Theses and Dissertations from UMD
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Item THREE ESSAYS ON QUANTUM TECHNOLOGY APPLICATIONS(2024) Stein, Amanda; Wang, Ping; Information Studies; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation examines quantum technology applications in three essays. Essay 1 portrays how companies are beginning to innovate with quantum computing in four case studies. The cases employ and enrich the Diffusion of Innovations theory as a conceptual framework for quantum computing innovation adoption and management. Essay 2 follows the evolution of quantum sensing with two cases of how organizations currently use the technology and plan to use it in the future. These cases illustrate how people and organizations use their discourse to develop an organizing vision for adopting and applying quantum sensing. Essay 3 focuses on the relationships between quantum technology and artificial intelligence through a literature review using grounded theory. The essay provides examples on how the two technologies interact and recommendations to stakeholders for future advancement. In summary, while the science and engineering side of quantum technologies is still developing, understanding how quantum technologies are and will be applied can help inform business and public policies.Item Quantum Algorithms for Nonconvex Optimization: Theory and Implementation(2024) Leng, Jiaqi; Wu, Xiaodi XW; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Continuous optimization problems arise in virtually all disciplines of quantitative research. While convex optimization has been well-studied in recent decades, large-scale nonconvex optimization problems remain intractable in both theory and practice. Quantum computers are expected to outperform classical computers in certain challenging computational problems. Some quantum algorithms for convex optimization have shown asymptotic speedups, while the quantum advantage for nonconvex optimization is yet to be fully understood. This thesis focuses on Quantum Hamiltonian Descent (QHD), a quantum algorithm for continuous optimization problems. A systematic study of Quantum Hamiltonian Descent is presented, including theoretical results concerning nonconvex optimization and efficient implementation techniques for quantum computers. Quantum Hamiltonian Descent is derived as the path integral of classical gradient descent algorithms. Due to the quantum interference of classical descent trajectories, Quantum Hamiltonian Descent exhibits drastically different behavior from classical gradient descent, especially for nonconvex problems. Under mild assumptions, we prove that Quantum Hamiltonian Descent can always find the global minimum of an unconstrained optimization problem given a sufficiently long runtime. Moreover, we demonstrate that Quantum Hamiltonian Descent can efficiently solve a family of nonconvex optimization problems with exponentially many local minima, which most commonly used classical optimizers require super-polynomial time to solve. Additionally, by using Quantum Hamiltonian Descent as an algorithmic primitive, we show a quadratic oracular separation between quantum and classical computing. We consider the implementation of Quantum Hamiltonian Descent for two important paradigms of quantum computing, namely digital (fault-tolerant) and analog quantum computers. Exploiting the product formula for quantum Hamiltonian simulation, we demonstrate that a digital quantum computer can implement Quantum Hamiltonian Descent with gate complexity nearly linear in problem dimension and evolution time. With a hardware-efficient sparse Hamiltonian simulation technique known as Hamiltonian embedding, we develop an analog implementation recipe for Quantum Hamiltonian Descent that addresses a broad class of nonlinear optimization problems, including nonconvex quadratic programming. This analog implementation approach is deployed on large-scale quantum spin-glass simulators, and the empirical results strongly suggest that Quantum Hamiltonian Descent has great potential for highly nonconvex and nonlinear optimization tasks.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 Engineering a Control System for a Logical Qubit-Scale Trapped Ion Quantum Computer(2023) Risinger, Andrew Russ; Monroe, Christopher R; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum computing is a promising field for continuing to develop new computing capabilities, both in its own right and for continued gains as Moore's Law growth ends.Trapped ion quantum computing is a leading technology in the field of quantum computing, as it combines the important characteristics of high fidelity operations, individual addressing, and long coherence times. However, quantum computers are still in their infancy; the first quantum computers to have more than a handful of quantum bits (qubits) are less than a decade old. As research groups push the boundaries of the number of qubits in a system, they are consistently running into engineering obstacles preventing them from achieving their goals. There is effectively a knowledge gap between the physicists who have the capability to push the field of quantum computing forward, and the engineers who can design the large-scale & reliable