Limits and security of free-space quantum communications

The study of free-space quantum communications requires tools from information theory, optics, and turbulence theory. Here we combine these to bound the ultimate rates for key entanglement distribution through a link, where propagation systems is generally affected by diffraction, atmospheric extinction, turbulence, pointing errors, background noise. Besides establishing limits, also show that composable secret-key rate achievable suitable (pilot-guided postselected) coherent-state protocol sufficiently close therefore showing suitability channels high-rate distribution. Our paper provides analytical assessing finite-size security protocols in general conditions standard assumption stable communication channel (as typical fiber-based connections) more challenging scenario fading links).Received 9 September 2020Accepted 5 March 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.013279Published American Physical Society under terms Creative Commons Attribution 4.0 International license. Further this work must maintain attribution author(s) published article's title, journal citation, DOI.Published SocietyPhysics Subject Headings (PhySH)Research AreasQuantum cryptographyQuantum entanglementQuantum Information