# Laser Technology in Photonic Applications for Space
## 3 Lasers in Space
### Communication
#### 3.2.3 Quantum Communication  
Quantum technologies have the potential to make communications more secure through quantum encryption and quantum key distribution (QKD). Such optical quantum setups in space are limited by the high stability required of the devices and the low losses required to achieve proper satellite to ground communication.  
Quantum communication technologies are still in the first stages of development, nevertheless the technological feasibility has already been demonstrated and reported. [PERSON] et al. [106], reported in 2017 a satellite-relayed QKD downlink from the low Earth Orbit Micius satellite to different ground stations located in China and Austria with a maximum distance of 7600 Km. The link used 848.6 nm photons emitted from eight fibre-coupled laser diodes (four for signal and four for decoy states) pulsed at 100 MHz with 0.2 ns pulse width. A following experiment from [PERSON] et al. [107], achieved also an important milestone, allowing an entanglement distribution over 1200 km in Chinaby using two satellite-to-ground downlinks. Authors reported moreover that the entangled-photon source used was based on a Sagnac interferometer with a periodically poled KTiOPO\({}_{4}\) crystal and represented a spaceborn design robust again vibration and temperature. [PERSON] et al. [108], demonstrated experimentally for the first time in 2018 the feasibility of quantum communications between high-orbiting satellites from a Global Navigation Satellite System (GNSS) and a ground station at a distance of approximately 20.000 km.  
Some groups are doing similar studies to implement QKD in smaller-size satellites [109; 110] down to 50 Kg on the Small Optical TrAnsponder (SOTA) [111], whilst others study other aspects, such as the uplink communication using a 3U CubeSat, as explained in [PERSON] et al. [112], Working on stable and robust sources is also important, as reported by [PERSON] et al. [113], Here, the group reports an Entangled Photon Source (EPS) for a future QKD test. Although the device is only qualified as Engineering Qualification Model (EQM), the article describes the device suitability for future space missions. The designed setup is based on a sagnac interferometer double size pumped periodically poled potassium titanyl phosphate (ppKTP) crystal that provides 284,000 photon pair counts per second at 810 nm for an 8 mW pump power.  
One of the current bottle necks for the implementation of quantum and laser communication is the turbulence in the atmosphere. Optical signals can be degraded during their propagation, which affects the interpretation of the detected signals. A way to enhance quantum state-of-the-art communication instruments will be the use of Adaptive Optics (AO). Using AO in space would help applying corrections to laser communication links between Optical Ground Stations (OGS) and satellites to compensate the errors [114], or in intersatellite links through the atmosphere, where atmospheric turbulence affects the communication signal and can increase bit error rates [115]. The efficiency of protocols, such as QKD, are highly dependent on the transmission, so AO can compensate for defects due to turbulence, which is particularly challenging in communication links, such as ground-to-satellite [116]. This AO concept currently allows the mobile and stationary Adaptive Optical Ground Stations (AOGS) to operate full-duplex uplink/downlinks with LEO and GEO with data rates of up to 5.525 Gbps [101] and will be necessary to improve quantum communication in space.