Authenticated Multiuser Quantum Direct Communication using Entanglement Swapping

We present Authenticated Multiuser Quantum Direct Communication(MQDC) protocols using entanglement swapping. Quantum direct communication is believed to be a safe way to send a secret message without quantum key distribution. The authentication process in our protocol allows only proper users to participate in communication. In the communication stage after the authentication, any two authorized users among n users can communicate each other even though there is no quantum communication channels between them. For this protocol, we need only n quantum communication channels between the authenticator and n users. It is similar to the present telephone system in which there are n communication channels between telephone company and users and any two designated users can communicate each other using telephone line through the telephone company. The securities of our protocols are analysed to be the same as those of other quantum key distribution protocols.