Limits and Security of Free-Space Quantum Communications

TL;DR

This paper establishes fundamental limits and practical protocols for secure quantum communication over free-space channels, considering atmospheric effects and noise, and demonstrates the near-optimality of certain coherent-state protocols.

Contribution

It combines quantum information, optics, and turbulence theory to derive ultimate rate bounds and evaluates the performance of a practical protocol close to these limits.

Findings

Derived bounds for key and entanglement distribution rates in free-space quantum links.

Showed that a pilot-guided coherent-state protocol approaches these bounds.

Provided analytical tools for security assessment in various channel conditions.

Abstract

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 is typical in fiber-based connections) to the more challenging scenario of a fading channel (as is typical in free-space links).