Improvement of a controlled quantum secure direct communication protocol

A security loophole exists in Gao et al.'s controlled quantum secure direct communication protocol. By employing the security loophole, the receiver can obtain the secret message sent by the sender without the permission of the controller in their protocol. In order to avoid this loophole, we present an improved protocol in this paper. In the improved protocol, entangled particles are prepared at random in two GHZ-like states, which ensure that the receiver is not able to recover the secret message without knowing the initially entangled state. Compared with the other improved version whose security depends on the perfect quantum channel, our improved protocol is secure in a noisy quantum channel. Therefore, our protocol is more practical.