Quantum privacy amplification for quantum secure direct communication
  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. 
Fu-Guo Deng and Gui Lu Long