# Satellite-mediated quantum atmospheric links

## TL;DR

This paper analyzes how Earth's atmosphere affects satellite-based quantum communication links, focusing on signal distortions, transmittance fluctuations, and security implications for quantum protocols.

## Contribution

It provides a detailed geometric and statistical model of atmospheric effects on satellite quantum links, aiding secure quantum communication design.

## Key findings

- Transmittance fluctuations saturate at large zenith angles.
- Atmospheric effects strongly depend on zenith angle.
- Derived transmittance probability distribution for security analysis.

## Abstract

The establishment of quantum communication links over a global scale is enabled by satellite nodes. We examine the influence of Earth's atmosphere on the performance of quantum optical communication channels with emphasis on the downlink scenario. We derive the geometrical path length between a moving low Earth orbit satellite and an optical ground station as a function of the ground observer's zenith angle, his geographical latitude, and the meridian inclination angle of the satellite. We show that the signal distortions due to regular atmospheric refraction, atmospheric absorption, and turbulence have a strong dependence on the zenith angle. The observed saturation of transmittance fluctuations for large zenith angles is explained. The probability distribution of the transmittance for slant propagation paths is derived, which enables us to perform the security analysis of decoy state protocols implemented via satellite-mediated links.