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Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors


We report the first quantum key distribution (QKD) experiment to enable the creation of secure keys over 42 dB channel loss and 200 km of optical fibre. We used the differential phase shift QKD (DPS-QKD) protocol, implemented with a 10-GHz clock frequency and superconducting single-photon detectors (SSPD) based on NbN nanowires. The SSPD offers a very low dark count rate (a few Hz) and small timing jitter (60 ps, full width at half maximum, FWHM). These characteristics allowed us to achieve a 12.1 bit s–1 secure key rate over 200 km of fibre, which is the longest terrestrial QKD over a fibre link yet demonstrated. Moreover, this is the first 10-GHz clock QKD system to enable secure key generation. The keys generated in our experiment are secure against both general collective attacks on individual photons and a specific collective attack on multiphotons, known as a sequential unambiguous state discrimination (USD) attack.

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Figure 1: Secure key rate as a function of channel loss.
Figure 2: Experimental set-up for 10-GHz clock DPS-QKD
Figure 3: An SSPD.
Figure 4: Histogram of received 10-GHz clock signal.
Figure 5: DPS-QKD experimental results.


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The authors thank E. Diamanti, M. M. Fejer, G. N. Gol'tsman, E. Ip, J. M. Kahn, G. Kalogerakis, L. G. Kazovsky, N. Y. Kim, C. Langrock, R. V. Roussev and Y. Tokura for their support during this research. Financial support was provided by the CREST and SORST programs of the Japan Science and Technology Agency (JST), the National Institute of Information and Communications Technology (NICT) of Japan, the MURI Center for Photonic Quantum Information Systems (ARO/ARDA DAAD19-03-1-0199), DTO, DARPA and the NIST Quantum Information Science Initiative.

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H. Takesue designed and performed the experiments, analysed the data and wrote the paper, S. W. Nam performed the experiments and analysed the data, Q. Zhang performed the experiments, R. H. Hadfield performed the experiments, T. Honjo analysed the data, K. Tamaki analysed the data, and Y. Yamamoto planned the experiments.

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Correspondence to Hiroki Takesue.

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The authors declare no competing financial interests.

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Takesue, H., Nam, S., Zhang, Q. et al. Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors. Nature Photon 1, 343–348 (2007).

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