Nonequilibrium quasiparticle distribution in superconducting resonators: Effect of pair-breaking photons

SciPost Physics, vol. 17

Abstract

Many superconducting devices rely on the finite gap in the excitation spectrum of a superconductor: thanks to this gap, at temperatures much smaller than the critical one the number of excitations (quasiparticles) that can impact the device’s behavior is exponentially small. Nevertheless, experiments at low temperature usually find a finite, non-negligible density of quasiparticles whose origin has been attributed to various non-equilibrium phenomena. Here, we investigate the role of photons with energy exceeding the pair-breaking threshold 2\Delta 2 Δ as a possible source for these quasiparticles in superconducting resonators. Modeling the interacting system of quasiparticles, phonons, sub-gap and pair-breaking photons using a kinetic equation approach, we find analytical expressions for the quasiparticles’ density and their energy distribution. Applying our theory to measurements of quality factor as function of temperature and for various readout powers, we find they could be explained by assuming a small number of photons above the pair-breaking threshold. We also show that frequency shift data can give evidence of quasiparticle heating.

Authors 2

1. Paul B. Fischer Aachen JARA-Institute for Quantum Information

   RWTH Aachen University · Forschungszentrum Jülich

   Affiliation as printed

   Forschungszentrum Jülich

   RWTH Aachen University

   JARA Institute for Quantum Information, RWTH Aachen University, 52056 Aachen, Germany

2. Gianluigi Catelani

   Forschungszentrum Jülich · Technology Innovation Institute

   Affiliation as printed

   Forschungszentrum Jülich

   Technology Innovation Institute

   Quantum Research Center, Technology Innovation Institute, Abu Dhabi 9639, UAE