Quasiparticle number fluctuations in superconductors

TL;DR

This paper develops a comprehensive theoretical framework for understanding quasiparticle number fluctuations in superconductors, using master equations to model intrinsic and extrinsic fluctuation processes, with applications to experimental measurements.

Contribution

It introduces a multivariate master equation approach to describe quasiparticle fluctuations, extending previous models to include extrinsic loss mechanisms.

Findings

The theory accurately describes intrinsic quasiparticle fluctuations.

It accounts for extrinsic loss processes affecting fluctuations.

Application to time-resolved measurements in superconducting aluminum.

Abstract

We present a general theory of quasiparticle number fluctuations in superconductors. The theory uses the master equation formalism. First, we develop the theory for a single occupation variable. Although this simple system is insufficient to describe fluctuations in a physical superconductor, it is illustrative, allowing this discussion to serve as a self-contained introduction. We go on to develop a multivariate theory that allows for an arbitrary number of levels with transitions of arbitrary size between levels. We specialize the multivariate theory for two particular cases. First, we consider intrinsic quasiparticle fluctuations. In a previous Letter, these results were used to describe time-resolved measurements of thermodynamic fluctuations in a superconducting Al box [C.M. Wilson, L. Frunzio and D.E. Prober, Phys. Rev. Lett. 87, 067004 (2001)]. Finally, we extend these results to include fluctuations due to extrinsic loss processes.