Efficient Generation Protocol For Logical Entangled States

Quantum communication is a way to transmit information by using the basic principles of quantum mechanics(e.g.,quantum unclonable principle,Heisenberg uncertainty relation,etc.).Quantum communication mainly includes quantum teleportation(QT),quantum key distribution(QKD)and quantum secure direct communication(QSDC).Quantum entanglement is an important resource in quantum communication.In various applications,the communicating parties should distribute the entanglement at long distance.However,the presence of channel noise may lead to photon transmission loss and quantum state decoherence when photons are transmitted in a practical noisy quantum channel.As a result,the entanglement quality will be degraded significantly after the long distance transmission,which can largely reduce the communication efficiency of quantum communication protocols and even threatens their security.To solve the above problems,logical coding is proposed,which has strong resistance to photon loss and errors.The first work in this paper presents a generation protocol for three-layer logic GreenbergerHorne-Zeilinger(GHZ)states.This protocol uses a crossed Kerr medium to construct a quantum bit parity checker.With the help of the quantum bit parity checker,we can generate the three-layer-logic GHZ states with near certainty using auxiliary single photons.The protocol requires only two different cross-Kerr media and no complex Toffoli gates,which is easier to operate than previous protocols for logical state-generation.Based on the above features,the protocol has potential applications in future quantum communication.The second work of this paper presents a generation protocol for the dual-rail encoded Bell state scheme prepared under linear optical conditions using the imperfect single photon states from the practical parametric down-conversion entanglement source(SPDC source).The scheme employs three kinds of variable beam splitters(VBS).We show that the two-photon component of the photon state generated by the SPDC source can also be used to prepare the target dual-rail encoded state,and therefore,the preparation probability of the target state can be effectively improved.The scheme can be realized under current experimental conditions.It has strong applicability,and may play an important role in future quantum information processing.