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IEEE Transactions on Applied Superconductivity

SQUID-Readout for the Superconducting Nanowire Single-Photon Detector

A. Kirste, Alexey Semenov, Heinz‐Wilhelm Hübers, D. Drung, M. Peters, T. Schurig, K. Ilin, M. Siegel

Published Jun 1, 2009• 6 citations
Publisher

Abstract

Superconducting single-photon detectors from thin niobium nitride nanostrips exhibit a near-infrared cut-off of the wavelength-independent quantum efficiency along with a moderate energy resolution in the wavelength range around the cut-off. For the wavelength range before the cut-off, we have developed the electro-thermal model that describes the initial growth, diminution and disappearance of the normal domain created by a photon in the superconducting nanostrip. The response parameters extracted from the model fit were used to optimize the signal-to-noise ratio of the SQUID-based readout and to achieve in the temperature range from 1 to 4 K a photon counting frequency of 20 MHz and a dark count rate less than four in one hour.

DOI
10.1109/tasc.2009.2018144

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Superconducting single-photon detectors from thin niobium nitride nanostrips exhibit a near-infrared cut-off of the wavelength-independent quantum efficiency along with a moderate energy resolution in the wavelength range around the cut-off. For the wavelength range before the cut-off, we have developed the electro-thermal model that describes the initial growth, diminution and disappearance of the normal domain created by a photon in the superconducting nanostrip. The response parameters extracted from the model fit were used to optimize the signal-to-noise ratio of the SQUID-based readout and to achieve in the temperature range from 1 to 4 K a photon counting frequency of 20 MHz and a dark count rate less than four in one hour.
Superconducting single-photon detectors from thin niobium nitride nanostrips exhibit a cut-off of the wavelength-independent quantum efficiency along with a moderate energy resolution in the near-infrared spectral range. Before the cut-off, the intrinsic quantum efficiency of the detector reaches ≈30% of the ultimate value, which is physically limited to the absorbance of the detector structure. The intrinsic quantum efficiency is most likely controlled by non-homogeneities of the niobium nitride films. We have developed an electro-thermal model of the detector response that allowed us to optimize the SQUID-based readout and to achieve, in the temperature range from 1 to 6 K, the photon count rate 3 × 107 s−1 and a dark count rate less than 10−4 s−1.
Abstract Superconducting Nanowire Single-Photon Detectors (SNSPDs) are key building blocks for photonic quantum technologies due to their ability to detect single photons with ultra-high efficiency, low dark counts and fast temporal resolution. Superconducting materials exhibiting high uniformity, large absorption cross-section and atomic-scale thickness are desirable to extend single-photon detection from the near-infrared up to the terahertz regime, where existing material choices are especially constrained. Substrate independence would further open the way to integrating detectors onto functional materials and heterostructures, enhancing performance and enabling proximal readout of a wide range of individual excitations. Here, we pattern the prototypical two-dimensional superconductor niobium diselenide (NbSe 2 ) into few-layer nanowires less than 100 nm wide and demonstrate single-photon detection at 780 nm and 1550 nm. At the same time, the dark-count rate remains below 1 Hz up to the switching current and we achieve a timing jitter below 50 ps. We estimate via a diffusive hot-spot model that materials and geometric parameters would allow a theoretical cut-off wavelength in the millimetre range, assuming state-of-the-art device parameters. Our results open up routes toward quantum-limited detectors integrated into quantum-photonic circuits and quantum devices, with the potential for novel detection capabilities and unprecedented energy sensitivity.
Single photon detectors based on meanders from 5-nm thin B1 niobium nitride nanostrips show moderate spectral resolution along with the spectral cut-off of the quantum efficiency occurring in the near-infrared spectral range. Beyond the cut-off there is a gradual decrease of the quantum efficiency. To describe the response beyond the cut-off we combine the previously developed hot-spot detection mechanism with the photon-induced unbinding of the vortex-antivortex pairs, which appear in the superconducting film below the Kosterlitz-Thouless phase transition. The detector responds to the absorption of a single photon with a voltage pulse whose amplitude depends on the photon energy. We report the observation of a photon-energy dependent statistical distribution of the pulse amplitude that defines the energy resolution of the detector. We analyzed the detector response to continuous and femtosecond-pulse radiation using broadband microwave amplifiers and a fast single-shot oscilloscope as well as integrating SQUID amplifiers. Our analysis suggests that both the amplitude and the duration of the response pulses are jointly controlled via the kinetic inductance of the meander line and the Joule power dissipated in the meander by the bias current.
Superconducting Nanowire Single Photon Detectors (SNSPD) are very fast, low noise devices, that detect single photons over a broad spectral range with practical quantum efficiency. The quantum efficiency drops at a certain deposited energy abruptly, the so called cutoff energy or the corresponding cutoff wavelength. The most common material to fabricate these nanowires is niobium nitride (NbN). A promising material, that has not been used for this purpose before is tantalum nitride (TaN). Many material properties of TaN and NbN are similar, but TaN has a smaller energy gap. This should make the device more sensitive to photons with lower energies. In this thesis, SNSPDs made of NbN and TaN were characterized, the quantum efficiencies were measured and the cutoff criteria were determined and compared. With the use of TaN the cutoff wavelength could be increased significantly. Measurements of quantum efficiency and dark-count rates at several temperatures and operation conditions are presented and discussed. Furthermore, the cutoff criteria are determined and compared between the materials mentioned as well as the experimental results are compared to the theoretical predictions of the detection model.
Niobium nitride superconducting nanowire single-photon detectors were fabricated on thermally oxidized silicon substrates with large active areas of 30 μm× 30 μm.To achieve non-constricted detectors,we improved the film growth and electron beam lithography process to fabricate uniform 100-nm wide Nb N nanowires with a fill factor of 50%.The devices showed 72.4% system detection efficiency(SDE) at 100-Hz dark count rate(DCR) and 74-ps timing jitter,measured at the fiber communication wavelength of 1550 nm.The highest SDE which is 81.2% when the DCR is ～ 700 c/s appears at the wavelength of 1650 nm.
The spectral detection efficiency and the dark count rate of superconducting nanowire single-photon detectors (SNSPD) have been studied systematically on detectors made from thin NbN films with different chemical compositions. Reduction of the nitrogen content in the 4 nm thick NbN films results in a decrease of the dark count rates more than two orders of magnitude and in a red shift of the cut-off wavelength of the hot-spot SNSPD response. The observed phenomena are explained by an improvement of uniformity of NbN films that has been confirmed by a decrease of resistivity and an increase of the ratio of the measured critical current to the depairing current. The latter factor is considered as the most crucial for both the cut-off wavelength and the dark count rates of SNSPD. Based on our results we propose a set of criteria for material properties to optimize SNSPD in the infrared spectral region.