Continuous Measurement of Quasiparticle Trapping in Aluminum Nanobridge Josephson Junctions*

Non-equilibrium quasiparticle (QP) populations are a significant source of decoherence in many superconducting quantum circuits. 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. We discuss simulated and experimental 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).