The role of averages in CV-QKD over fast fading channels

TL;DR

This paper investigates the security of CV-QKD protocols over fast-fading channels, comparing two models for eavesdropping analysis, and shows that the secret key rate depends heavily on the chosen channel fluctuation treatment.

Contribution

It introduces and compares two analytical models for assessing CV-QKD security over fast-fading channels, emphasizing the impact of channel fluctuation treatment on key rate calculations.

Findings

SKR varies significantly with the eavesdropping model used.

Analytical expressions for both models are derived.

Choosing the appropriate model is crucial for accurate security assessment.

Abstract

This work presents a study of continuous-variable quantum key distribution (CV-QKD) protocols over fast-fading channels, typically found in free-space communication links. Two eavesdropping models are considered to evaluate their security under collective attacks: \textit{Holevo bound average} (HBA) and \textit{covariance matrix average} (CMA). In the HBA approach, the Holevo bound is averaged over the channel transmittance. In contrast, the CMA method calculates the Holevo bound from the average covariance matrix. Analytical expressions are developed for both strategies. The two methods also differ in how they calculate the mutual information between the legitimate parties. The results demonstrate that the SKR is significantly influenced by how you treat channel fluctuations, highlighting the importance of choosing the model that better describes the actual implementation of the protocol.