Abstract: Mode-pairing quantum key distribution (MP-QKD) can break through the linear bound of secure key rates under repeaterless conditions without phase locking. However, most existing MP-QKD protocols assume that the photon number distribution is known and the light source is reliable, which is difficult to fulfill in practical implementations. To overcome this issue, a novel photon number distribution monitoring scheme, referred to as novel light source monitoring (NLSM), is proposed in this work. This scheme adopts a passive structure to monitor the light source, thereby eliminating the modulation errors introduced by active components in traditional monitoring techniques. Simulation results indicate that the implementation of NLSM scheme has minimal impact on the secure key rate and achieves transmission performance comparable to the ideal scenario. Moreover, compared with the existing active monitoring schemes, NLSM scheme demonstrates stronger robustness and stability under practical conditions, such as modulation inaccuracies and light source fluctuations. These advantages make NLSM scheme a promising solution for improving the security and practicality of MP-QKD systems under actual light source conditions, providing essential support for large-scale deployment of MP-QKD systems.

Key words: quantum communication, quantum key distribution, mode-pairing, light source monitoring, photon number distribution