Probing non-equilibrium quasiparticle populations in superconducting quantum circuits.

Non-equilibrium quasiparticle (QP) populations in superconducting quantum circuits are a limiting source of decoherence. When properly phase biased, nanobridge Josephson junctions provide sub-gap bound states for QPs and so function as QP traps [1]. The occupation of a trap state by a QP alters the inductive contribution of the junction to its host circuit. Thus, the population of QPs trapped in the junction can be directly measured by probing the resonance of a capacitively shunted nanobridge with standard superconducting qubit hardware. By measuring QP trapping in real time and performing statistical analyses, one can infer information about the QP generation, annihilation, and trapping mechanisms. We discuss these single-shot fidelity measurements of QPs trapping in the junctions of a nanobridge SQUID embedded in a superconducting resonator. We further discuss the use of our device as a tool for measuring QP properties and for testing methods of QP mitigation. [1] E. M. Levenson-Falk, F. Kos, R. Vijay, L. Glazman, and I. Siddiqi, Phys. Rev. Lett. 112, 047002 (2014).