Theses and Dissertations from UMD
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Item Ultra-high impedance superconducting circuits(2023) Mencia, Raymond; Manucharyan, Vladimir E; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chains of Josephson junctions are known to produce some of the largest kinetic per unit-length inductance, which can exceed the conventional geometric one by about 104. However, the maximum total inductance is still limited by the stray capacitance of the chain, which results in parasitic self-resonances. This stray capacitance is unnecessarily large in most circuits due to the high dielectric constant of silicon or sapphire substrates used. Here, we explore a regime of ultra-high impedance superconducting circuits by introducing the technique of releasing the Josephson chain off the substrate. The ultra-high impedance regime (Z > 4xRQ ~ 25.8 kOhms) is realized by combining a maximal per-unit-length inductance with a minimal stray capacitance and demonstrating the highest impedance electromagnetic structures available today. We begin with suspended “telegraph” transmission lines, composed of 30,000+ junctions, and show that the wave impedance can exceed 5 x RQ (33 kOhms) while the line still maintains a negligible DC resistance. To quantify the effects of parasitic chain modes in ultra-high impedance circuits, we use high-inductance fluxonium qubits. We show that chain modes are ultra-strongly coupled to the qubit but can be moved to a higher frequency with the Josephson chain releasing technique. Finally, we create a superconducting quasicharge qubit (blochnium), dual of transmon, whose impedance reaches over 30 x RQ (200 kOhms) with no evidence of parasitic modes below 10 GHz. This qubit completes the periodic table of superconducting atoms and demonstrates the dual nature of a small Josephson junction in ultra-high impedance circuits, which we probe in a DC experiment in the final chapter.Item OPTICAL AND ELECTRICAL RESPONSE OF SUPERCONDUCTING RESONATORS FOR A HYBRID QUANTUM SYSTEM(2021) Voigt, Kristen; Wellstood, Frederick C.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I describe my contributions towards a hybrid quantum system that would have coupled 87Rb atoms to a superconducting device. I first discuss my work coupling an optical fiber to a translatable thin-film LC lumped-element superconducting Al microwave resonator operating at 100 mK in a dilution refrigerator. The LC resonators had resonance frequencies f0 of 6.15 GHz, quality factors Q of 1.5 x 105 to 6.5 x 105 at high powers, and were mounted inside a superconducting aluminum 3D cavity with a resonance frequency of 7.5 GHz and Q of 8 x 103. An optical microfiber (60 µm diameter) passed through a hole in the 3D cavity near the LC resonator. The 3D cavity was mounted on an x-z attocube-translation stage that allowed the LC resonator to be moved relative to the fiber. The resonator’s f0 and Q were affected both by the fiber dielectric perturbing the resonator’s electric field and from scattered light from the fiber. I measured both effects as a function of fiber-resonator position. I modeled the resonator’s optical response by accounting for optical production, recombination, and diffusion of quasiparticles and the non-uniform position-dependent illumination of the resonator. Using the model, I extracted key parameters describing quasiparticles in the resonator. The hybrid quantum system requires the 87Rb and LC resonator resonance to be tuned to the same frequency. I describe our LC resonator tuning method which moves a superconducting Al pin into the resonator’s electric field, decreasing the resonator capacitance and increasing its resonance frequency up to 137 MHz. This was done at 15 mK using an attocube translation stage. I also investigated two-level system (TLS) defects in an LC resonator by applying a dc voltage. I describe a model in which the TLS causes a capacitive perturbation to the resonator rather than the ‘standard’ electric-dipole coupling model. I use this model of a capacitive TLS or cTLS, to describe intermittent telegraph noise measured in the transmission S21 through the resonator. I measured shifts in f0 of more than 6 kHz corresponding to a cTLS fluctuating its capacitance contribution by 430 zF.