Proximity effects in gap-engineered superconducting qubits

Bogoliubov quasiparticles are an intrinsic source of errors for superconducting qubits [1]. These excitations harm the qubit coherence in two ways: i) they can exchange energy with the qubit when tunneling across the junction, and ii) dynamically lead to dephasing, for instance during burst events, by reducing the Josephson energy and so the qubit frequency. In qubits with gap-asymmetric junctions tunnelling-related errors can be efficiently suppressed at low temperatures [2, 3, 4]. Moreover, gap-engineered qubits can provide a probe to study nonequilibrium quasiparticle distributions [3, 5, 6, 7]. In this presentation, we discuss the impact of the proximity effect, absent for gap-symmetric films, on gap-engineered qubits. The proximity effect between the two superconducting layers produces deviations from the ideal BCS density of states (DoS). In particular, a single gap characterizes the two films, with “subgap” states in the higher order parameter film, and broadening of the BCS divergences. The modified DoSs in the two films determine a weaker suppression of the transition rates with decreasing temperature. Moreover, the diverging relaxation rate for qubit transition energy resonant to gap asymmetry is regularized by the proximity effect, and so the related discontinuity in the frequency shift.

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