Decoherence of superconducting qubits caused by quasiparticle tunneling

TL;DR

This paper develops a theory for quasiparticle-induced decoherence in superconducting qubits, showing that such effects are generally small but can be significant in specific designs like the split transmon, impacting qubit coherence.

Contribution

It introduces a theoretical framework for understanding quasiparticle tunneling effects on qubit dephasing and broadening, highlighting conditions where these effects are measurable or negligible.

Findings

Quasiparticle tunneling causes small dephasing rates in most qubit types.

Split transmon design could enable measurement of quasiparticle-induced dephasing.

Frequency fluctuations due to Andreev state occupation are more detectable in small-area junctions.

Abstract

In superconducting qubits, the interaction of the qubit degree of freedom with quasiparticles defines a fundamental limitation for the qubit coherence. We develop a theory of the pure dephasing rate \Gamma_{\phi} caused by quasiparticles tunneling through a Josephson junction and of the inhomogeneous broadening due to changes in the occupations of Andreev states in the junction. To estimate \Gamma_{\phi}, we derive a master equation for the qubit dynamics. The tunneling rate of free quasiparticles is enhanced by their large density of states at energies close to the superconducting gap. Nevertheless, we find that \Gamma_{\phi} is small compared to the rates determined by extrinsic factors in most of the current qubit designs (phase and flux qubits, transmon, fluxonium). The split transmon, in which a single junction is replaced by a SQUID loop, represents an exception that could make possible the measurement of \Gamma_{\phi}. Fluctuations of the qubit frequency leading to inhomogeneous broadening may be caused by the fluctuations in the occupation numbers of the Andreev states associated with a phase-biased Josephson junction. This mechanism may be revealed in qubits with small-area junctions, since the smallest relative change in frequency it causes is of the order of the inverse number of transmission channels in the junction.