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Imaging and Applied Optics

Quantum Communications along the Canary Strongly-Turbulent Optical Link

Ivan Capraro, Davide Bacco, Alberto Dall’Arche, Davide G. Marangon, Francesca Gerlin, Andrea Tomaello, Giuseppe Vallone, Paolo Villoresi

Published Jan 1, 2013• 0 citations
Publisher

Abstract

Investigations on the role of turbulence for Quantum Communications along the 143-km link in both spatial and temporal domain indicate how to exploit constructively its effects. Novel applications with relevant background noise may be envisaged.
DOI
10.1364/pcdvt.2013.ptu1f.4

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Abstract Since the invention of the laser in the 60s, one of the most fundamental communication channels has been the free-space optical channel. For this type of channel, a number of effects generally need to be considered, including diffraction, refraction, atmospheric extinction, pointing errors and, most importantly, turbulence. Because of all these adverse features, the free-space channel is more difficult to study than a stable fiber-based link. For the same reasons, only recently it has been possible to establish the ultimate performances achievable in quantum communications via free-space channels. Differently from previous literature, mainly focused on the regime of weak turbulence, this work considers the free-space optical channel in the more challenging regime of moderate-to-strong turbulence. This regime may occur in long-distance free-space links on the ground, in uplink to high-altitude platform systems (HAPS) and, more interestingly, in downlink from near-horizon satellites. In such a regime we rigorously investigate ultimate limits for quantum communications and show that composable keys can be extracted using continuous variable quantum key distribution. In particular, we apply our results to downlink from satellites at large zenith angles, for which not only turbulence is strong but also refraction causes non-trivial effects in terms of trajectory elongation.
The study of free-space quantum communications requires tools from quantum information theory, optics and turbulence theory. Here we combine these tools to bound the ultimate rates for key and entanglement distribution through a free-space link, where the propagation of quantum systems is generally affected by diffraction, atmospheric extinction, turbulence, pointing errors, and background noise. Besides establishing ultimate limits, we also show that the composable secret-key rate achievable by a suitable (pilot-guided and post-selected) coherent-state protocol is sufficiently close to these limits, therefore showing the suitability of free-space channels for high-rate quantum key distribution. Our work provides analytical tools for assessing the composable finite-size security of coherent-state protocols in general conditions, from the standard assumption of a stable communication channel (as typical in fiber-based connections) to the more challenging scenario of a fading channel (as typical in free-space links).