Trusted Noise in Continuous-Variable Quantum Key Distribution: a Threat and a Defense

TL;DR

This paper investigates how trusted preparation and detection noise affect the security of continuous-variable quantum key distribution, revealing both detrimental and beneficial effects depending on the protocol and noise configuration.

Contribution

It provides a detailed analysis of the nonlinear impact of trusted noise on security, including new insights into the negative role of detection noise in direct-reconciliation and the positive effects in reverse-reconciliation.

Findings

Trusted preparation noise affects security in both protocols.

Detection noise can be detrimental in direct-reconciliation.

Adding trusted noise can improve robustness against channel noise.

Abstract

We address the role of the phase-insensitive trusted preparation and detection noise in the security of a continuous-variable quantum key distribution, considering the Gaussian protocols on the basis of coherent and squeezed states and studying them in the conditions of Gaussian lossy and noisy channels. The influence of such a noise on the security of Gaussian quantum cryptography can be crucial, even despite the fact that a noise is trusted, due to a strongly nonlinear behavior of the quantum entropies involved in the security analysis. We recapitulate the known effect of the preparation noise in both direct and reverse-reconciliation protocols, as well as the detection noise in the reverse-reconciliation scenario. As a new result, we show the negative role of the trusted detection noise in the direct-reconciliation scheme. We also describe the role of the trusted preparation or detection noise added at the reference side of the protocols in improving the robustness of the protocols to the channel noise, confirming the positive effect for the coherent-state reverse-reconciliation protocol. Finally, we address the combined effect of trusted noise added both in the source and the detector.