Nonequilibrium regimes for quasiparticles in superconducting qubits

Bogoliubov quasiparticles are a source of errors for superconducting qubits. These unwanted excitations can exchange energy with the qubit while tunneling across the Josephson junction, leading to decoherence. In junctions with gap-asymmetric electrodes, quasiparticles trapped in the low-gap electrode can be harmless when the gap-asymmetry exceeds the qubit transition energy [1], as confirmed by measurements of correlated errors in multi-qubit devices [2]. For this reason, understanding the energy distribution of nonequilibrium quasiparticles is under active consideration, with recent research showing that Fermi-like distribution can coexist with nonequilibrium density over an extended temperature range [3]. In this presentation, we consider a theoretical model based on effective chemical potentials [4]. Besides capturing the temperature crossover from low-temperature nonequilibrium to thermalization, our description predicts different nonequilibrium regimes depending on the gap asymmetry, the generation rate, and the temperature. Such regimes are characterized by the relation between the chemical potentials. We discuss possible protocols to probe the different regimes and argue how some standard assumptions can lead to a wrong estimate of the qubit parameters. Finally, we discuss potential experimental consequences, also in the context of magnetic-field resilient transmons [5].