Counting nonequilibrium quasiparticles in a superconductor

Superconductivity is based on the Cooper pairing of electrons, which enables flow of electrical current without resistance, expulsion of magnetic fields in the Meissner effect, as well as electrical circuits obeying the rules of quantum mechanics. Such superconducting circuits are currently under intensive study as building blocks of quantum computers, among other applications. The performance of many superconducting devices is limited by spurious unpaired electrons known as nonequilibrium quasiparticles. In this thesis, we study nonequilibrium quasiparticle excitations in mesoscopic superconductors using charge detectors based on single-electron devices. In the first part of this thesis, we present background material on superconductivity and single-electron devices, and review the literature on excess nonequilibrium quasiparticles. We then describe the experimental methods used to fabricate and measure the devices studied in this thesis, presenting a new technique for fabricating high-quality aluminum oxide tunnel barriers to a semiconducting nanowire covered with epitaxially grown aluminum, as well as a method for analyzing the switching rates of a random telegraph signal such as that resulting from tunneling of electrons. The next part of this thesis focuses on charge detectors based on single-electron devices. We have improved our measurement setup by integrating a Josephson parametric amplifier with ultralow added noise. As an application of single-electron devices and charge detection in stochastic thermodynamics, we quantify the work that can be extracted beyond the free-energy difference from a single-electron box. We also measure in real time Cooper pair splitting, in which the two electrons of a Cooper pair simultaneously tunnel to spatially separated normal metal islands whose charge is monitored. In the last part, we present results on quasiparticle excitations. We have found that the origins of quasiparticles on a mesoscopic superconductor may be dominated by the backaction of charge detectors. Investigating the backaction mechanism, we find that it is most likely mediated by nonequilibrium phonons. We then show that a mesoscopic superconductor can be used as a detector of Cooper-pair-breaking radiation, such as phonons, in a mode where the response is self-calibrating: inferring the rate of Cooper pairs broken does not require detailed knowledge of the device parameters. Finally, we show that a superconductor can be completely free of quasiparticles for times up to seconds, five orders of magnitude longer than the timescales for qubit operations. This is done by monitoring the instantaneous quasiparticle number on a mesoscopic island with a fast charge detector. In our experiment, the Cooper pair breaking rate decayed over timescales of weeks, which rules out all the previously suggested origins of quasiparticle excitations.