From quantum jumps to quasiparticle population*

Superconducting quasiparticles (QP) play a dominant role in the relaxation of the fluxonium qubit in the vicinity of the half-flux-quantum bias point. Recent experiments integrating the fluxonium with a quantum-limited amplifier have measured quantum jump trajectories between the ground state and the first excited state. These trajectories show a change in the characteristic lifetime of the fluxonium qubit as a function of time, arising from a change in the number of QP's in the sample [1]. Using a simple model of QP dynamics and their effect on the fluxonium qubit, we can access the QP population with temporal resolution better than a 100 microsecond. Such rapid monitoring of QP dynamics is essential for understanding the sources of QP's and ultimately suppressing them.\\[4pt] [1] ``Non-Poissonian quantum jumps of a fluxonium qubit due to quasiparticle excitations,'' U. Vool, I.M. Pop et al. to be published in PRL 2014.