Theses and Dissertations from UMD
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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 Variable Qubit-Qubit Coupling Via a Tunable LC Resonator(2018) Ballard, Cody James; Wellstood, Frederick C.; Lobb, Christopher J.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation examines the design, fabrication, and characterization of a superconducting lumped-element tunable LC resonator that is used to vary the coupling between two superconducting qubits. Some level of qubit-qubit coupling is needed to perform gating operations. However, with fixed coupling, single qubit operations become considerably more difficult due to dispersive shifts in their energy levels transitions that depend on the state of the other qubit. Ideally, one wants a system in which the qubit-qubit coupling can be turned off to allow for single qubit operations, and then turned back on to allow for multi-qubit gate operations. I present results on a device that has two fixed-frequency transmon qubits capacitively coupled to a tunable thin-film LC resonator. The resonator can be tuned in situ over a range of 4.14 GHz to 4.94 GHz by applying an external magnetic flux to two single-Josephson junction loops, which are incorporated into the resonator’s inductance. The qubits have 0-to-1 transition frequencies of 5.10 GHz and 4.74 GHz. To isolate the system and provide a means for reading out the state of the qubit readout, the device was mounted in a 3D Al microwave cavity with a TE101 mode resonance frequency of about 6.1 GHz. The flux-dependent transition frequencies of the system were measured and fit to results from a coupled Hamiltonian model. With the LC resonator tuned to its minimum resonance frequency, I observed a qubit-qubit dispersive shift of 2χ_qq≈ 0.1 MHz, which was less than the linewidth of the qubit transitions. This dispersive shift was sufficiently small to consider the coupling “off”, allowing single qubit operations. The qubit-qubit dispersive shift varied with the applied flux up to a maximum dispersive shift of 2χ_qq≈ 6 MHz. As a proof-of-principle, I present preliminary results on performing a CNOT gate operation on the qubits when the coupling was “on” with 2χ_qq≈ 4 MHz. This dissertation also includes observations of the temperature dependence of the relaxation time T1 of three Al/AlOx/Al transmons. We found that, in some cases, T1 increased by almost a factor of two as the temperature increased from 30 mK to 100 mK. We found that this anomalous behavior was consistent with loss due to non-equilibrium quasiparticles in a transmon where one electrode in the tunnel junction had a smaller volume and slightly smaller superconducting energy gap than the other electrode. At sufficiently low temperatures, non-equilibrium quasiparticles accumulate in the electrode with a smaller gap, leading to an increased density of quasiparticles at the junction and a corresponding decrease in the relaxation time. I present a model of this effect, use the model to extract the density of non-equilibrium quasiparticles in the device, and find the values of the two superconducting energy gaps.Item Piezoelectric Microbeam Resonators Based on Epitaxial Al0.3Ga0.7As Films(2005-11-22) Li, Lihua; DeVoe, Don; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this work, piezoelectric resonators based on single crystal Al0.3Ga0.7As films are implemented. The combination of Si doped Al0.3Ga0.7As as electrode layers and moderate piezoelectric properties of updoped Al0.3Ga0.7As film leads to lattice matched single crystal resonators with high attainable quality factors and capability of integration with high speed circuits. To validate the fabrication process, simple cantilever beam structures are developed and characterized by laser Doppler vibrometry. In order to achieve higher center frequencies, a clamped-clamped (c-c) beam design is explored. Important resonator parameters including resonance frequency, quality factor, and power handling ability are investigated. Measured quality factors of c-c beams were found to be limited by anchor losses to the substrate. A free-free (f-f) beam design is proposed in order to alleviate the energy dissipation due to anchor losses. Fabricated f-f beam devices show increased quality factors compared to the c-c beam design. Another improvement is the adoption of bimorph configuration instead of unimorph configuration. Compared to unimorph cantilever beam design, bimorph cantilevers showed 80% to 120% of increase in displacement with the same driving voltage without significant change in quality factors. The quality factors of flexural mode resonators in atmospheric pressure are low due to the effect of air damping. For this reason, proper working of flexural mode resonators requires a vacuum package which imposes unwanted complexity in packaging. To solve this problem, length-extensional mode resonators (bar resonators) are proposed to take advantage of low air shear damping. Bar resonators with lengths ranging from 1000 micro-m to 100 mico-m have been fabricated and tested. Measured resonant frequencies range from 2.5 MHz to 72 MHz with good matching to theoretical predictions. The quality factors of bar resonators at their first resonant frequency are measured in air and in high vacuum, showing values between 4,300 - 8,900 and 8,000 - 17,000, respectively, with corresponding measured motional resistances of 7.3 kohm - 10.5 kohm and 4.0 kohm - 7.8 kohm, respectively. The developed bar resonators showed excellent power handling ability up to -10 dBm which is much higher than equivalent electrostatic resonators.