  We report an experimental implementation of free-space quantum secure direct
communication based on single photons. The quantum communication scheme uses
phase encoding, and the asymmetric Mach-Zehnder interferometer is optimized so
as to automatically compensate phase drift of the photons during their
transitions over the free-space medium. An information transmission rate of 500
bps over a 10-meter free space with a mean quantum bit error rate of
0.49%$\pm$0.27% is achieved. The security is analyzed under the scenario that
Eve performs collective attack and photon number splitting collective attack.
Our results show that quantum secure direct communication is feasible in free
space.
