstrate that reservoir computers trained with noise or LMNT regularization are stable and replicate the true system climate, while LMNT allows for greater ease of regularization parameter tuning when using reservoir computers.
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    All-Sky Search for Very-High-Energy Emission from Primordial Black Holes and Gamma-Ray Bursts with the HAWC Observatory
    (2023) Engel, Kristi Lynne; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Transient sources of very-high-energy gamma rays are short-lived astrophysical phenomena often associated with catastrophic events that change their brightness over relatively short timescales. The search for and study of such objects, especially in the TeV energy regime, has the possibility to shed light not only on the physics at play within the enigmatic, chaotic environments that produce such emission, but also to answer several remaining questions in fundamental physics. In this dissertation, we leverage the sensitivity and characteristics of the High-Altitude Water Cherenkov (HAWC) Observatory in pursuit of gaining insight into these areas. The HAWC Observatory, located on the side of the side of the Sierra Negra volcano in Puebla, Mexico at an altitude of 4,100 m above sea level, is an extensive-air-shower array sensitive to gamma rays from ~0.1 to >100 TeV that has been in operation since March of 2015. It has a wide field of view of ~2 sr at any one time and, combined with its large operational duty cycle (>95%), observes 2/3 of the sky every day. HAWC operates using the water-Cherenkov detection technique with 1,200 photomultiplier tubes (PMTs) in two different sizes to detect Cherenkov emission from secondary air-shower particles. Herein, we present an improved characterization for the larger of these two PMT models for inclusion within the Monte Carlo simulation of the HAWC Observatory, as well as the custom testing apparatus designed and constructed for this purpose. With HAWC's wide field of view, near-continuous uptime, and large archival dataset, it serves as an ideal observatory with which to search for transient sources of all kinds. We apply these advantages to perform searches for two types of transient sources--- Primordial Black Holes (PBHs) and Gamma-Ray Bursts (GRBs). The first of these, a search for emission signatures of evaporating PBHs, is performed on 959 days of HAWC data for remaining lifetimes of 0.2, 1, 10, and 100 s, assuming radiation development according to the Standard Emission Model. We show that previous attempts to perform searches for transient searches similar to PBHs with HAWC were oversampling at detrimental levels and improve upon that method to achieve greater statistical rigor. Finding no significant emission for any duration, we place upper limits at the 99% confidence level on the local burst rate density. For the second of these source types, we apply the low-energy improvements recently made to the HAWC data reconstruction procedure to search for very-high-energy emission within the first 0.1, 1, 10, and 100 s of emission for 93 GRBs within HAWC's field of view at their reported T0 over the first 7 years of HAWC operations. This search is performed using permutations of four different assumed redshift values and four different assumed spectral indices. Finding no significant emission for any duration under any set of assumption parameters, we place upper limits at the 95% confidence level on the intrinsic flux for all GRBs. For those GRBs with external flux models available from other gamma-ray detectors, we compare the HAWC limits to those models in order to constrain the possible emission in the TeV regime with respect to that at lower energy values. We also perform a follow-up execution of this analysis with start times shifted to match external model start times which differed from T0. Again finding no significant emission, we place upper limits at the 95% confidence level on the intrinsic flux for all parameter sets and for all external start times for those GRBs HAWC was most likely to have seen. Finally, we speculate about the future of searches for PBHs and GRBs with the next-generation wide-field-of-view instrument, the Southern Wide-field Gamma-ray Observatory (SWGO), presenting projected performance for these two types of transient sources.
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    Search for emerging-jet signatures in pp collisions at 13 TeV with CMS using a fully data-based method for background extraction
    (2023) Chen, Yi-Mu; Belloni, Alberto; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    For this thesis, I present a search for emerging jets produced in proton-proton collisions at a center-of-mass energy of 13 TeV. This search examines a hypothetical dark QCD sector that couples to the standard model through a scalar mediator. The scalar mediator decays into a standard model quark and a dark sector quark. As the dark sector quark showers and hadronizes, it produces long-lived dark mesons that subsequently decay into SM particles, resulting in a jet with multiple displaced vertices, known as an emerging jet. This search extends the existing efforts of CMS by including the possibility of a flavored coupling between the standard model sector and dark sector, which results in emerging jets containing both long-lived and prompt decays. This search looks for pair production of the scalar mediators at the LHC, yielding events with two SM jets and two emerging jets. As the detector signature left by such dark sector showers significantly deviates from the assumptions made when designing the detector, a fully data-based method is used for evaluating the number of standard model events that can be mistaken as signal events. The search is carried out on data collected by the CMS experiment corresponding to an integrated luminosity of 138 /fb , and we observe no significant excess. The results are interpreted using two dark sector models and exclude mediator masses up to 1750 GeV for an unflavored dark QCD model and up to 2000 GeV for a flavored-aligned dark QCD model.
