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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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.
The authors declare no competing financial interests.
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Ndagano, B., Perez-Garcia, B., Roux, F. et al. Characterizing quantum channels with non-separable states of classical light. Nature Phys 13, 397–402 (2017). https://doi.org/10.1038/nphys4003
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