CV QKD with discretized modulations in the strong noise regime

TL;DR

This paper analyzes the limits of quantum key distribution using discretized modulations in noisy environments, showing that strong signals are insecure and identifying conditions for secure communication.

Contribution

It introduces a detailed analysis of discretized modulation QKD protocols under strong noise, highlighting the impact of signal strength on security.

Findings

Strong signals make QKD insecure due to classicality from eavesdroppers

Security depends on noise regime, with different thresholds in weak and strong noise

QKD becomes feasible only when signal exceeds a critical strength in low-noise conditions

Abstract

We consider a general family of quantum key distribution (QKD) protocols utilizing displaced thermal states with discretized modulations. Separating the effects of the Gaussian channel and the non-Gaussian distribution, we have studied the dependence of the secret key generation rate on the magnitude of modulations (the strength of the modulated signal). We show that in the limit of strong signal, QKD is impossible: from the perspective of an efficient eavesdropper, the ensemble of transmitted states is effectively classical. This constitutes a quantum correction to performance of finite-length QKD protocols. We demonstrate that two regimes must be distinguished: weak and strong thermal noise. In the case of strong noise, the security boundary is mostly determined by the weak-signal limit. When the noise is weak, however, QKD may become possible only when the signal strength exceeds some critical value.