Quantum Secure Direct Communication Network with Two-Step Protocol

An efficient quantum secure direct communication network protocol with the two-step scheme is proposed by using the Einstein--Podolsky--Rosen (EPR) pair block as the quantum information carrier. The server, say Alice, prepares and measures the EPR pairs in the quantum communication and the users perform the four local unitary operations to encode their message. Anyone of the legitimate users can communicate another one on the network securely. Since almost all of the instances in this scheme are useful and each EPR pair can carry two bits of information, the efficiency for qubits and the source capacity both approach the maximal values.