# Adaptive optics benefit for quantum key distribution uplink from ground to a satellite

## TL;DR

This paper models how adaptive optics can significantly improve the efficiency of quantum key distribution from ground to satellite by compensating atmospheric turbulence, thereby enhancing secure communication distances.

## Contribution

It demonstrates the potential of adaptive optics with laser guide stars to substantially increase link transmissivity in ground-to-satellite quantum communication.

## Key findings

- Modest 3dB improvement with co-aligned beacon
- Up to 7dB improvement using laser guide star
- AO can significantly boost secret key rates in high-loss regimes

## Abstract

For quantum communications, the use of Earth-orbiting satellites to extend distances has gained significant attention in recent years, exemplified in particular by the launch of the Micius satellite in 2016. The performance of applied protocols such as quantum key distribution (QKD) depends significantly upon the transmission efficiency through the turbulent atmosphere, which is especially challenging for ground-to-satellite uplink scenarios. Adaptive optics (AO) techniques have been used in astronomical, communication, and other applications to reduce the detrimental effects of turbulence for many years, but their applicability to quantum protocols, and their requirements specifically in the uplink scenario, are not well established. Here, we model the effect of the atmosphere on link efficiency between an Earth station and a satellite using an optical uplink, and how AO can help recover from loss due to turbulence. Examining both low-Earth-orbit and geostationary uplink scenarios, we find that a modest link transmissivity improvement of about 3dB can be obtained in the case of a co-aligned downward beacon, while the link can be dramatically improved, up to 7dB, using an offset beacon, such as a laser guide star. AO coupled with a laser guide star would thus deliver a significant increase in the secret key generation rate of the QKD ground-to-space uplink system, especially as reductions of channel loss have favourably nonlinear key-rate response within this high-loss regime.