A novel secure communication protocol is presented, based on an entangled pair of qubits and allowing asymptotically secure key distribution and quasi-secure direct communication. Since the information is transferred in a deterministic manner, no qubits have to be discarded. The transmission of information is instantaneous, i.e. the information can be decoded during the transmission. The security against arbitrary eavesdropping attacks is provided. In case of eavesdropping attacks with full information gain, the detection rate is 50% per control transmission. The experimental realization of the protocol is feasible with relatively small effort, which also makes commercial applications conceivable. Introduction.-Cryptographic schemes based on quantum mechanics are usually non-deterministic [1, 2, 3]. Alice, the sender, can encode a classical bit into a quantum state, which is then sent to Bob, but she cannot determine the bit value that Bob will finally decode. Inspite of that, such non-deterministic communication can be used to establish a shared secret key between Alice and Bob, consisting of a sequence of random bits. This secret key can then be used to encrypt a message which is sent through a classical public channel. Recently, a novel quantum communication protocol has been presented [4] that allows secure direct communication, where the message is deterministically send through the quantum channel, but can only be decoded after a final transmission of classical information. We present a communication scheme, the "ping-pong protocol", that also allows for deterministic communication. This protocol can be used for the transmission of either a secret key or a plaintext message. In the latter case, the protocol is quasi-secure, i.e. an eavesdropper is able to gain a small amount of message information before being detected. In case of a key transmission the protocol is asymptotically secure. In contrast to other quantum cryptographic schemes, the presented scheme is instantaneous, i.e. the information can be decoded during the transmission and no final transmission of additional information is needed. The basic idea of the protocol, encoding information by local operations on an EPR pair, has already been raised by Bennett and Wiesner [5]. In our protocol, we follow this idea, but abandon the dense coding feature in favour of a secure transmission.
In Reference REF , Boström and Felbinger not only proposed Ping-Pong protocol but also analyzed security of the protocol against an attack that an eavesdropper Eve performs any quantum operation on the quantum channel from Bob to Alice.