A Quantum Network System of QSS-QDC Using χ-Type Entangled States

A multiuser quantum direct communication network system for N users utilizing χ-type entangled states is proposed. The network system is composed of a communication center, N users, and N quantum lines linking the center and the N users. There is no quantum line among users, and therefore only N quantum lines are necessary for communication between users. Using one χ-type entangled state, in this protocol we are able to send two bits of information through direct communication and, at the same time, share two bits of quantum keys. The security of the protocol is then analyzed. (general)