Cluster state based controlled quantum secure direct communication protocol with controllable channel capacity

Controllable quantum secure direct communication is an important branch of quantum communication. In this paper, we propose a controlled quantum secure direct communication protocol with channel capacity controllable based on a five-particle cluster state. To start with, the sender Alice prepares the five-particle cluster state sequence and inserts decoy photon randomly, and then sends two parts of the particle sequence to the receiver Bob and the controller Cindy, and meanwhile keeps one part of the particle sequence himself. After Bob and Cindy receive the particle sequence, Alice performs a Z-based single-particle measurement and publishes the measurement results and the position information of the decoy photon through the classical channel. According to the information published by Alice, Bob and Cindy remove the decoy photon and perform a Bell-state measurement to their own part particle sequence. Three sides of communication complete the first safety examination of the channel by checking the bit error rate of the measurement results. After that, the controller Cindy determines the channel capacity by selecting the measurement base (Z basis or X basis) to measure its own particle sequence, and then announces the measured results with classical channel. The sender Alice inserts decoy photon and codes the information by doing a unitary transformation to its own particle sequence and then sends the receiver Bob and tells him the position information of the decoy photon with classical channel. Combining the information published by Cindy with the information transmitted by Alice, Bob can complete the second safety examination of the channel and decode the information Alice has sent by removing decoy photon and performing a Bell-state measurement of his own two groups of particle with appropriate rules. Through an analysis of the entangled structural properties of the five-particle cluster state, it has been confirmed that this protocol is designed to make full use of the entanglement properties of the five-particle cluster in different entangled structures. Therefore the protocol can obviously be generalized into the two-way controlled quantum secure direct communication by simply changing the rules of the measurement and the particles used for unitary coding. Through analyzing the security of this protocol, it reveals that this protocol can effectively both prevent eavesdroppers from eavesdropping useful information and monitor this kind of act, and therefore the controlled quantum secure direct communication can theoretically be established in a certain noise environment.