Economically Improving Message-Unilaterally-Transmitted Quantum Secure Direct Communication to Realize Two-Way Communication

We present a subtle idea to economically improve message-unilaterally-transmitted quantum secure direct communication (QSDC) protocols to realize two-way secure direct communication. Suppose Alice and Bob securely share a secret key. When the message sender, say, Alice, wants to send her secret messages to Bob, she can first use the secret key to encrypt her secret messages, then she publicly sends the encrypted messages to the message receiver, say, Bob, via a classical channel. After receiving the encrypted messages, by using the secret key Bob can decrypt the encrypted messages to securely obtain the secret messages which Alice wants to send him. According to the classical one-time-pad method, if the secret key is used one time, then both the secret messages and the secret key are secure. Quantum key distribution (QKD) is an ingenious application of quantum mechanics, in which two remote legitimate users (Alice and Bob) establish a shared secret key through the transmission of quantum signals. Hence, much attention has been focused on QKD after the pioneering work of Bennett and Brassard published in 1984 [1]. Till now there have been many theoretical QKDs [2-20]. Recently, a novel concept, quantum secure direct communication (QSDC) was proposed [15,16,19]. In those QSDC protocols [15,16,19], secret messages can be transmitted directly without first creating a secret key to encrypt them and quantum mechanics ensures the security of the transmitted secret messages. Nevertheless, so far all the QSDC protocols [15,16,19,21] are message-unilaterally-transmitted communication protocols, alternatively, two legitimate parties can not simultaneously transmit their different secret messages to each other (dialogue) in a set of quantum communication device. In general, convenient bidirectional simultaneous mutual communications (dialogue) are very useful and usually desired. In fact, the goal to realize a two-way secure direct communication can be achieved by adopting the strategy of using two sets of message-unilaterally-transmitted QSDC device between two parties. Later in this short paper, we will show the above mentioned strategy of two-way secure direct communication is uneconomical and we will present a subtle idea to economically improve a message-unilaterally-transmitted QSDC to realize two-way secure direct communication.