# Fano fluctuations in superconducting nanowire single-photon detectors

## TL;DR

This paper models how Fano fluctuations influence the photon counting rate in superconducting nanowire single-photon detectors, explaining the smearing of response curves and matching experimental data across various conditions.

## Contribution

It introduces a theoretical framework describing Fano fluctuation effects on SNSPD response, including integral modeling and analysis of width and singularities, validated by experiments.

## Key findings

- Fano fluctuations cause smearing of the PCR vs bias current curve.
- The model accurately predicts the width of the response in narrow wires.
- Good agreement with experimental data across temperatures and photon energies.

## Abstract

Because of their universal nature, Fano fluctuations are expected to influence the response of superconducting nanowire single-photon detectors (SNSPDs). We predict that photon counting rate ($PCR$) as a function of bias current ($I_B$) in SNSPDs is described by an integral over a transverse coordinate-dependent complementary error function. The latter describes smearing of local responses due to Fano fluctuations of the amount of energy deposited into electronic system. The finite width, $\sigma$, of the $PCR$ vs $I_B$ arises from fluctuations in the energy partition between quasiparticles and phonons during the energy down-conversion cascade. In narrow-nanowire SNSPDs the local responses are uniform, and the effect of Fano-fluctuations on $\sigma$ is dominant. In wide-nanowire SNSPDs with strong coordinate dependence of local responses due to vortex-antivortex unbinding and vortex entry from edges, Fano-fluctuations smear singularities imprinted by vorticity on the transition part of $PCR$ curve. We demonstrate good agreement between theory and experiments for a series of bath temperatures and photon energies in narrow-wire WSi SNSPDs. The time-resolved hotspot relaxation curves predicted by Fano fluctuations match the Lorentzian shapes observed in experiments over the whole range of bias currents investigated except for their tails.