A novel secure communication protocol

Quantum key distribution (QKD) is a protocol which is provably secure. Eve cannot gain any information from the qubits transmitted from Alice to Bob without disturbing their state. First , the no-cloning theorem forbids Eve to perfectly clone Alice’s qubit. Second, in any attempt to distinguish between two non-orthogonal quantum states, information gain is only possible at the expense of introducing disturbance to the signal [1]. Based on the postulate of quantum measurement [2] and no-cloning theorem [3], different QKD protocols are presented [4-8]. However, these types of cryptographic schemes are usually nondeterministic. Recently, K. Bostrom and T. Felbinger presented a protocol, which allows for deterministic communication using entanglement [9]. Cai presented an protocol which allows for deterministic communication using a single qubit which is prepared in mixed state [10]. In this paper, we show a novel protocol which is a deterministic secure direct communication protocol using single qubit in mixed state. Surprisingly, even Alice sends two or more qubits which are prepared in the same state to Bob every time, this protocol is also secure. It is well known that we can prepare a photon in states {|0 >, |1 >} or {|φ0 >, |φ1 >}in its polarization degree of freedom, where