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 SUPERCONDUCTING RADIO FREQUENCY MATERIALS SCIENCE THROUGH NEAR-FIELD MAGNETIC MICROSCOPY(2020) Oripov, Bakhrom Gafurovich; Anlage, Steven M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducing Radio-Frequency (SRF) cavities are the backbone of a new generation of particle accelerators used by the high energy physics community. Nowadays, the applications of SRF cavities have expanded far beyond the needs of basic science. The proposed usages include waste treatment, water disinfection, material strengthening, medical applications and even use as high-Q resonators in quantum computers. A practical SRF cavity needs to operate at extremely high rf fields while remaining in the low-loss superconducting state. State of the art Nb cavities can easily reach quality factors Q>2x10^10 at 1.3 GHz. Currently, the performance of the SRF cavities is limited by surface defects which lead to cavity breakdown at high accelerating gradients. Also, there are efforts to reduce the cost of manufacturing SRF cavities, and the cost of operation. This will require an R&D effort to go beyond bulk Nb cavities. Alternatives to bulk Nb are Nb-coated Copper and Nb3Sn cavities. When a new SRF surface treatment, coating technique, or surface optimization method is being tested, it is usually very costly and time consuming to fabricate a full cavity. A rapid rf characterization technique is needed to identify deleterious defects on Nb surfaces and to compare the surface response of materials fabricated by different surface treatments. In this thesis a local rf characterization technique that could fulfill this requirement is presented. First, a scanning magnetic microwave microscopy technique was used to study SRF grade Nb samples. Using this novel microscope the existence of surface weak-links was confirmed through their local nonlinear response. Time-Dependent Ginzburg-Landau (TDGL) simulations were used to reveal that vortex semiloops are created by the inhomogenious magnetic field of the magnetic probe, and contribute to the measured response. Also, a system was put in place to measure the surface resistance of SRF cavities at extremely low temperatures, down to T=70 mK, where the predictions for the surface resistance from various theoretical models diverge. SRF cavities require special treatment during the cooldown and measurement. This includes cooling the cavity down at a rate greater than 1K/minute, and very low ambient magnetic field B<50 nT. I present solutions to both of these challenges.Item Dissipation in a superfluid atom circuit(2017) Lee, Jeffrey Garver; Hill, Wendell T; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bose-Einstein condensates of weakly interacting dilute atomic gases provide a unique system with which to study phenomena associated with superfluidity. The simplicity of these systems allows us to study the fundamental physics of superfluidity without having to consider the strong interactions present in other superfluid systems such as superconductors and liquid helium. While condensate-based studies have been around for 20 years, our novel approach to confining ultracold atoms has opened a completely new range of parameter space to investigate. Armed with an ability for straightforward creation of arbitrary, time-dependent potential landscapes in which to study superfluid interactions, we were able to take a closer look at predictions of superfluid behavior that are decades old, but until now have never been tested directly. The purpose of this research was to draw direct analogies between superfluid BEC systems, which we term superfluid atom circuits, and existing superconducting circuits, thus allowing us to take advantage of much of the existing knowledge that has come from this well-studied field. Specifically, existing circuits and devices that have been created with superconductors give us insight into what might be possible someday with atom-circuit devices and inspiration to create them. In these experiments, we employed two different atom circuits; one classical (thermal ideal gas) and one quantum (ultracold superfluid). Our results show that each system is equivalent to an electronic circuit consisting of a capacitor being discharged through an inductor in series with some dissipative element. In the thermal system, dissipation can be described in terms of simple resistive flow with the resistance equivalent to ballistic, Sharvin resistance seen in electronic circuits. The superfluid measurements show that the dissipation is best described as a resistance-shunted Josephson junction, which is an analogue to similar devices in superconducting circuits. Additionally, the specific geometry of the atom circuit we used in our superfluid system allowed us to investigate directly a predicted mechanism responsible for the dissipation in superfluids caused by the generation of collective excitations, namely vortices. Direct observation of this mechanism has not previously been possible in superfluid helium and superconducting systems.Item Radio Frequency Superconducting Quantum Interference Device Meta-atoms and Metamaterials: Experiment, Theory, and Analysis(2016) Zhang, Daimeng; Anlage, Steven M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metamamterials are 1D, 2D or 3D arrays of articial atoms. The articial atoms, called "meta-atoms", can be any component with tailorable electromagnetic properties, such as resonators, LC circuits, nano particles, and so on. By designing the properties of individual meta-atoms and the interaction created by putting them in a