Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item NONEQUILIBRIUM STATISTICAL PHYSICS OF FEEDBACK-CONTROLLED AND AUTONOMOUS INFORMATION-THERMODYNAMIC SYSTEMS(2024) Bhattacharyya, Debankur; Jarzynski, Christopher; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis investigates the nonequilibrium dynamics of a variety of systems evolving under control protocols. A control protocol can involve feedback based on measurements performed by an external agent, or it can be a predefined protocol that does not rely on explicit measurements of the system’s state. In the context of information thermodynamics, the former setup belongs to the paradigm of non-autonomous or feedback-controlled Maxwell's demons, and the latter to the paradigm of autonomous demons. The thesis begins with a framework for analyzing non-autonomous feedback control, when the control protocol is applied by an agent making continuous measurements on the system. A multiple-timescales perturbation theory, applicable when there exists an appropriate separation of timescales, is developed. This framework is applied to a classical two-state toy model of an information engine – a device that uses feedback control of thermal fluctuations to convert heat into work. Additionally, quantum trajectory simulations are used to study a feedback-controlled model of Maxwell's demon in a double quantum dot system. Next, a modeling scheme for converting feedback-controlled Maxwell's demons to autonomous (non-feedback) systems is developed. This scheme explicitly accounts for the thermodynamic costs of information processing, by incorporating an information reservoir, modeled as a sequence of bits. This modeling scheme is then applied for converting the classical analogue of the non-autonomous double quantum dot Maxwell's demon, discussed previously, to an autonomous model. Using analytical, semi-analytical and stochastic simulation-based approaches, it is shown that the obtained model can act either as an information engine, or as a “Landauer eraser”, and then the phase diagrams that identify these regimes of behavior are constructed. Finally, fast-forward shortcuts to adiabaticity for classical Floquet-Hamiltonian systems is developed, and applied to a periodically driven asymmetric double well (without feedback control). Tools from dynamical systems theory are then used to characterize the system’s angle-variable dynamics.Item Optimized simulations of fermionic systems on a quantum computer(2024) Wang, Qingfeng; Monroe, Christopher; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum computing holds promise for simulating microscopic phenomena, offering profound implications across disciplines such as chemistry, condensed matter physics, and high-energy physics, particularly in the accurate simulation of fermions. However, practical implementation requires the optimization of quantum programs to mitigate quantum noise and decoherence effects. Given the constraints of near-term quantum computers, the Variational Quantum Eigensolver (VQE) emerges as a key approach for estimating molecular ground state energies, crucial for determining chemical properties. This work aims to present advancements in optimizing VQE simulations to minimize quantum computational resources. Specifically, this work explores various optimization strategies, including the utilization of second-order perturbation correction to recover additional energy beyond VQE estimates and select critical ansatz terms. Additionally, circuit optimization techniques are investigated, focusing on achieving shorter equivalent ansatz circuits, particularly for physically-inspired VQE ansatz, through methods such as generalized fermion-to-qubit transformations and Pauli string orderings. Furthermore, this work demonstrates the advantage of a better initial state on a trapped-ion quantum computer.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 RELAXATION TIME FLUCTUATIONS IN TRANSMONS WITH DIFFERENT SUPERCONDUCTING GAPS(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.Item Foundational Theoretical Issues of Quantum Heat Engines and Hot Entanglement(2022) Arisoy, Onat; Hu, Bei-Lok; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study open quantum systems in the contexts of quantum heat engines, refrigerators and quantum entanglement in systems in contact with high temperature reservoirs. Emphasizing the underlying theoretical foundations rather than practical aspects such as enhancement in efficiency of engines and refrigerators or in entanglement measures for a particular set of system and bath parameters, this work focuses on some important aspects of open quantum systems and quantum thermodynamics and provide in-depth analysis of them using relatively simple yet robust models in non- equilibrium statistical mechanics. These examples include a refrigerator for quantum many-body systems in the Markovian regime, a single harmonic oscillator quantum Otto cycle with its generalization to squeezed thermal baths and the entanglement dynamics of two coupled harmonic oscillators each having its own separate thermal baths in the non-Markovian regime. The investigation of these setups in a unified context in this dissertation also brings up the discussion on the validity of Markovian approximation for open quantum systems and the qualitative differences in Markovian versus non-Markovian open system dynamics, which is addressed on multiple occasions throughout the cases we study. Our analysis of quantum Otto cycle with squeezed thermal reservoirs show that the efficiency of the cycle does not change due to the squeezing in the bath in contrast to previous works studying the same cycle restricted by Markovian assumptions. In our investigation of the effects of time-dependent coupling in a system of two harmonic oscillators with two separate baths in both Markovian and non-Markovian regimes, we find that the driving-induced instability of the solutions of the Langevin equations of the oscillator system is necessary to sustain entanglement at late times with hot reservoirs which mayrender hot entanglement untenable. The effects of Markovianity/non-Markovianity, non-thermal reservoirs, contact with multiple reservoirs and time-dependent system Hamiltonians in quantum thermodynamics are addressed in this thesis.Item Towards Low-frequency Squeezed Light and Its Applications with Four-wave Mixing in Rubidium Vapor(2020) Wu, Meng-Chang; Lett, Paul D.; Rolston, Steven L.