Ultimate classical communication rates of quantum optical channels

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V. Giovannetti, R. Garcia-Patrón, N. J. Cerf, A. S. Holevo
Nature Photonics 8, 796-800 (2014)

Optical channels, such as fibers or free-space links, are ubiquitous in today's telecommunication networks. A complete physical model of these channels must necessarily take quantum effects into account to determine their ultimate performances. Single-mode, phase-insensitive bosonic Gaussian channels have been extensively studied over past decades, given their importance for practical applications. In spite of this, a long-standing unsolved conjecture on the optimality of Gaussian encodings has prevented finding their classical communication capacity.

In their work, Giovannetti and co-workers solve this conjecture by proving that the vacuum state achieves the minimum output entropy of these channels. This establishes the ultimate achievable bit rate under an energy constraint, as well as the long awaited proof that the single-letter classical capacity of these channels is additive. This result represents a breakthrough in quantum information theory, solving a long-standing open conjecture.