lattice, one can create a metamaterial with intriguing properties not found in nature. My Ph. D. work examines the meta-atoms based on radio frequency superconducting quantum interference devices (rf-SQUIDs); their tunability with dc magnetic field, rf magnetic field, and temperature are studied. The rf-SQUIDs are superconducting split ring resonators in which the usual capacitance is supplemented with a Josephson junction, which introduces strong nonlinearity in the rf properties. At relatively low rf magnetic field, a magnetic field tunability of the resonant frequency of up to 80 THz/Gauss by dc magnetic field is observed, and a total frequency tunability of 100% is achieved. The macroscopic quantum superconducting metamaterial also shows manipulative self-induced broadband transparency due to a qualitatively novel nonlinear mechanism that is different from conventional electromagnetically induced transparency (EIT) or its classical analogs. A near complete disappearance of resonant absorption under a range of applied rf flux is observed experimentally and explained theoretically. The transparency comes from the intrinsic bi-stability and can be tuned on/ off easily by altering rf and dc magnetic fields, temperature and history. Hysteretic in situ 100% tunability of transparency paves the way for auto-cloaking metamaterials, intensity dependent filters, and fast-tunable power limiters. An rf-SQUID metamaterial is shown to have qualitatively the same behavior as a single rf-SQUID with regards to dc flux, rf flux and temperature tuning. The two-tone response of self-resonant rf-SQUID meta-atoms and metamaterials is then studied here via intermodulation (IM) measurement over a broad range of tone frequencies and tone powers. A sharp onset followed by a surprising strongly suppressed IM region near the resonance is observed. This behavior can be understood employing methods in nonlinear dynamics; the sharp onset, and the gap of IM, are due to sudden state jumps during a beat of the two-tone sum input signal. The theory predicts that the IM can be manipulated with tone power, center frequency, frequency difference between the two tones, and temperature. This quantitative understanding potentially allows for the design of rf-SQUID metamaterials with either very low or very high IM response.Item Degenerate mixtures of rubidium and ytterbium for engineering open quantum systems(2015) Vaidya, Varun Dilip; Porto, James V; Rolston, Steven L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the last two decades, experimental progress in controlling cold atoms and ions now allows us to manipulate fragile quantum systems with an unprecedented degree of precision. This has been made possible by the ability to isolate small ensembles of atoms and ions from noisy environments, creating truly closed quantum systems which decouple from dissipative channels. However in recent years, several proposals have considered the possibility of harnessing dissipation in open systems, not only to cool degenerate gases to currently unattainable temperatures, but also to engineer a variety of interesting many-body states. This thesis will describe progress made towards building a degenerate gas apparatus that will soon be capable of realizing these proposals. An ultracold gas of ytterbium atoms, trapped by a species-selective lattice will be immersed into a Bose-Einstein condensate (BEC) of rubidium atoms which will act as a bath. Here we describe the challenges encountered in making a degenerate mixture of rubidium and ytterbium atoms and present two experiments performed on the path to creating a controllable open quantum system. The first experiment will describe the measurement of a tune-out wavelength where the light shift of $\Rb{87}$ vanishes. This wavelength was used to create a species-selective trap for ytterbium atoms. Furthermore, the measurement of this wavelength allowed us to extract the dipole matrix element of the $5s \rightarrow 6p$ transition in $\Rb{87}$ with an extraordinary degree of precision. Our method to extract matrix elements has found use in atomic clocks where precise knowledge of transition strengths is necessary to account for minute blackbody radiation shifts. The second experiment will present the first realization of a degenerate Bose-Fermi mixture of rubidium and ytterbium atoms. Using a three-color optical dipole trap (ODT), we were able to create a highly-tunable, species-selective potential for rubidium and ytterbium atoms which allowed us to use $\Rb{87}$ to sympathetically cool $\Yb{171}$ to degeneracy with minimal loss. This mixture is the first milestone creating the lattice-bath system and will soon be used to implement novel cooling schemes and explore the rich physics of dissipation.Item TUNABLE NONLINEAR SUPERCONDUCTING METAMATERIALS: EXPERIMENT AND SIMULATION(2015) Trepanier, Melissa; Anlage, Steven M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I present