| Abstract: | A theoretical scheme for alternative and secure direct communication is proposed.The communication is based on controlled quantum teleportation via W state.After insuring the security of the quantum channels (a set of qubits in the W states),Alice encodes the secret messages directly on a sequence of particle states and transmits them to Bob supported by Charlie,or to Charlie supported by Bob,using controlled quantum teleportation.Bob or Charlie can read out the encoded messages directly by the measurement on his qubits.In this scheme,the controlled quantum teleportation transmits Alice's message without revealing any information to a potential eavesdropper.Because there is no a transmission of the qubits carrying the secret messages between legitimate parties in the public channel,it is completely secure for direct secret communication if perfect quantum channel is used.We also discuss that when one of the three legitimate parties does not cooperate with others because of some reasons,the remaining two persons can still communicate with each other by entanglement purification.The main feature of this scheme is that the communicate between two sides is alternative but depends on the agreement of the third,and more over,the communication between two sides does not suspend with loss of one of three qubits (i.e.,one of the three legitimate parties does not cooperate with others). |
