Entanglement of Gaussian states and the applicability to quantum key distribution over fading channels

TL;DR

This paper investigates how entanglement of Gaussian states affects quantum key distribution over atmospheric fading channels, proposing methods to improve security and robustness against channel noise and fluctuations.

Contribution

It introduces a fading discrimination and post-selection method that enhances entanglement and security in free-space quantum communication under realistic atmospheric conditions.

Findings

Post-selection improves entanglement and security in fading channels

Optimal strategies significantly increase protocol robustness to excess noise

Results are stable against finite data and imperfect channel estimation

Abstract

Entanglement properties of Gaussian states of light as well as the security of continuous variable quantum key distribution with Gaussian states in free-space fading channels are studied. These qualities are shown to be sensitive to the statistical properties of the transmittance distribution in the cases when entanglement is strong or when channel excess noise is present. Fading, i.e. transmission fluctuations, caused by beam wandering due to atmospheric turbulence, is a frequent challenge in free-space communication. We introduce a method of fading discrimination and subsequent post-selection of the corresponding sub-states and show that it can improve the entanglement resource and restore the security of the key distribution over a realistic fading link. Furthermore, the optimal post-selection strategy in combination with an optimized entangled resource is shown to drastically increase the protocol robustness to excess noise, which is confirmed for experimentally measured fading channel characteristics. The stability of the result against finite data ensemble size and imperfect channel estimation is also addressed.