Quantum Secure Direct Communication Based on Four-Qubit Cluster States

A novel quantum secure direct communication scheme based on four-qubit cluster states is proposed. In this scheme, the quantum channel between the sender Alice and the receiver Bob consists of an ordered sequence of cluster states which are prepared by Alice. After ensuring the security of quantum channel, Alice prepares the encoded Bell-state sequence, and performs Bell-basis measurements on the qubits at hand. Then Alice tells the measured results to Bob, and Bob also performs Bell-basis measurements on his own qubits. Finally Bob can get the secret information through the analysis of their measured results. In our scheme, the qubits carrying the secret message do not need to be transmitted in public channel. We show this scheme is determinate and secure.