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Title:Meissner qubit: architecture, characterization and vortex-probing applications
Author(s):Ku, Jaseung
Director of Research:Bezryadin, Alexey
Doctoral Committee Chair(s):Eckstein, James
Doctoral Committee Member(s):Thaler, Jon; Clark, Bryan
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Quantum computing
Superconducting qubit
Abrikosov vortex
Abstract:Quantum computing has drawn enormous attention in physics community in both scientific and technological perspectives. Among several promising architectures for quantum computer are superconducting qubits based on Josephson junctions. Over the last few decades, there has been a dramatic improvement on the coherence time by a few orders of magnitude from a few nanoseconds to about a hundred microsecond. Such improvements were possible due to the elimination or suppression of various decoherence sources. Thus, it is critical to investigate the origin of such decoherence to extend our understanding and practically improve the coherence time. For example, Abrikosov vortices knowingly could be one of the decoherence source, and yet the quantitative research on the interaction of a superconducting qubit with such vortices has been lacking. We present a new type of transmon split-junction qubit which can be tuned by Meissner screening currents in the adjacent superconducting film electrodes. The qubits were measured using a 3D microwave cavity in the dispersive regime at the base temperature 45 mK. The measurement protocols were based on the circuit quantum electrodynamics (cQED) architecture and so-called high-power measurement. The achieved period of oscillation with magnetic field was much smaller than in usual SQUID-based transmon qubits, thus a strong effective field amplification has been realized. The best measured relaxation time was of the order of 50 μs and the dephasing time about 40 μs. This Meissner qubit allows an efficient coupling to superconducting vortices, which were induced by external applied magnetic field. We intend to present a quantitative analysis of the radiation-free energy relaxation in the qubits coupled to the Abrikosov vortices. The estimated relaxation rate combined with vortex counting process provided a good agreement with the experimental results. Also, the observation of coherent quantum oscillations provides strong evidence that vortices can exist in coherent quantum superposition of different position states. According to our suggested model, the wave function collapse is defined by Caldeira-Leggett dissipation associated with viscous motion of the vortex cores.
Issue Date:2016-04-20
Rights Information:Copyright 2016 Jaseung Ku
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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