Excitation of superconducting qubits from hot nonequilibrium quasiparticles

J. Wenner, Yi Yin, Erik Lucero, R. Barends, Yu Chen, B. Chiaro, J. Kelly, M. Lenander, Matteo Mariantoni, A. Megrant, C. Neill, P. J. J. O’Malley, D. Sank, A. Vainsencher, H. Wang, T. C. White, A. N. Cleland, and John M. Martinis

Accepted Thursday Mar 14, 2013

Superconducting qubits probe environmental defects such as non-equilibrium quasiparticles, an important source of decoherence. We show that "hot" non-equilibrium 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-neligable increase in the qubit excited state probability Pe. By injecting hot quasiparticles into a qubit, we experimentally measure an increase of Pe in semi-quantitative agreement with the model and rule out the typically assumed thermal distribution.