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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.
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cutoff frequency... spin qubit
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Oscillator
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coupled oscillators
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Deficits in attention regulation are associated with a wide array of negative outcomes including decreased academic achievement, school dropout, and increased risk for disruptive disorders. Current studies have identified possible biological markers of attention regulation including variability in heart rate oscillations. Changes in heart rate oscillations during a continuous performance task for preschoolers from a Head Start population were examined. Results indicated that sustained attention produced expected changes in the low frequency band and respiratory sinus arrhythmia. Changes in respiratory sinus arrhythmia predicted children's behavioral functioning and attentional functioning. Neither heart rate oscillation distinguished between attentional and behavioral functioning.
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frequency-doubling
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high frequency
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AIAA Scitech 2020 Forum 6-10 January 2020 Orlando, FL,This work presents a computational study of an oscillating NACA0012 airfoil’s response to vertical gusts at low Reynolds numbers. The gust is created by a cross-flow ducted floor jet and its interaction with a freestream flow causes the jet to bend downstream, thus creating a blockage effect and modifying the effective angle of attack (AoA) over an airfoil in the freestream flow. The interaction of the gust with the airfoil causes large unsteady forces, which exceed the peak static lift coefficient. As the gust becomes fully developed near the airfoil region, the airfoil exhibits a leading edge vortex formation and dynamic-stall-like phenomenon while remaining at a fixed zero degree AoA. The gust-wing interactions under dynamic pitching conditions are also studied by varying the reduced frequencies. The study shows that the effects of the gust can be mitigated by increasing the reducing frequency of the flapping wing. As a byproduct, larger lift and thrust will be produced.,
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Since 2006 it has been discovered experimentally that the superconducting state spontaneously breaks time-reversal symmetry (TRS) in several materials, such as Sr2RuO4, UPt3, URu2Si2, PrOs4Sb12, and Bi/Ni bilayers. This dissertation studies three physical phenomena related to time-reversal symmetry breaking (TRSB) in these superconductors. The experimental evidence for TRSB comes from the magneto-optical polar Kerr effect, which is determined by the high frequency ac Hall conductivity. However, these superconductors are also expected to exhibit a spontaneous dc Hall effect in the absence of an applied magnetic field. In the first part of this dissertation we propose a method for measuring the low frequency Hall conductivity in superconductors with TRSB. The method is based on a Corbino disk geometry where an oscillating co-axial magnetic field induces circular electric field, which, in turn, induces radial charge oscillations due to the Hall conductivity. In the second part, we propose an explanation for the polar Kerr effect observed in the Hidden-Order phase of the heavy-fermion superconductor URu2Si2. Using a Ginzburg-Landau model for a complex order parameter, we show that the system can have a metastable ferromagnetic state, which produces the Kerr signal, even if the Hidden-Order state respects TRS. We predict that applying a reversed magnetic field should reset the system to the non-magnetic ground state, resulting in zero Kerr signal. In the third part of the dissertation, we investigate the conditions for the existence of a Majorana bound state on a vortex in a 2D d+id superconductor with strong spin-orbit coupling. This TRSB pairing was proposed earlier for the Ni/Bi bilayer. We find that the Majorana bound state can exist for a d+id pairing under conditions similar to those for s-wave pairing.
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Semiconductor lasers have been one of the major building blocks of fiber-optics based communication systems. For the past two decades, specifications of these lasers have been tailored to particular applications by defining certain performance parameters that do not necessarily overlap from one application to another. In order to commoditize laser sources by increasing volume and reducing cost, the laser performance parameters needed to be generic and not customized. In this simulation work, we modify and enhance essential performance parameters of a simple, low-cost laser with generic specifications and tailor it to specific applications that normally require advanced and complicated laser structures, by using electronic feedback for instantaneous impairment correction, while at the same time maintain a compact size for the laser and the supporting circuitry around it. We essentially use the same laser chip to cover the three distinct applications being analyzed. Maintaining generic laser chip specifications while only modifying the feedback loop circuit design parameters can facilitate photonic hybrid integration. Specific applications targeted are, linear optical-analog transmission for wireless backhauling by reducing the laser relative intensity noise while maintaining and enhancing transmitter linearity, improving laser modulation response and increasing the laser relaxation-oscillation frequency, another application covers laser linewidth reduction to target long haul transmission and coherent detection and finally an application that requires producing a self-pulsating laser for analog to digital conversion sampling application. The performance for all these application is analyzed in both the time and frequency domains. For the analog-signal laser, a laser RIN reduction of 15 dB was realized in the simulation and experiment. For the linewidth-reduction application, the simulation also shows a reduction of linewidth from 2.4 MHz to 24 Hz can be achieved along with simultaneous reduction of 1/f noise at low frequencies, and carrier effect noise at high frequencies to flatten the laser FM response. As for the pulse-generation application, an analysis on controlling the self-pulsation characteristics of semiconductor DFB laser was performed. A detailed analysis of the noise effects on the jitter performance of this system is also carried out. This work also establishes methods to optimize and control a pulsing DFB-laser system for photonic analog-to-digital conversion application by tuning the laser drive current with the potential to replace mode-locked fiber lasers in many applications from RF photonics to chaos communication and optical signal processing.
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