  Fast development of supercomputer and perspective quantum computer is posing
increasing serious threats to communication security. Based on the laws of
quantum mechanics, quantum communication offers provable security of
communication, and is a promising solution to counter such threats. Quantum
secure direct communication (QSDC) is one of the important branches of quantum
communication. Different from other branches of quantum communication, it
transmits secret information directly. Recently, remarkable progress has been
made in the proof-of-principle experimental demonstrations of QSDC. However, it
remains a technical feast to march QSDC into practical application. Here, we
report an implementation of practical quantum secure communication system. The
security is analyzed in the Wyner wiretap channel theory. The system uses a
coding scheme based on concatenation of low density parity check (LDPC) codes,
which works in a regime with realistic environment of high noise and high loss.
The present system operates with a repetition rate of 1 MHz, and at a distance
of 1.5 kilometers. The secure communication rate is 50 bps, which can
effectively send text message and files such as image and sounds with a
reasonable size.
