Quantum Secure Communication Protocol With The Improvements

The combination of quantum mechanics and classical information produces a newsubject—quantum information (QI). Being one important branch of QI, quantumsecret communication is a new application of the fundamental properties of quantummechanics in the secret communication field, hence the studies in this field naturallyown important significance in theory. Compared with the usual quantum cryptography(QC) based on the quantum key distribution (QKD), quantum secure directcommunication (QSDC) has obvious convenience and effectiveness, as a consequence,the latter has more demands on security than the former. Since the quantum securecommunication based on the QKD has already been achieved in reality, it is believedthat the QSDC would have wider applicable prospects due to its superiorities to theusual QKD-based secure communication. In the domain of the QSDC, we have doneseveral research works concerning the protocols and their security. These works areoutlined as follows:1. (a) The ping-pong protocol has been improved by using quantum swapping andthe properties of the local unitary operations. As a result, the attacks on the ping-pongprotocol without eavesdropping can be successfully prevented. (b) Throughimproving Wojcik's eavesdropping scheme on the ping-pong protocol in the lossquantum channel, we have pushed up the insecurity upper bound of the transmissionefficiency of the ping-pong protocol from the original 60% to the present 80%. Thisthreshold has been further increased to 100% in our extensive study. This implies thateven in an ideal quantum channel the ping-pong protocol is insecure. To preventpossible eavesdropping attacks, the two measuring bases method should beincorporated.2. Using entanglement swapping and the properties of unitary operations, wepropose a secure direct communication protocol by further incorporating theadvantages of 'block' transmissions. Comparisons between the present protocol andtwo-step protocol are made. Although the transmission efficiency of our protocol seems to be reduced, the security has been greatly strengthened. Once the security ofthe quantum channel is ascertained, the success of the message transmission will beanticipative. Hence, the efficiency of the present protocol does not reduce on average.3. The application of the ping-pong protocol to the bidirectional communication isstudied. We find the task of "quantum dialogue" can be accomplished by using thisprotocol, i.e., secret messages can be simultaneously exchanged between twolegitimate users. By revisiting Ba An Nguyen's "quantum dialogue" scheme, we findthe protocol is insecure under the intercept-resend attacks, that is to say, the secretmessages will be completely eavesdropped while the eavesdropping is not detected.We have improved the Ba An Nguyen's scheme and accordingly the intercept-resendattacks can be detected, thus the quantum dialogue can be really achieved.