Excitation of Superconducting Qubits from Hot Nonequilibrium Quasiparticles

Superconducting qubits probe environmental defects such as nonequilibrium quasiparticles, an important source of decoherence. We show that “hot” nonequilibrium quasiparticles, with energies above the superconducting gap, affect qubits differently from quasiparticles at the gap, implying qubits can probe the dynamic quasiparticle energy distribution. For hot quasiparticles, we predict a non-negligible increase in the qubit excited state probability $P_e$. By injecting hot quasiparticles into a qubit, we experimentally measure an increase of $P_e$ in semiquantitative agreement with the model and rule out the typically assumed thermal distribution.