Quantum secure direct communication with optimal quantum superdense coding by using general four-qubit states

From the perspective of quantum circuit, a construction framework and a measurement framework of a general kind of four-qubit states are sketched, respectively. By utilizing the properties of this kind of states, a quantum secure direct communication (QSDC) protocol is put forward, which adopts the idea of optimal quantum superdense coding to achieve a maximal efficiency and high resources capacity. The security of the proposed protocol is discussed in detail and it is proved to be secure theoretically. Moreover, the sufficient and necessary condition of which multipartite states are suitable for optimal quantum superdense coding in quantum secure direct communication is figured out.