Energy decay and frequency shift of a superconducting qubit from non-equilibrium quasiparticles

Understand

Quasiparticles are an important decoherence mechanism in superconducting qubits, and can be described with a complex admittance that is a generalization of the Mattis-Bardeen theory.

• By injecting non-equilibrium quasiparticles with a tunnel junction, we verify qualitatively the expected change of the decay rate and frequency in a phase qubit.
• With their relative change in agreement to within 4% of prediction, the theory can be reliably used to infer quasiparticle density.
• We describe how settling of the decay rate may allow determination of whether qubit energy relaxation is limited by non-equilibrium quasiparticles.