As the size of superconducting structures in one or more dimensions are more and more reduced fluctuations in the superconducting phase become increasingly important. These fluctuations lead to noise in thin-film or nano-structured detectors and ultimately limit their sensitivity. Fluctuating quasi-particle numbers and temporarily created vortex-antivortex pairs are two possible noise-sources near the superconducting phase transition. We considered thermodynamic fluctuations occurring with a probability proportional to exp(-ΔF/k_BT), where ΔF is the change in free energy, k_B the Boltzmann constant and T the temperature. ΔF itself is temperature and current dependent in a non-trivial way. We will present model calculations and compare the bias-current and temperature dependence with experimental dark count rates obtained for a NbN single-photon detector. The detector consisted of a thin and narrow meander, which was biased with a direct current slightly less than the temperature dependent critical current. Sufficiently large fluctuations caused voltage transients very similar to those caused by absorbed visible and near-infrared photons and are the major contribution to the measured dark count rates.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/1745 must be provided.

1. Influence of line width on critical current of Nb and NbN thin-film bridges