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study a variety of mechanisms that introduce noise into squeezed light generated by a four-wave mixing (4WM) process in Rb vapor. The noise from the seeding beam itself is a general noise that appears in any squeezed light generated from a seeding process. This noise dominates in the squeezed light at acoustic and lower measurement frequencies. A second excess noise source is observed in the twin beams pumped by either a diode laser system or a Ti:sapphire laser system. This excess noise is much stronger in the diode laser systems. It is present in the twin beams at measurement frequencies when the 4WM gain is reduced toward unity. Most of this excess noise can be removed with a dual-seeded 4WM scheme. A third noise source we examine is from a two-beam coupling that degrades the squeezing of the dual-seeded 4WM process at low frequencies of the order of the atomic transition linewidth. This noise can be avoided by seeding skew rays in the 4WM gain region. This gives us an insight to solve this "cross talk" problem by imaging the source in the 4WM gain region. In addition to studying noise sources, we propose a gain-independent calibration scheme that relies on higher order correlation function for the absolute calibration of photodiodes. Having low frequency squeezing is really important if we record quantum images with a CCD camera, which has a slow shutter speed. Also, it's been very difficult for people to get low-frequency squeezing. We obtain a record level of low-frequency squeezing using a simple dual-seeding technique. With this study of noise sources we are closer to having a portable quantum light source using diode lasers.Item ATOMICALLY PRECISE FABRICATION AND CHARACTERIZATION OF DONOR-BASED QUANTUM DEVICES IN SILICON(2019) Wang, Xiqiao; Silver, Richard M; Appelbaum, Ian; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atomically precise donor-based quantum devices in silicon are a promising candidate for scalable solid-state quantum computing and analog quantum simulation. This thesis demonstrates success in fabricating state-of-the-art silicon-phosphorus (Si:P) quantum devices with atomic precision. We present critical advances towards fabricating high-fidelity qubit circuitry for scalable quantum information processing that demands unprecedented precision and reproducibility to control and characterize precisely placed donors, electrodes, and the quantum interactions between them. We present an optimized atomically precise fabrication scheme with improved process control strategies to encapsulate scanning tunneling microscope (STM)-patterned devices and technological advancements in device registration and electrical contact formation that drastically increase the yield of atomic-precision fabrication. We present an atomic-scale characterization of monolayer step edges on Si (100) surfaces using spatially resolved scanning tunneling spectroscopy and quantitatively determine the impact of step edge density of states on the local electrostatic environment. Utilizing local band bending corrections, we report a significant band gap narrowing behavior along rebonded SB step edges on a degenerately boron-doped Si substrate. We quantify and control atomic-scale dopant movement and electrical activation in silicon phosphorus (Si:P) monolayers using room-temperature grown locking layers (LL), sputter profiling simulation, and magnetotransport measurements. We explore the impact of LL growth conditions on dopant confinement and show that the dopant segregation length can be suppressed below one Si lattice constant while maintaining good epitaxy. We demonstrate weak-localization measurement as a high-resolution, high-throughput, and non-destructive method in determining the conducting layer thickness in the sub-nanometer thickness regime. Finally, we present atomic-scale control of tunnel coupling using STM-patterned Si:P single electron transistors (SET). We demonstrate the exponential scaling of tunnel coupling down to the atomic limit by utilizing the Si (100) 2×1 surface reconstruction lattice as a natural ruler with atomic-accuracy and varying the number of lattices counts in the tunnel gaps. We analyze resonant tunneling spectroscopy through atomically precise tunnel gaps as we scale the SET islands down to the few-donor quantum dot regime. Finally, by combining single/few-donor quantum dots with atomically defined single electron transistors as charge sensors, we demonstrate single electron charge sensing in few-donor quantum dots and characterize the tunnel coupling between few-donor quantum dots and precision-aligned single electron charge sensors.Item Exciton Photophysics at Fluorescent Quantum Defects(2018) Kim, Mijin; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluorescent quantum defect is an emerging synthetic structure that can be covalently attached to a semiconducting single-walled carbon nanotube. Incorporation of fluorescent quantum defect breaks the symmetry of carbon nanotubes at a defect center, creating new optically allowed, low-lying states in the electronic structure of carbon nanotube. Exciting electronic and optical properties arise from the defects, including the generation of new photoluminescence features, which can be used for applications, such as chemical sensing, bioimaging, and quantum light source. As excitons dominate the optical properties of carbon nanotubes, understanding the exciton photophysics in a defect-tailored carbon nanotube is essential to efficiently harness the emission properties of fluorescent quantum defects. In this dissertation, I aim to understand the exciton photophysics in fluorescent quantum defects in order to explain the origins and behavior of novel phenomena arising from them. First, the structure-property relationships of fluorescent quantum defects are discussed; these guide the systematic tuning of defect-induced emission and the binding energy of defect-trapped excitons. Then, the discussion moves to the exciton dynamics at fluorescent quantum defects. Particularly, I describe how the chemical nature of defects or the density of defects influences the thermal detrapping energy of excitons. The exciton-electron interaction at a fluorescent defect is also discussed. Our results suggest that a fluorescent quantum defect colocalizes an exciton and an electron as a tri-charge carrier and the brightening at the defect can be chemically tuned. Finally, I introduce super-resolved, hyperspectral photoluminescence spectroscopy, enabling both direct probing of a single fluorescent defect and the quantitative evaluation of the brightening of dark excitons.