关键词: 量子密码，量子安全直接通信，W态，幺正变换

Abstract: A novel two-party quantum secure direct communication protocol was proposed. By using ordered four-particle W state as carrier of information, encoding the secret message with unitary transformation, we can directly transmit secret message with local Be11-basis measurement and classical communication. In ideal channel, the protocol is secure against incoherent attack. hhe advantage of this scheme is that using W state is more little than GHZ state in energy loss and no quantum bit carrying secret message is transmitted in quantum channel.

Key words: Quantum cryptography, Quantum secure direct communication, W state, Unitary transformation