Continuous Real-Time Detection of Quasiparticle Trapping in Aluminum Nanobridge Josephson Junctions

Nonequilibrium quasiparticles are ubiquitous in superconducting electronics. These quasiparticles can trap in the internal Andreev bound states of a phase-biased Josephson junction, providing a mechanism for studying their presence and behavior. We characterize a quasiparticle trapping detector device based on a two-junction aluminum nanobridge superconducting quantum interference device incorporated into a transmission-line resonator. When flux-biased, distinct resonant frequencies develop depending on the trapped quasiparticle number. We demonstrate continuous detection of up to 3 trapped quasiparticles, with detection of a trapped quasiparticle with signal-to-noise ratio of 27 in 5 $\mu$s. We describe initial measurements of quasiparticle behavior and discuss the possible optimization and application of such detector devices.