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    Partially Covariant Quantum Theory of Gravitation
    (1972) Moncrief, Vincent E.; Nutku, Yavuz; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
    In this thesis it is argued that a strict law of conservation of probability is necessary for the unambiguous interpretation of any proposed quantum theory of gravitation. After a brief review of the current canonicnl methods for quantizing the gravitational field we conclude that they do not guarantee conservation of probability owing to the difficulty of finding a suitable intrinsic time coordinate. In an attempt to circumvent this problem we have proposed an alternative method of quantization which has a conventional Schrodinger equation and therefore a law of probability conservation. This result is achieved by imposing a weaker form of the quantum constraint equations than that of the conventional theory. In order to justify this approach it is necessary to show that, in spite of the weak form of the constraint equations, the Einstein theory is recovered in the classical limit . A partial proof of the desired result is given. The proposed quantum theory is developed somewhat by considering the interaction of matter and gravitational fields. Quantum analogs of the covariant conservation laws are derived for the special case of a massive spin-zero field. Charge conservation is also considered and an invariant scheme for defining the number of particles and anti-particles is developed.
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    (2023) Murray, Joseph Edward; Lobb, Christopher; Butera, Robert E; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Scanning tunneling microscopy (STM) provides an opportunity to study the physical and electromagnetic properties of surfaces at the atomic scale. When performed at low temperatures, in high magnetic fields, and with a variety of different probes, it offers a wide range of methods by which novel materials of great practical and theoretical interest can be evaluated, characterized, and even fabricated with atomic precision. This thesis describes three independent STM studies performed at cryogenic temperatures. In the first, I present an in-situ modification to our 4K STM which permits us to current-bias our samples during STM operation. The modification can be used to study non-equilibrium effects such as spin accumulation induced by a current through a spin Hall material and the spin-momentum locking which is present at the surface of topological insulators. Next, I examined oxygen-doped aluminum films with anomalously high kinetic inductance. A suggested explanation was the migration of oxygen to the grain boundaries, forming a percolation network separated by Josephson links. To determine the coupling between grains, I studied the films using milliKelvin STM performed with a superconducting tip. Finally, transport measurements performed by our collaborators indicated the possible presence of a topological Hall effect in thin films of Cr2Te3, induced by the presence of topologically non-trivial magnetic textures called magnetic skyrmions. In order to provide more decisive evidence, I studied the films using spin-polarized STM at 4K. In addition to the experimental studies, this work explores the theoretical underpinnings of the novel materials which constitute the frontier of our current understanding of condensed matter physics. An emphatically pedagogical viewpoint is adopted throughout as part of a continuing effort to bridge the gap between experiment and theory.
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    Variational Algorithms and Resources for Near-Term Quantum Simulation
    (2023) Sewell, Troy Jacob; Jordan, Stephen P; Sau, Jay; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The difficultly 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.
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    Theoretical Developments in Lattice Gauge Theory for Applications in Double-beta Decay Processes and Quantum Simulation
    (2023) Kadam, Saurabh Vasant; Davoudi, Zohreh; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    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.
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    Hunting Inflationary Fossils in Primordial Inhomogeneities
    (2023) Bodas, Arushi; Sundrum, Raman; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    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 10^(13) 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) 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. Since H can be as high as 5*10^(13) GeV, these oscillatory features present a unique opportunity to directly detect very heavy particles with masses ~ H. 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 bring heavier particles with masses up to 60H within the reach of observations. In the final part of the thesis, spanning chapters 3 and 4, we explore the viability of gravitational wave backgrounds (GWB) as novel data sources for unexplored 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.
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    (2023) Ghosh, Sohitri; Taylor, Jacob M.; Shawhan, Peter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    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 involves 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.
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    Phase Transitions in Random Quantum Circuits
    (2023) Niroula, Pradeep; Gorshkov, Alexey; Gullans, Michael J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Random circuits have emerged as an invaluable tool in the quantum computing toolkit. On the one hand, the task of sampling outputs from a random circuit has established itself as a promising approach to experimentally demonstrate the 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 entanglement phase transition that occurs in monitored 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 fully capture the non-classicality necessary to achieve universal quantum computation. A family of measures, termed "magic", is used to quantify the extent to which a quantum state 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 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 mapping 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 demonstrate the existence of an error-mitigation threshold in dimensions D>=2.
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    Thermodynamics of quantum gravitational ensembles
    (2023) Banihashemi, Batoul; Jacobson, Theodore A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The discovery of black hole thermodynamics and its extension to cosmological horizonsdemonstrated 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 horizons and de Sitter space. The main questions addressed in this dissertation are: how is the entropy of de Sitterspace 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, 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 configuration.