Efficient evaluation of decoherence rates in complex Josephson circuits

Abstract

A complete analysis of the decoherence properties of a Josephson junction qubit is presented. The qubit is of the flux type and consists of two large loops forming a gradiometer and one small loop, and three Josephson junctions. The contributions to relaxation (T_1) and dephasing (T_ϕ) arising from two different control circuits, one coupled to the small loop and one coupled to a large loop, is computed. We use a complete, quantitative description of the inductances and capacitances of the circuit. Including two stray capacitances makes the quantum mechanical modeling of the system five dimensional. We develop a general Born-Oppenheimer approximation to reduce the effective dimensionality in the calculation to one. We explore T_1 and T_ϕalong an optimal line in the space of applied fluxes; along this "S line" we see significant and rapidly varying contributions to the decoherence parameters, primarily from the circuit coupling to the large loop.

16 pages, 20 figures; v2: minor revision

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• Weakly Flux-Tunable Superconducting Qubit
• Lecture Notes on Quantum Electrical Circuits
• Charge-SQUID and Tunable Phase-slip Flux Qubit
• Computational modeling of decay and hybridization in superconducting circuits
• Design of an inductively shunted transmon qubit with tunable transverse and longitudinal coupling