Curator's Take
AI Commentary
This article provides the first direct quantification of quasiparticle‑driven transitions in a fluxonium qubit by deliberately injecting quasiparticles on chip, filling a long‑standing gap in our understanding of decoherence for this promising architecture. By showing that the external‑flux dependence of the rates hinges on superconducting‑gap asymmetry across the junction array and the small junction, the work reconciles earlier conflicting estimates of quasiparticle densities and offers a more accurate model for designing low‑loss fluxonium circuits. The findings will help engineers predict and mitigate quasiparticle loss as fluxoniums move toward larger‑scale quantum processors.
— Mark Eatherly
Summary
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 external magnetic flux dependence of the quasiparticle-induced transition rates, it is necessary to account for the superconducting gap asymmetry across the Josephson junctions. A comparison between theory and experiment constrains the relative quasiparticle contributions of the junction array and the small junction and helps explain previously reported discrepancies between the bounds on the quasiparticle densities inferred for these two circuit elements.