  Quantum secure direct communication (QSDC) is an important quantum
communication branch, which realizes the secure information transmission
directly without encryption and decryption processes. Recently, two table-top
experiments have demonstrated the principle of QSDC. Here, we report the first
long-distance QSDC experiment, including the security test, information
encoding, fiber transmission and decoding. After the fiber transmission of 0.5
km, quantum state fidelities of the two polarization entangled Bell states are
91% and 88%, respectively, which are used for information coding. We
theoretically analyze the performance of the QSDC system based on current
optical communication technologies, showing that QSDC over fiber links of
several tens kilometers could be expected. It demonstrates the potential of
long-distance QSDC and supports its future applications on quantum
communication networks.
