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Słysz, W., Wegrzecki, M., Bar, J., Grabiec, P., Górska, M., Zwiller, V., et al. (2007). Fibre-coupled, single photon detector based on NbN superconducting nanostructures for quantum communications. J. Modern Opt., 54(2-3), 315–326.
Abstract: We present a novel, two-channel, single photon receiver based on two fibre-coupled, NbN, superconducting, single photon detectors (SSPDs). The SSPDs are nanostructured superconducting meanders and are known for ultrafast and efficient detection of visible-to-infrared photons. Coupling between the NbN detector and optical fibre was achieved using a micromechanical photoresist ring placed directly over the SSPD, holding the fibre in place. With this arrangement, we obtained coupling efficiencies up to ∼30%. Our experimental results showed that the best receiver had a near-infrared system quantum efficiency of 0.33% at 4.2 K. The quantum efficiency increased exponentially with the photon energy increase, reaching a few percent level for visible-light photons. The photoresponse pulses of our devices were limited by the meander high kinetic inductance and had the rise and fall times of approximately 250 ps and 5 ns, respectively. The receiver's timing jitter was in the 37 to 58 ps range, approximately 2 to 3 times larger than in our older free-space-coupled SSPDs. We stipulate that this timing jitter is in part due to optical fibre properties. Besides quantum communications, the two-detector arrangement should also find applications in quantum correlation experiments.
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Verevkin, A., Pearlman, A., Slysz, W., Zhang, J., Currie, M., Korneev, A., et al. (2004). Ultrafast superconducting single-photon detectors for near-infrared-wavelength quantum communications. J. Modern Opt., 51(9-10), 1447–1458.
Abstract: The paper reports progress on the design and development of niobium-nitride, superconducting single-photon detectors (SSPDs) for ultrafast counting of near-infrared photons for secure quantum communications. The SSPDs operate in the quantum detection mode, based on photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-width superconducting stripe. The devices are fabricated from 3.5 nm thick NbN films and kept at cryogenic (liquid helium) temperatures inside a cryostat. The detector experimental quantum efficiency in the photon-counting mode reaches above 20% in the visible radiation range and up to 10% at the 1.3–1.55 μn infrared range. The dark counts are below 0.01 per second. The measured real-time counting rate is above 2 GHz and is limited by readout electronics (the intrinsic response time is below 30 ps). The SSPD jitter is below 18 ps, and the best-measured value of the noise-equivalent power (NEP) is 2 × 10−18 W/Hz1/2. at 1.3 μm. In terms of photon-counting efficiency and speed, these NbN SSPDs significantly outperform semiconductor avalanche photodiodes and photomultipliers.
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Smirnov, K., Divochiy, A., Vakhtomin, Y., Morozov, P., Zolotov, P., Antipov, A., et al. (2018). NbN single-photon detectors with saturated dependence of quantum efficiency. Supercond. Sci. Technol., 31(3), 035011 (1 to 8).
Abstract: The possibility of creating NbN superconducting single-photon detectors with saturated dependence of quantum efficiency (QE) versus normalized bias current was investigated. It was shown that the saturation increases for the detectors based on finer films with a lower value of Rs300/Rs20. The decreasing of Rs300/Rs20 was related to the increasing influence of quantum corrections to conductivity of superconductors and, in turn, to the decrease of the electron diffusion coefficient. The best samples have a constant value of system QE 94% at Ib/Ic ~ 0.8 and wavelength 1310 nm.
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Lipatov, A., Okunev, O., Smirnov, K., Chulkova, G., Korneev, A., Kouminov, P., et al. (2002). An ultrafast NbN hot-electron single-photon detector for electronic applications. Supercond. Sci. Technol., 15(12), 1689–1692.
Abstract: We present the latest generation of our superconducting single-photon detector (SPD), which can work from ultraviolet to mid-infrared optical radiation wavelengths. The detector combines a high speed of operation and low jitter with high quantum efficiency (QE) and very low dark count level. The technology enhancement allows us to produce ultrathin (3.5 nm thick) structures that demonstrate QE hundreds of times better, at 1.55 μm, than previous 10 nm thick SPDs. The best, 10 × 10 μm2, SPDs demonstrate QE up to 5% at 1.55 μm and up to 11% at 0.86 μm. The intrinsic detector QE, normalized to the film absorption coefficient, reaches 100% at bias currents above 0.9 Ic for photons with wavelengths shorter than 1.3 μm.
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Il'in, K. S., Verevkin, A. A., Gol'tsman, G. N., & Sobolewski, R. (1999). Infrared hot-electron NbN superconducting photodetectors for imaging applications. Supercond. Sci. Technol., 12(11), 755–758.
Abstract: We report an effective quantum efficiency of 340, responsivity >200 A W-1 (>104 V W-1) and response time of 27±5 ps at temperatures close to the superconducting transition for NbN superconducting hot-electron photodetectors (HEPs) in the near-infrared and optical ranges. Our studies were performed on a few nm thick NbN films deposited on sapphire substrates and patterned into µm-size multibridge detector structures, incorporated into a coplanar transmission line. The time-resolved photoresponse was studied by means of subpicosecond electro-optic sampling with 100 fs wide laser pulses. The quantum efficiency and responsivity studies of our photodetectors were conducted using an amplitude-modulated infrared beam, fibre-optically coupled to the device. The observed picosecond response time and the very high efficiency and sensitivity of the NbN HEPs make them an excellent choice for infrared imaging photodetectors and input optical-to-electrical transducers for superconducting digital circuits.
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