Strengthening practical continuous-variable quantum key distribution systems against measurement angular error

TL;DR

This paper analyzes how measurement angular errors in practical continuous-variable quantum key distribution systems affect security and proposes calibration methods to mitigate these effects, enhancing system performance.

Contribution

It provides a detailed interpretation of measurement angular error, its impact on security, and introduces an estimation and compensation scheme to improve practical CV-QKD security.

Findings

Measurement angular error degrades security significantly.

The proposed calibration method effectively reduces the impact of angular errors.

Security can be substantially improved with proper compensation techniques.

Abstract

The optical phase shifter that constantly rotates the local oscillator phase is a necessity in continuous-variable quantum key distribution systems with heterodyne detection. In previous experimental implementations, the optical phase shifter is generally regarded as an ideal passive optical device that perfectly rotates the phase of the electromagnetic wave of $90^\circ$. However, the optical phase shifter in practice introduces imperfections, mainly the measurement angular error, which inevitably deteriorates the security of the practical systems. Here, we will give a concrete interpretation of measurement angular error in practical systems and the corresponding entanglement-based description. Subsequently, from the parameter estimation, we deduce the overestimated excess noise and the underestimated transmittance, which lead to a reduction in the final secret key rate. Simultaneously, we propose an estimation method of the measurement angular error. Next, the practical security analysis is provided in detail, and the effect of the measurement angular error and its corresponding compensation scheme are demonstrated. We conclude that measurement angular error severely degrades the security, but the proposed calibration and compensation method can significantly help improve the performance of the practical CV-QKD systems.