Quasiparticle Poisoning of Fluxonium Qubits

Non-equilibrium quasiparticles are one of the major sources of decoherence in superconducting qubits. Moreover, quasiparticle poisoning due to high-energy particle impacts can cause correlated errors and thus impede quantum error correction. While there has been substantial progress in understanding the quasiparticle effects in transmons, further research is required for fluxonia. In this study, we inject pair-breaking phonons to create quasiparticles at the Josephson junctions of the fluxonium circuit. By controlling quasiparticle injection, we aim to distinguish the quasiparticle-induced effects from other decoherence channels. We obtain the qubit energy relaxation rate as a function of injection pulse parameters to gain insights into the poisoning dynamics. Additionally, we measure the dependence of energy relaxation and dephasing times on the external flux bias in the presence of injected quasiparticles to differentiate the effects of quasiparticle tunneling across the small and array junctions.