A pr 2 00 4 Secure Bidirectional Quantum Communication Protocol without Quantum Channel

In this letter we propose a theoretical deterministic secure direct bidirectional quantum communication protocol by using swapping quantum entanglement and local unitary operations, in which the quantum channel for photon transmission can be discarded, hence any attack with or without eavesdropping or even the destructive attack without scruple is impossible. Much attention [1-6] has been focused on the study of the quantum key distribution (QKD) after the pioneering work of Bennett and Brassard published in 1984 [7], for the shared QKD can be used to encrypt the secret messages which is sent through a classical channel. As a matter of fact, the deterministic secure direct communication is more attractive and usually desired due to its obvious convenience. However, because of its more demanding on the security than QKDs, a proposal of a deterministic secure direct communication protocol is usually quite difficult. Till the end of 2003, only three deterministic deterministic secure direct communication protocols had been proposed by using the quantum entanglement of a photon pair [4-6]. Moreover, recently Zhang et al has proposed another deterministic secure direct communication protocol by using the quantum entanglement swapping of two photon pairs [8]. These four protocols mentioned are all message-unilaterally-transmitted protocols. Very recently, inspired by the deterministic secure direct protocol (i.e., the ping-pong protocol) proposed by Boström and Felbinger [5], a de-terministic secure direct bidirectional simultaneous communication protocol is proposed by Zhang et al in a subtle way [9]. This is the first bidirectional secure quantum communication protocol. After this, according to the subtle idea presented in [9] the improvement on the two-step secure communication protocol [6] is also finished [10]. Hence, to our best knowledge, there are only six deterministic secure direct communication protocols so far. These protocols (except for the one in [8]) have four common properties as follows. (a) In all the protocols, after the message sender's encoding by unitary operation on the photon, the photon must be transmitted form the message sender's side to the message receiver's side via a quantum channel. This offers opportunities for the hostile person to eavesdrop or to attack the secret messages. (b) All the protocols are essentially quasisecure. Alternatively, the eavesdropping can not be detected with a possibility of 100%. As a result, a part of information might be leaked to the eavesdropper. (c) All the protocols are insecure under the attack without eavesdropping [11]. Hence, a strategy like message authentification should …