Distinguishing Parity-Switching Mechanisms in a Superconducting Qubit
Spencer Diamond, Valla Fatemi, Max Hays, Heekun Nho, Pavel D. Kurilovich, T. Connolly, Vidul Joshi, Kyle Serniak, Luigi Frunzio, L. I. Glazman, Michel Devoret
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 that 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 interfilm QP dynamics.