Progress in satellite quantum key distribution

Abstract

Quantum key distribution (QKD) is a family of protocols for growing a private encryption key between two parties. Despite much progress, all ground-based QKD approaches have a distance limit due to atmospheric losses or in-fibre attenuation. These limitations make purely ground-based systems impractical for a global distribution network. However, the range of communication may be extended by employing satellites equipped with high-quality optical links. This manuscript summarizes research and development which is beginning to enable QKD with satellites. It includes a discussion of protocols, infrastructure, and the technical challenges involved with implementing such systems, as well as a top level summary of on-going satellite QKD initiatives around the world.

Squeezing-enhanced quantum key distribution over atmospheric channels

“…The simulation of transmittance (3) in appropriate turbulence regime is ultimately driven by beam wave-number k, propagation distance L and C 2 n structure constant of the refractive index of the air, as well as initial beam-spot radius W 0 . An important description of atmospheric links that incorporates most of aforementioned quantities is the Rytov parameter σ 2 R = 1.23C 2 n k 7 6 L 11 6 . Rytov parameter is defined as the normalized irradiance variance of a plane wave propagating through media with random index of refraction [55].…”

“…However, the dedicated fiber-optical infrastructure can be unavailable, e.g., in the case of movable stations, necessity of quick channel deployment or in hostile environments. Moreover, the extra-long-distance inter-continental quantum communication over satellites relies on the free-space channels [6]. Therefore, the free-space channels are an important physical medium for QKD implementations.The main issue faced by the discrete-variable (DV) QKD protocols, based on single-photon states or weak coherent pulses and the direct photon counting, is the sensitivity of the detectors to the background light, which adds noise to the measured data.…”

“…However, the dedicated fiber-optical infrastructure can be unavailable, e.g., in the case of movable stations, necessity of quick channel deployment or in hostile environments. Moreover, the extra-long-distance inter-continental quantum communication over satellites relies on the free-space channels [6]. Therefore, the free-space channels are an important physical medium for QKD implementations.…”

Progrés et défis pour la cryptographie quantique

“…En effet, les effets des pertes et des fluctuations dues à l'atmosphère sont limités à son épaisseur d'environ 10 kilomètres, et la lumière peut ensuite se propager de façon quasiment imperturbable vers des satellites à orbite basse ou éventuellement géostationnaires. Plusieurs défis liés par exemple à la diffraction ou à la précision du pointage se présentent pour des telles expériences et ont été abordés dans des études de faisabilité importantes [5]. Un pas majeur a été franchi en 2017 avec les expériences du premier satellite équipé avec des systèmes quantiques, développé par…”

Fading channel estimation for free-space continuous-variable secure quantum communication

“…The fluctuating (fading) channels on the other hand are important for CV QKD because channel fading is typically observed in the atmosphere, where it is caused by air turbulence. Therefore any implementation of free-space CV QKD which does not rely on fiber-optical infrastructure has to deal with the estimation of fading channels, especially QKD implementations aiming for long-distance extraterrestrial QKD through a satellite [23,24]. Importantly, the channel has to be estimated using the same quantum states as used for transmitting the key because otherwise (if bright probe pulses are used) an adversary could manipulate the signal and the bright pulses in different ways and therefore mimic the eavesdropping attacks [25].…”