# Security bound of continuous-variable measurement-device-independent quantum key distribution with imperfect phase reference calibration

## TL;DR

This paper analyzes the security of continuous-variable measurement-device-independent quantum key distribution under imperfect phase reference calibration, providing a security bound and tolerance limits to ensure practical security in realistic noisy channels.

## Contribution

It develops a comprehensive framework to model and analyze the security impact of imperfect phase reference calibration in CV-MDI-QKD, deriving a tight security bound against collective attacks.

## Key findings

- Imperfect phase reference calibration significantly affects CV-MDI-QKD security.
- A tight security bound to excess noise from calibration imperfections is established.
- The framework allows secure operation without modifying existing CV-MDI-QKD systems.

## Abstract

Phase reference calibration is a necessary procedure in practical continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) for the need of Bell-State Measurement (BSM). However, the phase reference calibration may become imperfect in practical applications. We explored the practical security of CV-MDI-QKD with imperfect phase reference calibration under realistic conditions of lossy and noisy quantum channel. Specifically, a comprehensive framework is developed to model and characterize the imperfection of practical phase reference calibration operation, which is mainly caused by the non-synchronization of two remote lasers in senders. Security analysis shows that the imperfect phase reference calibration has significant side effects on the performance and security of the CV-MDI-QKD protocol. A tight security bound to excess noise introduced by imperfect phase reference calibration is derived for reverse reconciliation against arbitrary collective attacks in the asymptotic limit, and the tolerance of the CV-MDI-QKD protocol to this excess noise is also obtained. This security analysis framework can eliminate the security hazards caused by imperfect phase reference calibration without changing the existing CV-MDI-QKD system structure.