Quantum secure direct communication based on fully passive source

In practical quantum communication, imperfect devices may introduce side channels, creating opportunities for eavesdroppers. Especially on the source side, the side channels created by active modulation may compromise the security of the protocol. We proposes a passively-sourced quantum secure direct communication (QSDC) protocol based on fully passive source. By passively modulating both the quantum state and the intensity of the decoy state, we can avoid active modulation operations at the source, thereby enhancing the robustness of QSDC against side-channel attacks. We developed a system model and conducted parameter optimization to obtain the maximum secrecy message transmission rate achievable by the protocol for each channel attenuation. At a channel attenuation of 2, 4, 6 dB (corresponding to a communication distance of 5, 10, 15 km), the secrecy message transmission rates are 5.76 * 10^-5, 9.92 * 10^-6, and 4.99 * 10^-7 bit/sec. And its maximum communication distance is about 16.875 km, which is about 94.4% of that of actively modulated QSDC.