Mediated semi-quantum secure direct communication

Quantum secure direct communication (QSDC) makes two quantum users transmit secret message directly without first producing a shared secret key. Semi-quantum secure direct communication is a particular case of QSDC when one of the two users is a classical user. How to develop a similar protocol if both users are classical? In this paper, we proposed a mediated semi-quantum secure direct communication protocol where both classical users can transmit secret message with the help of a fully quantum third party. Classical users can generate and measure qubits in the computational basis, so they must rely on the third party to prepare alternative bases and perform alternative measurements. The security analysis shows that the protocol can effectively prevent secret message from eavesdropping even if the third party is untrusted. Moreover, the analysis shows that TP’s measurement operation can be associated with linear optical Bell measurements, where only two of the four Bell states would be measured. Finally, we perform an efficiency analysis that shows the protocol can reduce quantum resources at the expense of efficiency.