Quantum Secure Direct Communication With A Constant Number Of Epr Pairs

In this paper, we propose a quantum secure direct communication (QSDC) protocol based on Einstein-Podolsky-Rosen (EPR) pairs. Previous QSDC protocols usually consume one EPR pair to transmit a single qubit. If Alice wants to transmit an n-bit message, she needs at least n/2 EPR pairs when a dense coding scheme is used. In our protocol, if both Alice and Bob preshare 2c + 1 EPR pairs with the trusted server, where c is a constant, Alice can transmit an arbitrary number of qubits to Bob. The 2c EPR pairs are used by Alice and Bob to authenticate each other and the remaining EPR pair is used to encode and decode the message qubit. Thus the total number of EPR pairs used for one communication is a constant no matter how many bits will be transmitted. It is not necessary to transmit EPR pairs before transmitting the secret message except for the preshared constant number of EPR pairs. This reduces both the utilization of the quantum channel and the risk. In addition, after the authentication, the server is not involved in the message transmission. Thus we can prevent the server from knowing the message.