An efficient and secure two-way asynchronous quantum secure direct communication protocol by using entangled states is proposed in this paper. Decoy photons are utilized to check eavesdropping; the securities of the protocol are equal to BB84 protocol. After ensuring the security of the quantum channel, both parties encode the secret message by using CNOT operation and local unitary operation separately. The two-way asynchronous direct transition of secret message can be realized by using Bell measurement and von Neumann measurement, combined with classical communication. Different from the present quantum secure direct communication protocols, the two parties encode secret message through different operations which is equivalent to sharing two asymmetric quantum channels, and the protocol is secure for a noise quantum protocol. The protocol is efficient in that all entangled states are used to transmit secret message.
It is found that the controlled quantum secure direct communication (CQSDC) based on a seven-qubit entangled state protocol [S. K. Wang, X. W. Zha and H. Wu, Int. J. Theor. Phys. 57, 48 (2018)] can be passively attacked. That is to say, this protocol has the security loophole of information leakage. To be specific, the first bit of the sender’s two-bit message can be deduced by an attacker Eve according to the first and the third bits of Alice’s announcement and the first bit of the controller Charlie’s announcement. Then, a way to fix this loophole is discussed.
AbstractQuantum secure direct communication (QSDC) based on entanglement can directly transmit confidential information. However, the inability to simultaneously distinguish the four sets of encoded entangled states limits its practical application. Here, we explore a QSDC network based on time–energy entanglement and sum-frequency generation. In total,15 users are in a fully connected QSDC network, and the fidelity of the entangled state shared by any two users is >97%. The results show that when any two users are performing QSDC over 40 km of optical fiber, the fidelity of the entangled state shared by them is still >95%, and the rate of information transmission can be maintained above 1 Kbp/s. Our result demonstrates the feasibility of a proposed QSDC network and hence lays the foundation for the realization of satellite-based long-distance and global QSDC in the future.
A 15-user quantum secure direct communication network
