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Twin-field quantum key distribution over a 511 km optical fibre linking two distant metropolitan areas


The basic principle of quantum mechanics1 guarantees the unconditional security of quantum key distribution (QKD)2,3,4,5,6 at the cost of forbidding the amplification of a quantum state. As a result, and despite remarkable progress in worldwide metropolitan QKD networks7,8 over the past decades, a long-haul fibre QKD network without a trusted relay has not yet been achieved. Here, through the sending-or-not-sending protocol9, we achieve twin-field QKD10 and distribute secure keys without any trusted repeater over a 511 km long-haul fibre trunk that links two distant metropolitan areas. The fibre trunk contains 12 fibres in the cable, three of which are used for the quantum channel, optical synchronization and frequency locking, respectively. The remaining nine are used for classical fibre communication. Our secure key rate is around three orders of magnitude greater than that expected if the previous QKD field-test system was applied over the same length. Efficient quantum-state transmission and stable single-photon interference over such a long-haul deployed fibre pave the way to large-scale fibre quantum networks.

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Fig. 1: Field deployment of SNS-TF-QKD.
Fig. 2: Fluctuation of deployed fibre.
Fig. 3: Secure key rates of the SNS-TF-QKD experiment.


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This work was supported by the National Key R&D Program of China (grant numbers 2020YFA0309800, 2017YFA0303900 and 2017YFA0304000), the National Natural Science Foundation of China, the Chinese Academy of Sciences (CAS), the Key R&D Plan of Shandong Province (grant numbers 2019JZZY010205 and 2020CXGC010105), the Taishan Scholar Program of Shandong Province and the Anhui Initiative in Quantum Information Technologies.

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Authors and Affiliations



Y.L., X.-B.W., Q.Z. and J.-W.P. conceived the research. W.-J.Z., H.Li., L.-X.Y. and Z.W. developed the SNSPD. J.-P.C., C.Z., Y.-H.L. and H.-F.J. implemented the stable laser system and the wavelength-locking system in the field. J.-P.C., C.Z., Z.-Y.H. and Q.Z. performed the field experiments. C.J., X.-L.H. and X.-B.W. developed the theory and calculated the secure key rate. J.-P.C., S.-Z.M., H.Liu., Z.-Y.H., F.Z. and T.-Y.C. arranged and tested the field fibre. All authors performed the data analysis and prepared the manuscript.

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Correspondence to Xiang-Bin Wang, Qiang Zhang or Jian-Wei Pan.

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The authors declare no competing interests.

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Peer review informationNature Photonics thanks Charles C.-W. Lim and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figs. 1–5, Discussion and Tables 1–5.

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Chen, JP., Zhang, C., Liu, Y. et al. Twin-field quantum key distribution over a 511 km optical fibre linking two distant metropolitan areas. Nat. Photon. 15, 570–575 (2021).

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