In this work, Shibata et al. demonstrate how ultra-low noise detectors can allow the transmission distance for QKD in optical fiber to be extended. The optimum transmission wavelength for optical fiber is 1.55 μm. Off-the-shelf InGaAs semiconductor single-photon avalanche diodes are inherently noisy, leading to an elevated baseline error rate. The authors exploit superconducting nanowire single-photon detectors, which have emerged as a highly promising alternative for high-speed, low-noise, telecom-wavelength single-photon detection. In this work the NTT team have carefully engineered the nanowire environment to drive down the noise. Low temperature optical filters have been used to reject room temperature blackbody radiation and the detectors are cooled to a chilly 380 milliKelvin. The resultant dark count rate is just 0.01 Hz, orders of magnitude below previous implementations. In a laboratory QKD experiment, the authors have demonstrated that secure key transmission is feasible over up to 72 dB link loss (336 km distance) in optical fiber. This is equivalent to the distance between Washington DC and New York City. This work highlights the importance of emerging photonic technologies in QKD, and the potential of QKD as a real-world technology for secure communications between major cities.
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