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Overcoming the rate–distance limit of quantum key distribution without quantum repeaters

Abstract

Quantum key distribution (QKD)1,2 allows two distant parties to share encryption keys with security based on physical laws. Experimentally, QKD has been implemented via optical means, achieving key rates of 1.26 megabits per second over 50 kilometres of standard optical fibre3 and of 1.16 bits per hour over 404 kilometres of ultralow-loss fibre in a measurement-device-independent configuration4. Increasing the bit rate and range of QKD is a formidable, but important, challenge. A related target, which is currently considered to be unfeasible without quantum repeaters5,6,7, is overcoming the fundamental rate–distance limit of QKD8. This limit defines the maximum possible secret key rate that two parties can distil at a given distance using QKD and is quantified by the secret-key capacity of the quantum channel9 that connects the parties. Here we introduce an alternative scheme for QKD whereby pairs of phase-randomized optical fields are first generated at two distant locations and then combined at a central measuring station. Fields imparted with the same random phase are ‘twins’ and can be used to distil a quantum key. The key rate of this twin-field QKD exhibits the same dependence on distance as does a quantum repeater, scaling with the square-root of the channel transmittance, irrespective of who (malicious or otherwise) is in control of the measuring station. However, unlike schemes that involve quantum repeaters, ours is feasible with current technology and presents manageable levels of noise even on 550 kilometres of standard optical fibre. This scheme is a promising step towards overcoming the rate–distance limit of QKD and greatly extending the range of secure quantum communications.

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Fig. 1: Scheme to overcome the rate–distance limit of QKD.
Fig. 2: Schematics of the quantum distribution of encryption keys.
Fig. 3: Experimental characterization of phase drift and visibility.

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Acknowledgements

We acknowledge K. Tamaki for constructive criticism on the security argument. We acknowledge discussions with X. Ma, N. Lütkenhaus, B. Fröhlich, R. M. Stevenson, D. G. Marangon and A. J. Bennett.

Reviewer Information

Nature thanks X. Ma and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors

Contributions

M.L. and Z.L.Y. developed the TF-QKD scheme. Z.L.Y. and J.F.D. set up and performed the experiments, and all authors analysed the results. A.J.S. guided the work. M.L. wrote the manuscript with contributions from all authors.

Corresponding author

Correspondence to M. Lucamarini.

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The authors declare no competing interests.

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Supplementary information

Supplementary Information

This file contains the following sections: Fibre-based experiments; Protocol; Active feedback; Numerical simulations; Security argument; Notation; Reduction to a BB84-like scheme; Single-photon source; Actual schemes; Entanglement distillation and virtual schemes; Weak-coherent-pulse source; Dropping the assumption on random phase announcement; Collective beam-splitting attack; Considerations on the key rates in Fig. S3; Final considerations; and References.

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Lucamarini, M., Yuan, Z.L., Dynes, J.F. et al. Overcoming the rate–distance limit of quantum key distribution without quantum repeaters. Nature 557, 400–403 (2018). https://doi.org/10.1038/s41586-018-0066-6

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