Quantum secure direct communication with W state

A new theoretical scheme for quantum secure direct communication is proposed, where four-qubit symmetric W state functions as quantum channel. It is shown that two legitimate users can directly transmit the secret messages by using Bell-basis measurements and classical communication. The scheme is completely secure if the quantum channel is perfect. Even if the quantum channel is unsecured, it is still possible for two users to perform their secure communication. One bit secret message can be transmitted by sending a bit classical information.