Multi-intensity quantum secure direct communication relying on finite block-length

Quantum secure direct communication (QSDC) directly transmits confidential information over a quantum channel. In this context, it is important to take into account the multi-photon effect when using weak coherent laser pulses as the quantum source. Here we propose a multi-intensity QSDC (MI-QSDC) protocol where laser pulses having different intensities are used for secure communication in a fashion reminiscent of decoy states, where only signal intensity pulses are used for key exchange. Pulses of all intensities are used both for information transmission and eavesdropping detection in the MI-QSDC. The statistical fluctuations associated with practical finite block-length in QSDC are also considered and a tight secrecy capacity bound is given. The simulation results show that the proposed protocol is capable of communicating over a distance on the order of 100 km by relying on a finite block-length of sending 10 ¹⁴ pulses.