systems that enable pushing those envelopes. This thesis is an attempt to bridge that gap by framing trapped ion quantum computing in a manner accessible to engineers, as well as improving on the state-of-the-art in quantum computer digital and RF control systems. We also consider some of the practical and theoretical engineering challenges that arise when developing a leading-edge trapped ion quantum computer capable of demonstrating error-corrected logical qubits, using trapped Ytterbium-171 qubits.There are many fundamental quantum operations that quantum information theory assumes, yet which are quite complicated to implement in reality. First, we address the time cost of rearranging a chain of ions after a scrambling collision with background gases. Then we consider a gate waveform generator that reduces programming time while supporting conditional quantum gates. Next, we discuss the development of a digital control system custom-designed for quantum computing and quantum networking applications. Finally, we demonstrate experimental results of the waveform generator executing novel gate schemes on a chain of trapped ions. These building blocks together will unlock new capabilities in the field of trapped ion quantum computers.Item Non-local Transport Signatures and Quality Factors in the Realistic Majorana Nanowire(2022) Lai, Yi-Hua; Sau, Jay Deep; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Majorana zero modes (MZMs) can be fault-tolerant topological qubits due to their topological protection property and non-Abelian statistics. Over the last two decades, a deluge of theoretical predictions and experimental observations has been actively ongoing in the hope of implementing topological quantum computation upon MZMs. Among several solid-state systems, the most promising platform to realize MZMs is the one-dimensional semiconductor-superconductor nanowire (called ``Majorana nanowire'' in short), which is the focused system in this thesis. It is fundamental to identify MZMs as qubits to construct topological quantum computers. Therefore, the signatures of MZMs become highly crucial for verification. However, the earlier theoretical works demonstrate that topologically trivial Andreev bound states (ABSs) can mimic the hallmarks of MZMs in various aspects but do not carry topological properties. As a result, distinguishing MZMs from ABSs becomes significantly pivotal in the study of Majorana nanowire. One of the signatures the author studied is the robustness of the quantized zero-bias conductance peak (ZBCP) in the realistic Majorana nanowire. The importance of this signature becomes further enhanced, particularly after the 2018 Nature paper, which displayed the quantized Majorana conductance, got retracted. In Chapter 2, the proposed quality factors quantify the robustness of quantized ZBCPs. By comparing the numerical results between different scenarios, this study shows that the quality factor $F$ can help distinguish topological MZMs from trivial subgap bound states in the low-temperature limit. Another necessary signature of MZMs is the non-local correlation. In Chapter 3, the conductance correlation is demonstrated by modeling the comparing quantum-point-contact (QPC) conductance from each end. Both the pristine nanowire and the quantum-dot-hybrid-nanowire system are modeled and compared, which shows the significance of non-local end-to-end correlation for the existence of MZMs. The other approach to simultaneously examining the localization of states at both ends of the nanowire is through the Coulomb blockade (CB) measurement. The lack of sensitivity to the localized state at only one end makes the CB spectroscopy able to capture the non-local correlation feature of MZMs. However, CB transport in the Majorana nanowire is much more complicated to analyze than QPC transport because (a) Coulomb interaction is treated as equal to MZM physics without perturbation, and (b) there are many energy levels in the nanowire, which gives rise to an exponential complexity to solve the rate equations. In Chapter 4, a generalized version of Meir-Wingreen formula for the tunneling conductance of a two-terminal system is derived. This formula reduces the exponential complexity of the rate equations to as low as the linear complexity of QPC tunneling, thus allowing multiple energy levels to be included in the calculation. With dominant realistic effects in the model, the experimental features, such as the bright-dark-bright CB conductance pattern and decreasing oscillation conductance peak spacings (OCPSs) with the Zeeman field, will be simulated and explained theoretically. In short, the theoretical methods proposed in this thesis, including the quality factors, non-local correlation ZBCPs, and CB spectroscopy, are intended to distinguish MZMs from other topologically trivial bound states. Further investigations on the robustness of quantized conductance and non-local correlation analysis can clarify the ambiguous signals in the experiments and push the realization of topological quantum computation to the frontier.Item Defects and Strain in Silicon Metal-Oxide-Semiconductor (MOS) Quantum Dots(2021) Stein, Ryan M; Cumings, John; Stewart, Jr., Michael D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Silicon-based