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Description
Quasiparticle-induced decoherence in superconducting qubits, triggered by high-energy radiation impacts or sudden mechanical stress relief, is a leading source of correlated errors that limit the performance of fault-tolerant quantum error correction schemes. Although fluxonium qubits are a promising alternative to transmon qubits, quasiparticle effects in fluxonium devices remain far less explored. In this work, we directly measure quasiparticle-induced transition rates in fluxonium qubits under controlled on-chip quasiparticle injection. By accounting for the dynamics of quasiparticle relaxation, we extract quasi-instantaneous excitation and de-excitation qubit transition rates as a function of external flux after the injection. Remarkably, our results reveal a reversal of the excitation and de-excitation rates under non-equilibrium conditions as fluxonium approaches half-flux quantum, where the qubit transition energy becomes much smaller than the superconducting gap difference between the Josephson-junction leads. These findings shed light on previously reported discrepancies in bounds on quasiparticle densities between the small junctions and junction arrays, and underscore the importance of a modified theoretical treatment that explicitly incorporates the gap difference between superconducting leads, particularly for low-energy qubits.
