Quantum key distribution (QKD) offers ultimately secure communication based on the laws of quantum mechanics. For the realization of QKD network, it is necessary to increase the maximum distance of QKD. Previously, we have demonstrated the QKD over 200 km (42.1 dB loss) in fiber using superconducting nanowire single photon detectors (SSPDs) [1]. Here, we have successfully improved the S/N ratio of SSPDs by 30 dB [2], and demonstrated the QKD over 336 km (72 dB loss) in fiber [3], which is the today’s world record of QKD distance.

In QKD, faint single photons are usually used as a carrier of information. When the fiber length increases, the loss of the fiber increase and the signals of single photons are masked by the noise. It is impossible to distribute the secure key when the quantum bit error rate (QBER) exceeds a certain level. Therefore, we need a single-photon detector with higher S/N ratio for the long distance QKD. We find that the origin of the dark count rate (DCR) of SSPD is the blackbody radiation at room temperature which propagate through an optical fiber, and introduce the cold optical bandpass filter to block the radiation [2]. It is possible to improve the S/N ratio of SSPDs more than 30 dB using the cold optical filter. Figure 1 shows the system detection efficiency (η) and DCR of the SSPD. At a bias current of 21.5 μA, η = 4.4% and DCR < 0.01 cps is achieved.

Using the SSPDs with ultralow DCR, we perform the QKD experiment under the differential phase shift QKD scheme using a weak coherent light. Figure 2 summarizes the results of our experiment. At the loss of 72 dB (336 km fiber), we can generate the secure keys from the sifted keys since the QBER is still below 4.1% [3].