Quantum communications promise to revolutionize the way information is exchanged and protected. Unlike their classical counterpart, they are based on dim optical pulses that cannot be amplified by conventional optical repeaters. Consequently, they are heavily impaired by propagation channel losses, confining their transmission rate and range below a theoretical limit known as repeaterless secret key capacity. Overcoming this limit with today’s technology was believed to be impossible until the recent proposal of a scheme that uses phase-coherent optical signals and an auxiliary measuring station to distribute quantum information. Here, we experimentally demonstrate such a scheme for the first time and over significant channel losses, in excess of 90 dB. In the high loss regime, the resulting secure key rate exceeds the repeaterless secret key capacity, a result never achieved before. This represents a major step in promoting quantum communications as a dependable resource in today’s world.

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The authors acknowledge useful discussions with M. Curty about the protocol in ref. 26. M.M. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) and Toshiba Research Europe Ltd. M.P. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 675662. G.L.R. acknowledges financial support via the EPSRC funded Centre for Doctoral Training (CDT) in Integrated Photonic and Electronic Systems, Toshiba Research Europe Ltd and The Royal Commission for the Exhibition of 1851.

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Author notes

  1. These authors contributed equally: M. Minder, M. Pittaluga.


  1. Toshiba Research Europe Ltd, Cambridge, UK

    • M. Minder
    • , M. Pittaluga
    • , G. L. Roberts
    • , M. Lucamarini
    • , J. F. Dynes
    • , Z. L. Yuan
    •  & A. J. Shields
  2. Department of Engineering, Cambridge University, Cambridge, UK

    • M. Minder
    •  & G. L. Roberts
  3. School of Electronic and Electrical Engineering, University of Leeds, Leeds, UK

    • M. Pittaluga


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M.M. and M.P. developed the experimental set-up, performed the measurements and analysed the data. G.L.R. and J.F.D. supported the experimental work. Z.L.Y., M.L. and A.J.S. guided the work. M.L., M.M. and M.P. provided the simulations and wrote the manuscript, with contributions from all the authors.

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The authors declare no competing interests.

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Correspondence to M. Lucamarini.

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