Quasiparticle-induced transitions in a fluxonium qubit

Quasiparticles are a prominent decoherence source in superconducting qubits, but their effects are notoriously difficult to isolate in fluxonium. Unlike a transmon, fluxonium is insensitive to offset charge, precluding charge-parity detection of quasiparticle tunneling. We address this challenge by measuring the excitation and de-excitation rates in a fluxonium qubit under controlled on-chip quasiparticle injection. We show that to accurately model the magnetic flux dependence of the transition rates it is necessary to account for the superconducting gap asymmetry across the Josephson junctions. The comparison between theory and experiment allows us to separate the contribution of quasiparticles in the inductor from that in the small junction, addressing previously reported discrepancies between inferred quasiparticle densities in these device parts.