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Physica status solidi. C, Conferences and critical reviews/Physica status solidi. C, Current topics in solid state physics

Fluctuations and dark count rates in superconducting NbN single‐photon detectors

Andreas Engel, Alexei D. Semenov, Heinz‐Wilhelm Hübers, K. Ilin, M. Siegel

Published Mar 1, 2005• 14 citations
Publisher

Abstract

We measured the temperature- and current-dependence of dark count rates of a superconducting singlephoton detector. The detector's key element is a 84 nm wide meander strip line fabricated from a 5 nm thick NbN film. Due to its reduced dimensions various types of fluctuations can cause temporal and localized transitions into a resistive state leading to dark count events. Adopting a recent refinement of the hotspot model we achieve a satisfying description of the experimental dark count rates taking into account fluctuations of the Cooper-pair density and current-assisted unbinding of vortex-antivortex pairs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI
10.1002/pssc.200460809

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We measured the temperature- and current-dependence of dark count rates of a superconducting singlephoton detector. The detector's key element is a 84 nm wide meander strip line fabricated from a 5 nm thick NbN film. Due to its reduced dimensions various types of fluctuations can cause temporal and localized transitions into a resistive state leading to dark count events. Adopting a recent refinement of the hotspot model we achieve a satisfying description of the experimental dark count rates taking into account fluctuations of the Cooper-pair density and current-assisted unbinding of vortex-antivortex pairs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
We present results on dark count rates and spectral sensitivities of superconducting single-photon detectors in the visible and near-infrared spectral range. The active detector element is a nanometre-sized (a few nanometres thick and less than 100nm wide) meander line carrying a supercurrent. The superconducting materials are NbN and Nb, respectively. The NbN detector exhibited a flat spectral sensitivity up to about 2.4μm. Fluctuations of the superconducting order parameter are considered as a major source of dark count events. A simple model and its limitations to explain the observed dark count rates is discussed.
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.
We present our studies on dark counts, observed as transient voltage pulses, in current-biased NbN superconducting single-photon detectors (SSPDs), as well as in ultrathin (~4 nm), submicrometer-width (100 to 500 nm) NbN nanobridges. The duration of these spontaneous voltage pulses varied from 250 ps to 5 ns, depending on the device geometry, with the longest pulses observed in the large kinetic-inductance SSPD structures. Dark counts were measured while the devices were completely isolated (shielded by a metallic enclosure) from the outside world, in a temperature range between 1.5 and 6 K. Evidence shows that in our two-dimensional structures the dark counts are due to the depairing of vortex-antivortex pairs caused by the applied bias current. Our results shed some light on the vortex dynamics in 2D superconductors and, from the applied point of view, on intrinsic performance of nanostructured SSPDs.
We perform measurements of the switching current distributions of three w ≈ 120 nm wide, 4 nm thick NbN superconducting strips which are used for single-photon detectors. These strips are much wider than the diameter of the vortex cores, so they are classified as quasi-two-dimensional (quasi-2D). We discover evidence of macroscopic quantum tunneling by observing the saturation of the standard deviation of the switching distributions at temperatures around 2 K. We analyze our results using the Kurkijärvi-Garg model and find that the escape temperature also saturates at low temperatures, confirming that at sufficiently low temperatures, macroscopic quantum tunneling is possible in quasi-2D strips and can contribute to dark counts observed in single photon detectors. At the highest temperatures the system enters a multiple phase-slip regime. In this range single phase-slips are unable to produce dark counts and the fluctuations in the switching current are reduced.