Detecting parity effect in a superconducting device in the presence of parity switches

TL;DR

This paper demonstrates a superconducting device that exhibits a persistent parity effect despite quasiparticle tunneling, with real-time detection revealing the influence of measurement backaction.

Contribution

It introduces a superconducting device showing a parity effect under quasiparticle tunneling and identifies backaction from the charge detector as a cause of pair breaking.

Findings

Parity effect persists with single quasiparticle presence

Backaction from charge detector causes pair breaking

Device is quasiparticle-free 97% of the time

Abstract

We present a superconducting device showing a clear parity effect in the number of electrons, even when there is, on average, a single nonequilibrium quasiparticle present and the parity of the island switches due to quasiparticles tunneling in and out of the device at rates on the order of 100 Hz. We detect the switching by monitoring in real time the charge state of a superconducting island connected to normal leads by tunnel junctions. The quasiparticles are created by Cooper pairs breaking on the island at a rate of a few kHz. We demonstrate that the pair breaking is caused by the backaction of the single-electron transistor used as a charge detector. With sufficiently low probing currents, our superconducting island is free of quasiparticles 97% of the time.