experimental and numerical simulation results for two types of nonlinear tunable superconducting metamaterials: 2D arrays of rf SQUIDs (radio frequency superconducting quantum interference devices) as magnetic metamaterials and arrays of Josephson junction-loaded wires as electric metamaterials. The effective inductance of a Josephson junction is sensitive to dc current, temperature, and rf current. I took advantage of this property to design arrays of Josephson junction-loaded wires that present a tunable cutoff frequency and thus a tunable effective permittivity for propagating electromagnetic waves in a one-conductor waveguide. I measured the response of the metamaterial to each tuning parameter and found agreement with numerical simulations that employ the RCSJ (resistively and capacitively shunted junction) model. An rf SQUID is an analogue of an SRR (split ring resonator) with the gap capacitance replaced with a Josephson junction. Like the SRR the SQUID is a resonant structure with a frequency-dependent effective permeability. The difference between the SQUID and the SRR is that the effective inductance and thus effective permeability of the SQUID can be tuned with dc and rf flux, and temperature. Individual rf SQUID meta-atoms and two-dimensional arrays were designed and measured as a function of each tuning parameter and I have found excellent agreement with numerical simulations. There is also an interesting transparency feature that occurs for intermediate rf flux values. The tuning of SQUID arrays has a similar character to the tuning of individual rf SQUID meta-atoms. However, I found that the coupling between the SQUIDs increases the resonant frequency, decreases dc flux tuning, and introduces additional resonant modes. Another feature of arrays is disorder which suppresses the coherence of the response and negatively impacts the emergent properties of the metamaterial. The disorder was experimentally found to be mainly due to a dc flux gradient across the metamaterial. I investigated methods to recover the coherence, specifically by varying the coupling between the SQUID meta-atoms, increasing the amplitude of the applied rf flux, and increasing temperature. In this thesis I successfully demonstrate both electric and magnetic tunable superconducting metamaterials based on the Josephson effect. The tuning of these metamaterials occurs over a larger range, on faster time scales, and with lower losses than previous tunable metamaterials.Item High Reynolds Number Vertical Up-Flow Parameters For Cryogenic Two-Phase Helium I(2014) Mustafi, Shuvo; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The two phase flow characteristics of helium I are of interest since under most operational scenarios this cryogenic fluid exists in both liquid and vapor form because of its extremely low boiling point and latent heat of vaporization. There is a significant knowledge gap in the flow boiling parameters of helium (heat transfer coefficient, pressure drop and dryout heat flux) for high Reynolds number vertical up-flows (Re =10^5-10^6). This dissertation fills this gap and helps to expand the use of helium as an inert simulant for hydrogen. Since no prior correlations for the flow boiling parameters existed for vertical up-flows of helium at these Reynolds numbers, any predictions of these parameters were dependent on correlations that were tested at lower Reynolds numbers, or correlations based on other fluids. The thermophysical properties of helium I are significantly different from most other fluids; therefore the capability of prior correlations in predicting experimental observations was limited. As part of this research new correlations are proposed for the flow boiling parameters. This research begins the investigation of a new regime for two-phase helium I flows at Reynolds numbers above 3e5. The techniques described will enable future work to address other gaps in knowledge for helium I flows that still remain. The prior heat transfer coefficient correlation over-predicted the data that was collected for this research. The new correlation improves the agreement with data by a factor of 98. Two prior models for pressure drop, the separated flow model and the homogeneous flow model, under-predict the observed pressure drop. The newer versions of the separated flow and the homogeneous flow correlations improve agreement with the data by about a factor of 3 and by more than a factor of 2 respectively. The previous dryout heat flux correlation considerably over predicts the observed dryout heat flux. The new correlation improves agreement with the data by a factor of 21. Significant cryogenic challenges were overcome to collect the research data. The strategies described for surmounting the diverse challenges such as thermal acoustic oscillations and low dryout heat flux could be used by future two-phase cryogenic flow researchers.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.