Distinguishing parity-switching mechanisms in a superconducting qubit

TL;DR

This study investigates the mechanisms behind quasiparticle-induced decoherence in superconducting qubits by analyzing flux-dependent charge-parity switching, revealing the roles of photon-assisted processes and quasiparticle trapping.

Contribution

It introduces a flux-dependent measurement method to distinguish photon-assisted parity switching from quasiparticle generation in superconducting qubits.

Findings

Parity-switching rate peaks depend on qubit state and flux.

Photon-assisted processes significantly contribute to quasiparticle generation.

Excess quasiparticles are mainly trapped in the low-gap film.

Abstract

Single-charge tunneling is a decoherence mechanism affecting superconducting qubits, yet the origin of excess quasiparticle excitations (QPs) responsible for this tunneling in superconducting devices is not fully understood. We measure the flux dependence of charge-parity (or simply, ``parity'') switching in an offset-charge-sensitive transmon qubit to identify the contributions of photon-assisted parity switching and QP generation to the overall parity-switching rate. The parity-switching rate exhibits a qubit-state-dependent peak in the flux dependence, indicating a cold distribution of excess QPs which are predominantly trapped in the low-gap film of the device. Moreover, we find that the photon-assisted process contributes significantly to both parity switching and the generation of excess QPs by fitting to a model that self-consistently incorporates photon-assisted parity switching as well as inter-film QP dynamics.