single electron devices (SEDs), fabricated using gate-defined quantum dots are some of the world’s most sensitive devices. Local charge fluctuations and disorder caused by defects in the oxide or substrate impurities can profoundly affect device operation. While most workers consider the above when fabricating SEDs in the Si MOS system, they do not typically consider strain. The fabrication process of the gate material usually results in a thin film under a significant amount of stress, which locally modulates the silicon conduction band. Additionally, the coefficient of thermal expansion mismatch between typical MOS gate materials, such as aluminum, and the underlying silicon substrate also produces strain, which further modifies the conduction band. For quantum dot devices measured at cryogenic temperatures, this local modification of the conduction band is strong enough to lead to the formation of unintentional quantum dots and to affect the tunnel coupling between dots. To realize the potential of quantum devices, gate-induced strain must be understood so as to be mitigated or exploited. In this work, we investigate the role of gate-induced strain in quantum dot devices by comparing measurements of the 4-terminal I(V) characteristics of tunnel barrier devices at cryogenic temperatures. From this, we demonstrate a new electrical measurement of gate-induced strain using tunnel junctions (TJs). Our COMSOL simulations of these devices show that the gate-induced strain will modify the barrier height, depending on both the magnitude and sign of inhomogeneous stress. We fabricate MOS devices on bulk silicon wafers with a variety of gate electrodes, including aluminum and titanium. By comparing nearly identical tunnel junction devices fabricated with two different gate materials, Al and Ti, we measure a relative strain difference consistent with our experimentally measured coefficients of thermal expansion. Our results show that the commonly used bulk parameters for simulating strain effects in silicon QDs do not work well in practice. Additionally, we present measurements of oxide defect densities (fixed charge and interface trap density) as a function of forming gas anneal temperature for three different gate metals: Al, Ti/Pd, and Ti/Pt. We also investigate the effect of these anneals on the mechanical properties of the gate material, such as the intrinsic film stress and coefficient of thermal expansion. The combination of our charge defect and mechanical measurements show that there is no way to simultaneously minimize the effects of both using the forming gas anneal. This result puts tension on designing fabrication processes for MOS QDs where one must choose between setting the anneal such that defects are minimized or the strain-induced modulation of the conduction band is minimized. Additionally, we find that our measured values of the coefficient of thermal expansion deviate significantly from the expected bulk values. This suggests that the common material parameters used to simulate gate-induced strain in MOS QD are not accurate. Building towards the goal of controlling non-idealities in silicon MOS QDs requires methods of measuring strain under relevant conditions while also finding ways to adjust processing to minimize the impact of other non-idealities. The work in thesis represents a significant step towards that goal. The devices presented easily lend themselves to future work exploring deposition parameters and anneals to manipulate inhomogeneous strain. Our method for measuring relative strain satisfies the sensitivity, spatial resolution and low-temperature requirements relevant for MOS QDs. Moreover, the fabrication and measurements are similar to those for QDs so that this method is directly relevant for QD devices. Our data provide an important step forward in assessing gate-induced strain in QD devices in-situ while highlighting the need for further experimental work and a greater theoretical understanding of the electrostatics and strain behavior.Item INTERACTING PHOTONS IN CIRCUIT QUANTUM ELECTRODYNAMICS: DECAY OF THE COLLECTIVE PHASE MODE IN ONE-DIMENSIONAL JOSEPHSON JUNCTION ARRAYS DUE TO QUANTUM PHASE-SLIP FLUCTUATIONS(2020) Grabon, Nicholas Christopher; Manucharyan, Vladimir; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Light does not typically scatter light, as witnessed by the linearity of Maxwell’s equations. In this work, we demonstrate two superconducting circuits, in which microwave photons have well-defined energy and momentum, but their lifetime is finite due to decay into lower energy photons. The circuits we present are formed with Josephson junction arrays where strong quantum phase-slip fluctuations are present either in all of the junctions or in only a single junction. The quantum phase-slip fluctuations are shown to result in the strong inelastic photon-photon interaction observed in both circuits. The phenomenon of a single photon decay provides a new way to study multiple long-standing many-body problems important for condensed matter physics. The examples of such problems, which we cover in this work include superconductor to insulator quantum phase transition in one dimension and a general quantum impurity problem. The