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APS

Entanglement between the charge and phase degrees of freedom in a superconducting qubit

Mun Dae Kim

Published Mar 1, 2011• 0 citations
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Superconducting circuits exhibit quantum properties on a macroscopic scale, and are natural candidates for solid state quantum computing. Their low-energy physics can be described in terms of the phase of the order parameter, a single collective degree of freedom associated with billions of coherently paired electrons. In practice, however, on top of the superfluid condensate there are single-particle excitations (quasiparticles) with a continuous energy spectrum; the quasiparticles are coupled to the phase degree of freedom. The presence of quasiparticles in the system sets serious constraints on the performance of superconducting charge qubits. In this thesis, we study the kinetics of superconducting quantum circuits, and discuss the fundamental limitations on the energy and phase relaxation times in the presence of quasiparticles.
We study a readout scheme of a superconducting flux qubit state with a Cooper pair box as a transmon. The qubit states consist of the superpositions of two degenerate states where the charge and phase degrees of freedom are entangled. Owing to the robustness of the transmon against external fluctuations, our readout scheme enables the quantum non-demolition and single-shot measurement of flux qubit states. The qubit state readout can be performed by using the nonlinear Josephson amplifiers after a π/2 rotation driven by an ac electric field.