Probing the effect of superconducting gap profile on quasiparticle dynamics in superconducting qubits

Single-charge tunneling is a decoherence mechanism affecting superconducting qubits. Suppressing the flux of high-energy photons reduces the rate of single-charge tunneling in these devices; however, there remains a nonequilibrium quasiparticle population due to undetermined Cooper-pair breaking mechanisms. We have found that these nonequilibrium quasiparticles are well-thermalized and become trapped in regions of the device with a lower superconducting gap. We measure single-charge tunneling rates in devices with different engineered superconducting gap profiles placed in a shared photon environment. This results in an improved understanding of the effect of superconducting gap profile on the spatial distribution and dynamics of quasiparticles in superconducting qubits.