photon lifetime data can be treated as a rare example of a verified and useful quantum many-body simulation.Item Raman coherence effects in a superconducting Jaynes-Cummings system(2015) Novikov, Sergey; Wellstood, Frederick C; Palmer, Benjamin S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation describes a study of Raman coherence effects using superconducting quantum circuits. Raman coherence can occur in a three-level system driven by two coherent electromagnetic fields. In a suitable system with a metastable state, the effect is typically manifest as coherent population trapping (CPT) and electromagnetically induced transparency (EIT). I derive the theoretical framework and show experimentally that in the case of a cascade three-level system based on transmon superconducting qubit states, an effect known as the Autler-Townes doublet (ATD), rather than CPT or EIT, occurs. I propose, model, and implement a quasi- system made of combined transmon-cavity levels, which has a meta-stable state required for CPT and EIT. I measure CPT, and demonstrate coherence of the dark state in the time domain. Instead of EIT, I observe a new phenomenon – electromagnetically suppressed transmission (EST). The large negative dispersion accompanying EST leads to superluminal pulse propagation in the system. My results suggest that quantum superconducting circuits provide a viable platform for studying quantum optics of multi-level systems.Item Quantum Information Processing with Trapped Ion Chains(2014) Manning, Timothy Andrew; Monroe, Christopher R; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Trapped atomic ion systems are currently the most advanced platform for quantum information processing. Their long coherence times, pristine state initialization and detection, and precisely controllable and versatile interactions make them excellent quantum systems for experiments in quantum computation and quantum simulation. One of the more promising schemes for quantum computing consists of performing single and multi-qubit quantum gates on qubits in a linear ion crystal. Some of the key challenges of scaling such a system are the individual addressing of arbitrary subsets of ions and controlling the growing complexity of motional mode interactions as the number of qubits increases or when the gates are performed faster. Traditional entangling quantum gates between ion qubits use laser pulses to couple the qubit states to the collective motion of the crystal, thereby generating a spin-spin interaction that can produce entanglement between selected qubits. The intrinsic limitations on the performance of gates using this method can be alleviated by applying optimally shaped pulses instead of pulses with constant amplitude. This thesis explains the theory behind this pulse shaping scheme and how it is implemented on a chain of Yb ions held in a linear radiofrequency `Paul' trap. Several experiments demonstrate the technique in chains of two, three, and five ions using various types of pulse shapes. A tightly focused individual addressing beam allows us to apply the entangling gates to a target pair of ions, and technical issues related to such tight focusing are discussed. Other advantages to the pulse shaping scheme include a robustness against detuning errors and the possibility of suppressing undesirable coupling due to optical spillover on neighboring ions. Combined with ion shuttling, we harness these features to perform sequential gates to different qubit pairs in order to create genuine tripartite entangled states and demonstrate the programmable quantum information processing capability of our system.Item Multi-junction effects in dc SQUID phase qubits(2013) Cooper, Benjamin Kevin; Wellstood, Frederick C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I discuss experimental and theoretical results on an LC filtered dc SQUID phase qubit. This qubit is an asymmetric aluminum dc SQUID, with junction critical currents 1.5 and 26.8 μA, on a sapphire substrate. The layout differs from earlier designs by incorporating a superconducting ground plane and weakly coupled coplanar waveguide microwave drive line to control microwave-qubit coupling. I begin with a discussion of quantizing lumped element circuit models. I use nodal analysis to construct a 2d model for the dc SQUID phase qubit that goes beyond a single junction approximation. I then discuss an extension of this ``normal modes'' SQUID model to include the on-chip LC filter with design frequency ∼ 180 MHz. I show that the filter plus SQUID model yields an effective Jaynes-Cummings Hamiltonian for the filter-SQUID system with coupling g / 2 π ∼ 32 MHz. I present the qubit design, including a noise model predicting a lifetime T1 = 1.2 μs for the qubit based on the design parameters. I characterized the qubit with measurements of the current-flux characteristic, spectroscopy, and Rabi oscillations. I measured T1 = 230 ns, close to the value 320 ns given by the noise model using the measured parameters. Rabi oscillations show a pure dephasing time Tφ = 1100 ns. The spectroscopic and Rabi data suggest two-level qubit dynamics are inadequate for describing the system. I show that the effective Jaynes-Cummings model reproduces some of the unusual features.