Noise of a superconducting photon detector

Abstract

Advantage of superconducting photon detectors is a much lower noise in comparison to semiconductor counterparts. The purpose of the present work is to advance understanding of the physics of noises in a superconducting nanostructures. We have studied the rate and spectrum of dark counts of an 80-nm wide, 5-nm thick superconducting NbN meander-line and analyzed experimental data considering various noise mechanisms: quantum phase-slips, discrete thermodynamic fluctuations of the superconducting order parameter, and topological fluctuations in the two-dimensional vortex gas. The latter mechanism, i.e. Kosterlitz-Thouless phase transition, is the only one adequately describing the observed noise performance. Besides that, this approach provides clear explanation of the peculiarities of the resistive transition. The other two mechanisms would result in a much stronger current and temperature dependence of the dark count rate than the experiment shows. Basing on our understanding of the noise source, we propose a novel concept of a superconductor submillimeter direct detector. The detector operates in the superconducting state and utilizes current-dependent high-frequency electrical noise that changes under irradiation. The novel aspect is the use of the noise, which generally hampers the performance of conventional detectors, as the physical quantity that itself senses radiation. Predicted noise-equivalent power is 10_-13 W Hz_-1/2 at 4 K and should improve at lower temperatures.