Microwave-induced excess quasiparticles in superconducting resonators measured through correlated conductivity fluctuations

TL;DR

This study measures how microwave power influences quasiparticle density and lifetime in aluminium superconducting resonators, revealing correlated fluctuations that enable highly sensitive detection of superconductor properties.

Contribution

It introduces a method to measure quasiparticle dynamics via correlated conductivity fluctuations, enhancing understanding of microwave-induced quasiparticle behavior.

Findings

Quasiparticle density decreases with reduced microwave power.

Quasiparticle lifetime increases as power decreases.

Fluctuations in the superconductor can be detected with near-vacuum noise sensitivity.

Abstract

We have measured the number of quasiparticles and their lifetime in aluminium superconducting microwave resonators. The number of excess quasiparticles below 160 mK decreases from 72 to 17 $\mu$m$^{-3}$ with a 6 dB decrease of the microwave power. The quasiparticle lifetime increases accordingly from 1.4 to 3.5 ms. These properties of the superconductor were measured through the spectrum of correlated fluctuations in the quasiparticle system and condensate of the superconductor, which show up in the resonator amplitude and phase respectively. Because uncorrelated noise sources vanish, fluctuations in the superconductor can be studied with a sensitivity close to the vacuum noise.