  Using quantum mechanics, secure direct communication between distant parties
can be performed. Over a noisy quantum channel, quantum privacy amplification
is a necessary step to ensure the security of the message. In this paper, we
present a quantum privacy amplification scheme for quantum secure direct
communication using single photons. The quantum privacy amplification procedure
contains two control-not gates and a Hadamard gate. After the unitary gate
operations, a measurement is performed and one photon is retained. The retained
photon carries the state information of the discarded photon, and hence reduces
the information leakage. The procedure can be performed recursively so that the
information leakage can be reduced to any arbitrarily low level.
