Fluctuations in the electron system of a superconductor exposed to a photon flux

TL;DR

This study investigates fluctuations in the electron system of a superconducting aluminium film exposed to 1.54 THz photons, revealing regimes dominated by photon arrival randomness and quasiparticle recombination, with implications for superconducting photon detection.

Contribution

It demonstrates measurement of electron fluctuations in a superconductor caused by photon flux, identifying regimes dominated by photon arrival and quasiparticle dynamics.

Findings

Fluctuations at high power reflect photon arrival randomness, enabling optical efficiency measurement.

At low power, fluctuations are dominated by quasiparticle recombination, allowing lifetime measurement.

Optical efficiency of 48% was achieved in the high-power regime.

Abstract

We report on fluctuations in the electron system, Cooper pairs and quasiparticles, of a superconducting aluminium film. The superconductor is exposed to pair-breaking photons (1.54 THz), which are coupled through an antenna. The change in the complex conductivity of the superconductor upon a change in the quasiparticle number is read out by a microwave resonator. A large range in radiation power can be chosen by carefully filtering the radiation from a blackbody source. We identify two regimes. At high radiation power, fluctuations in the electron system caused by the random arrival rate of the photons are resolved, giving a straightforward measure of the optical efficiency (48%). At low radiation power fluctuations are dominated by excess quasiparticles, the number of which is measured through their recombination lifetime.