Undoing the effect of loss on quantum entanglement

Journal name:
Nature Photonics
Year published:
Published online


Entanglement distillation, the purpose of which is to probabilistically increase the strength and purity of quantum entanglement, is a primary element of many quantum communication and computation protocols. It is particularly necessary in quantum repeaters in order to counter the degradation of entanglement that inevitably occurs due to losses in communication lines. Here, we distil the Einstein–Podolsky–Rosen state of light, the workhorse of continuous-variable entanglement, using noiseless amplification. The advantage of our technique is that it permits recovering a macroscopic level of entanglement, however low the initial entanglement or however high the loss may be. Experimentally, we recover the original entanglement level after one of the Einstein–Podolsky–Rosen modes has experienced a loss factor of 20. The level of entanglement in our distilled state is higher than that achievable by direct transmission of any state through a similar loss channel. This is a key step towards realizing practical continuous-variable quantum communication protocols.

At a glance


  1. Experimental set-up.
    Figure 1: Experimental set-up.

    Inset: Conceptual scheme. The NLA distillation event corresponds to the click in the single-photon detector placed in the beamsplitter output.

  2. Experimental results.
    Figure 2: Experimental results.

    a, A total of 10,000 samples of raw position quadrature data for the original two-mode squeezed state (top) and the distilled state after applying the two methods of degrading the entanglement (reducing the initial squeezing, middle; asymmetric loss, bottom). The NLA gain is 6.5 for the middle plot and 10 for the bottom. The degree of correlation is similar in all three cases. The quadratures exhibit non-classical correlations at a level below the shot noise (dashed circle). b,c, Analysis of the distilled states as a function of NLA gain. Left columns: the case of reduced initial squeezing. Right column: the case of asymmetric loss. In b, two-mode squeezing is displayed, measured by the variances of the sum (anti-squeezing) and difference (squeezing) of the position quadratures in the two channels of the distilled EPR state. In c, the logarithmic negativity is shown. The vertical axes in b are scaled in units of standard quantum limit. The theoretical curves for the case of low initial squeezing are calculated assuming the initial squeezing parameter of γ = 0.05, detection efficiencies in the undistilled and distilled channels of ηA = 0.5 and ηB = 0.5, respectively, and a single-photon preparation efficiency of η = 0.65 (refs 15,28,29). For the case of a loss channel, γ = 0.135 and ηA = 0.45; other parameters are the same. The efficiency parameters are defined in Supplementary Fig. 2.


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Author information

  1. These authors contributed equally to this work

    • Alexander E. Ulanov &
    • Ilya A. Fedorov


  1. Russian Quantum Center, 100 Novaya St., Skolkovo, Moscow 143025, Russia

    • Alexander E. Ulanov,
    • Ilya A. Fedorov,
    • Anastasia A. Pushkina,
    • Yury V. Kurochkin &
    • A. I. Lvovsky
  2. Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology, Chengdu, Sichuan 610054, China

    • Alexander E. Ulanov &
    • A. I. Lvovsky
  3. Moscow Institute of Physics and Technology, Institutsky Lane 9, Dolgoprudny 141700, Russia

    • Alexander E. Ulanov &
    • Anastasia A. Pushkina
  4. P.N. Lebedev Physics Institute, Leninskiy Prospect 53, Moscow 119991, Russia

    • Ilya A. Fedorov,
    • Anastasia A. Pushkina &
    • A. I. Lvovsky
  5. Centre for Quantum Computation and Communication Technology, School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia

    • Timothy C. Ralph
  6. Institute for Quantum Science and Technology, University of Calgary, Calgary Alberta T2N 1N4, Canada

    • A. I. Lvovsky
  7. Quantum Information Science Program, Canadian Institute for Advanced Research, Toronto, Ontario M5G 1Z8, Canada

    • A. I. Lvovsky


The experiment was conceived and designed by A.E.U., I.F., Y.K., T.C.R. and A.L., and performed by A.E.U., I.F., Y.K., A.A.P. and A.L. The data were analysed by A.E.U., I.F. and A.L. A.E.U., I.F., Y.K., T.C.R. and A.L. contributed to writing the paper.

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