Passive decoy state quantum secure direct communication.

The decoy state method is an effective approach for addressing the source imperfections in quantum communication. By employing pulses of varying intensities to estimate error rates and yields, this method requires active modulation of the source intensity. However, the active modulation operation may introduce side-channel vulnerabilities, potentially allowing an eavesdropper to execute Trojan horse attacks. The passive decoy state method circumvents this issue by eliminating the need for active intensity modulation. In this paper, we propose a passive decoy state quantum secure direct communication (QSDC) protocol that generates weak coherent states with random intensities using two phase-randomized coherent pulses. Leveraging linear optics, our protocol facilitates straightforward implementation and is robust against eavesdropping side-channel attacks targeting source intensity modulators.We establish a system model for the protocol and derive its secrecy message capacity. Additionally, we optimize the input intensities of the protocol and obtain the maximum secrecy message capacity for each channel attenuation value. Compared with the previous protocol, our protocol exhibits an approximately 25% enhancement in the secrecy message transmission rate within a channel attenuation range of 6 dB. We also compare the performance of the three-decoy-state protocol with that of the infinite-decoy-state protocol. Within a channel attenuation range of 6.8 dB, the secrecy message capacity of the three-decoy-state protocol achieves approximately 94%-96% of that of the infinite-decoy-state.