  Quantum secure direct communications (QSDC) can directly transmit secret
messages through quantum channel without keys. The imperfect photon source is a
major obstacle for QSDC's practical implementation. The unwanted vacuum state
and multi-photon components emitted from imperfect photon source largely reduce
QSDC's secrecy message capacity and even threaten its security. In the paper,
we propose a high-efficient passive decoy-state QSDC protocol with the heralded
single-photon source (HSPS). We adopt a spontaneous parametric down-conversion
source to emit entangled photon pairs in two spatial modes. By detecting the
photons in one of the two correlated spatial modes, we can infer the photon
number distribution of the other spatial mode. Meanwhile, our protocol allows a
simple passive preparation of the signal states and decoy state. The HSPS can
effectively reduce the probability of vacuum state and increase QSDC's secrecy
message capacity. Meanwhile, the passive decoy-state method can simplify the
experimental operations and enhance QSDC's robustness against the third-party
side-channel attacks. Under the communication distance of 10 km, the secrecy
message capacity of our QSDC protocol can achieve 81.85 times (average photon
number of 0.1) and 12.79 times (average photon number of 0.01) of that in the
original single-photon-based QSDC protocol without the HSPS. Our QSDC protocol
has longer maximal communication distance (about 17.975 km with average photon
number of 0.01). Our work serves as a major step toward the further development
of practical passive decoy-state QSDC systems.
