Quasiparticle injection dynamics in fluxonium qubits

Quasiparticle tunneling across the Josephson junctions of superconducting qubits is known to cause decoherence. While considerable progress has been made in understanding and mitigating the quasiparticle effects in transmons, the dynamics in fluxonium qubits are less understood. In this work, we create excess quasiparticles in the fluxonium qubits, both at the small and array junctions, by injecting pair-breaking phonons into the chip substrate. With this controlled injection, we can separate the effects caused by the injected quasiparticles from other major sources of decoherence. We measure the energy relaxation rate with injected quasiparticles as a function of external flux bias, allowing us to differentiate the effects of quasiparticle tunneling across the small and array junctions. Furthermore, we disentangle the changes in excitation and de-excitation rates due to the injected quasiparticles around the integer and half-integer flux bias points. This helps us elucidate the temporal dynamics of quasiparticle densities within the small and array junctions.