Deterministic Secure Direct Communication Using Ping-pong protocol without public channel

Based on an EPR pair of qubits and allowing asymptotically secure key distribution, a secure communication protocol is presented. Bob sends either of the EPR pair qubits to Alice. Alice receives the travel qubit. Then she can encode classical information by local unitary operations on this travel qubit. Alice send the qubit back to Bob. Bob can get Alice’s information by measurement on the two photons in Bell operator basis. If Eve in line, she has no access to Bob’s home qubit. All her operations are restricted to the travel qubit. In order to find out which opeartion Alice performs, Eve’s operation must include measurements. The EPR pair qubits are destroyed. Bob’s measurement on the two photons in Bell operator basis can help him to judge whether Eve exist in line or not. In this protocal, a public channel is not necessary. PACS numbers: 03.67.Hk, 03.65.Ud, 03.67.Dd The idea of quantum cryptography was first proposed in the 1970s by Stephen Wiesner [1] and by Charles H. Bennett of IBM and Gilles Brassard of The University of Montreal [5]. The motive that we build quantum-mechanical communications channels is not only to transmit information securely without being eavesdropped on but also to transmit information more efficiently. We consider that quantum channel is secure because that quantum physics establishes a set of negative rules stating things that cannot be done [2]: (1) One cannot take a measurement without perturbing the system. (2) One cannot determine simultaneously the position and the momentum of a particle with arbitrarily high accuracy. (3) One cannot simultaneously measure the polarization of a photon in the verticalhorizontal basis and simultaneously in the diagonal basis. (4) One cannot draw pictures of individual quantum processes. (5) One cannot duplicate an unknown quantum state. These characters of quantum physics let us have ability to exchange information securely. As is well known, operations on one particle of an Einstein-Podolsky-Rosen (EPR) pair cannot influence the marginal statistics of measurements on the other particle. Let’s suppose that Bob have two photons which are maximally entangled in their polarization degree of freedom