Controlled Quantum Secure Direct Communication Protocol Based on Five-Particle Cluster State and EPR Pair Entanglement Swapping

To transmit secure message, a controlled quantum secure direct communication protocol is proposed based on five-particle cluster state and EPR pair entanglement swapping. The sender and the controller own their EPR pair sequences respectively. The sender sends one photon of the EPR pair he owns to controller. The controller performs local unitary operation randomly on one photon of the EPR pair he owns and sends it to the receiver as his permission. The sender encodes his secret message on the other photon of the EPR pair he owns and sends it to the receiver. Because of EPR pair entanglement swapping, the two photons in the receiver´s hand entangled. Under the permission of the controller, the controller and the receiver measure their new EPR pairs in the Bell basis in turn. By comparing the measurement result of the receiver and the original state, the receiver can get the secret message. To ensure the security of the message transmitting, five photons of five-particle cluster state are all mixed in sending sequence to detect eavesdropping which can bring per qubit 88% eavesdropping detection probability. Compared with taking three photons of five-particle cluster state as detection photons, five photons in this paper will decrease the number of five-particle cluster state for detection greatly. The security of the present protocol is discussed: the detection probability of this protocol is more than 88%, and intercept-measure-resend attacks and correlation-elicitation (CE) attack can be defended effectively.