We proposed a passive continuous-variable quantum key distribution (CVQKD) using a locally generated local oscillator (LLO) at reception. Different from the LLO-based Gaussian-modulated coherent-state quantum key distribution, our protocol is implemented by taking advantage of a multimode thermal source, beam splitters, optical attenuators, and homodyne detectors rather than the amplitude and phase modulators, which could be a cost-effective solution for LLO-based CVQKD. We adopt a general LLO noise model to analyze the performance of our protocol. That is to say, our consideration for noise in the proposed scheme contains the excess noise caused by passive state preparation, the phase noise, and the photon-leakage noise from the reference path. Because our protocol waives the necessity of the use of amplitude and phase modulators, there is no need to bear the effect of modulation noise caused by finite dynamics. Simulation results show that the improvement of the mode-overlap coefficient can suppress the excess noise caused by passive state preparation and thus enhance the performance of our protocol. Because lower relative phase drift and the photon-leakage noise can be obtained by using the multiplexing technique, we can still achieve reasonable performance of our protocol with the mode-overlap coefficient values which are very close to the experimental reference. Furthermore, we perform the finite-size analysis for the proposed protocol, which is more practical than that achieved in the asymptotic limit. This work confirms the feasibility of passive CVQKD implemented in the LLO configuration within the metropolitan area.

• Received 24 September 2020
• Accepted 16 February 2021

DOI:https://doi.org/10.1103/PhysRevA.103.032604