Quasiparticle Poisoning of a Superconducting Island by Nonequilibrium Phonons

Quasiparticle excitations are detrimental to the operation of various superconducting devices. The density of quasiparticles should be exponentially suppressed at low temperatures, but commonly a quasiparticle density many orders of magnitude larger than the thermal expectation is observed. Phonons with energy greater than twice the superconducting gap can break Cooper pairs to form quasiparticles, and conversely two quasiparticles recombining to form a Cooper pair will emit such phonons. In our experiment, we demonstrate how the superconducting island of a single-electron transistor is poisoned with quasiparticles and the parity effect is consequently destroyed, when another heated superconducting island is used as an emitter of nonequilibrium phonons, while a thermal phonon emitter, a similarly heated normal metal island, creates much fewer quasiparticles.