  Quantum communication holds promise for absolutely security in secret message
transmission. Quantum secure direct communication is an important mode of the
quantum communication in which secret messages are securely communicated over a
quantum channel directly. It has become one of the hot research areas in the
last decade, and offers both high security and instantaneousness in
communication. It is also a basic cryptographic primitive for constructing
other quantum communication tasks such as quantum authentication, quantum
dialogue and so on. Here we report the first experimental demonstration of
quantum secure direct communication with single photons. The experiment is
based on the DL04 protocol, equipped with a simple frequency coding. It has the
advantage of being robust against channel noise and loss. The experiment
demonstrated explicitly the block data transmission technique, which is
essential for quantum secure direct communication. In the experiment, a block
transmission of 80 single photons was demonstrated over fiber, and it provides
effectively 16 different values, which is equivalent to 4 bits of direct
transmission in one block. The experiment has firmly demonstrated the
feasibility of quantum secure direct communication in the presence of noise and
loss.
