Appl. Phys. Express 6, 072801 (2013)
Superconducting single-photon detectors (SSPDs) are widely used as high-performance devices in many fields, including quantum information and quantum optics. The figure of merit of SSPDs is defined in terms of the dark count rate (DCR) as η/(DCR × Δt), where η is the detection efficiency and Δt is the timing jitter. The DCR is important because it governs the quantum bit error rate in quantum key distribution experiments. Now, Hiroyuki Shibata and colleagues from NTT Basic Research Laboratories, Japan, have demonstrated an SSPD composed of NbN that has an ultralow DCR of less than 1 cps. They found that the background signal at room temperature was produced by blackbody radiation propagating through the optical fibre to the SSPD. To reduce the DCR, they connected a coarse wavelength-division-multiplexer filter and a 1.6 μm rejection short-wavelength-pass filter in series to the SSPD and cooled the device to 0.38 K. On inserting the filters, at a bias current of 22 μA, the DCR of the system decreased from 14 cps to 0.1 cps, and the system detection efficiency decreased from 8.1% to 5.6%. As a consequence, the figure of merit increased by a factor of 97. Numerical calculations indicate that if the SSPD system is connected to a standard telecommunication optical fibre under the above-mentioned conditions, it can potentially support quantum key distribution over a distance of 300 km.
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