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Abstract

High-dimensional entanglement with spatial modes of light promises increased security and information capacity over quantum channels. Unfortunately, entanglement decays due to perturbations, corrupting quantum links that cannot be repaired without performing quantum tomography on the channel. Paradoxically, the channel tomography itself is not possible without a working link. Here we overcome this problem with a robust approach to characterize quantum channels by means of classical light. Using free-space communication in a turbulent atmosphere as an example, we show that the state evolution of classically entangled degrees of freedom is equivalent to that of quantum entangled photons, thus providing new physical insights into the notion of classical entanglement. The analysis of quantum channels by means of classical light in real time unravels stochastic dynamics in terms of pure state trajectories, and thus enables precise quantum error correction in short- and long-haul optical communication, in both free space and fibre.

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  • 31 March 2017

    In the version of this Article originally published, a credit line was missing for the image of Maxwell in Fig. 5. It should have read: 'Maxwell image credit: Bettmann / Contributor/ Bettmann / Getty Images.'

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Acknowledgements

We express our gratitude to L. Marrucci for providing us with q-plates. B.N. acknowledges financial support from the National Research Foundation of South Africa and the African Laser Centre. C.R.-G. acknowledges Claude Leon Foundation. B.P.-G., C.R.-G. and R.I.H.-A. acknowledge support from CONACyT.

Author information

Author notes

    • Yingwen Zhang

    Present address: Physics Department, Centre for Research in Photonics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.

Affiliations

  1. School of Physics, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa

    • Bienvenu Ndagano
    • , Benjamin Perez-Garcia
    • , Filippus S. Roux
    • , Melanie McLaren
    • , Carmelo Rosales-Guzman
    • , Othmane Mouane
    •  & Andrew Forbes
  2. Photonics and Mathematical Optics Group, Tecnológico de Monterrey, Monterrey 64849, Mexico

    • Benjamin Perez-Garcia
    •  & Raul I. Hernandez-Aranda
  3. National Metrology Institute of South Africa, Meiring Naude Road, Pretoria, South Africa

    • Filippus S. Roux
  4. CSIR National Laser Centre, PO Box 395, Pretoria 0001, South Africa

    • Yingwen Zhang
  5. School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa

    • Thomas Konrad

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Contributions

The conceptual idea was formulated by A.F. and T.K. The theoretical formalism was laid out by A.F., T.K., F.S.R., B.N. and B.P.-G. The classical experiments were carried out by B.N., B.P.-G., O.M. and C.R.-G., while the quantum experiment was carried out by Y.Z. All authors contributed to the data analysis and interpretation of the results. B.N. wrote the manuscript with inputs from all the authors. A.F. supervised the project.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Andrew Forbes.

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DOI

https://doi.org/10.1038/nphys4003

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