Energy decay and frequency shift of a superconducting qubit from   non-equilibrium quasiparticles
  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. 
M. Lenander, H. Wang, Radoslaw C. Bialczak, Erik Lucero, Matteo   Mariantoni, M. Neeley, A. D. O'Connell, D. Sank, M. Weides...
cond-mat.supr-con cond-mat.mes-hall