Frame Synchronization for Passive Continuous‐Variable Quantum Key Distribution with a Local Local Oscillator

Abstract Frame synchronization is essential in continuous‐variable quantum key distribution (CVQKD), typically relying on active modulation to establish a specific frame structure. In passive CVQKD systems, however, the elimination of modulation requirements poses a significant challenge for frame synchronization, especially in local‐local‐oscillator (LLO) configurations. This paper introduces a robust, modulation‐free frame synchronization method tailored for passive LLO CVQKD systems. This method leverages the complex cross‐correlation of quantum signals exchanged between Alice and Bob for synchronization purposes. Validated through an analytical model and numerical simulations, this approach demonstrates high synchronization success rates, robustness to noise, and resilience to both slow phase drifts between local oscillators and quantum signals and fast phase drifts stemming from two independent lasers. These results underscore the viability of the proposed method for implementing long‐distance CVQKD in real‐world scenarios.