Quantum Authenticated Direct Communication Using Bell States

Two protocols of quantum direct communication with authentication [Phys. Rev. A 73:042305, 2006] were recently proposed by Lee, Lim and Yang, based on the correlation of Greenberger-Horne-Zeilinger (GHZ) states. However, Zhang et al. showed that in the two protocols the authenticator Trent can eavesdrop the secret message by subtle strategies [Phys. Rev. A 75:026301, 2007]. In this paper, we propose two authenticated quantum direct communication (AQDC) protocols using Bell states. Users can identify each other by checking the correlation of Bell states. Alice can directly send a secret message to Bob without any previously shared secret using the remaining Bell states after authentication. The two proposed AQDC protocols are implemented under the condition that there is a quantum link between Alice and Bob and that there is no quantum link between Alice and Bob respectively, similar to the ones proposed by Lee, Lim and Yang [Phys. Rev. A 73:042305, 2006]. The proposed AQDC protocols not only fix the leaks in the AQDC protocols proposed by Lee, Lim and Yang, but also economize the quantum resource.