Improving the Security of Controlled Quantum Secure Direct Communication by Using Four Particle Cluster States Against an Attack with Fake Entangled Particles

摘要

A controlled quantum secure direct communication protocol (Zhang et al. Int. J. Theor. Phys. 48:2971-2976, 2009) by using four particle cluster states was proposed recently. The aim of Zhang et al. was that the successful realization of communication between Alice and Bob needed the cooperation of a controller, Charlie. However, we show that the controller Charlie's role could be excluded unknowingly. Using fake entangled particles and Bell measurement, the dishonest Bob who generates the initial signals can elicit Alice's secret message without the permission of Charlie. A possible improvement of the protocol is proposed.