Generating Quantum Entangled State And Implementing Teleportation Via Cavity QED System And Cross-Kerr Medium

Quantum information science is a combination of quantum mechanics and information science,including quantum cryptogram,quantum communication,quantum commutation and quantum measurement.Quantum information science takes many advandges which classical information science does not have.For example,improvement of the operation speed,security of the information,increase of information capacity, enhancement of detecting precision,and so on.Entangled state is a powerful tool in verifying the basic problem of quantum mechanics and it is also a significant resource in quantum information science.Entangled state of multiqubit is expecially important due to its structure and property.Entangled state of multiqubit can be used in quantum information processing(QIP),like controlled teleportation,controlled dense coding,multiparty communication,quantum cryptogram, distribution quantum computation,and so on.Entangled GHZ state,W state and Cluster state are some kinds of multiqubit entangled states.Cluster state attracts much attention since it is proposed.It is more stabile than GHZ state and robust to decoherence. Additionally,Cluster state can implement one-way quantum computation which only used single qubit measurement and solve the difficult problem of low fidelity in quantum computer based on lots of logic gates and of designing big quantum network.Teleportation is first proposed by Bennett in 1993 and is a useful and simple work in quantum computation and quanum information processing.It takes use of quantum property to realize information transmission and processing and improves the security of communication.It is an important branch of QIP.Cavity QED system is an ideal physical systems for quantum information processing and cross-kerr system also attracts people's attention for its simple and intuitional feature. In this dissertation,we investigate how to eliminate and make use of the decoherence to teleport quantum states and prepare quantum entangled states.Our main results include:1.Generation of Multiple-Photon GHZ State via Cavity-Assisted InteractionGreenberger-Horne-Zeilinger(GHZ)state is one of the most important kinds of multipartite entangled state and has been applied on quantum communication like secret sharing,multiqubit teleportation and controlled dense coding.Besides,photon is an ideal flying qubit and its two polarizations used as qubit has many advantages.For example, the number of qubit can readily be scaled up by generating many single-photon pulses, and single-qubit operation on the photonie qubit can be accurately performed through polarization rotation.Based on decoherenee of cavity decay,we propose a proposal to prepare multiple-photon GHZ state.N pairs single-photon pulses are successively injected and reflected from two sides of the cavity,which traps one atom.After atomic state being measured,2n-photon GHZ state is produced.In the ideal case,the successful probability of the scheme is close to unity.2.Generation of Four-Photon Cluster State with Coherent Light via Cross-Kerr NonlinearityA cluster state is one of the entangled states and owns unique properties,such as large persistency of entanglement,which means they are harder to be destroyed by local operations than GHZ-class states.The entangled state of the cluster also serves as a universal "substrate" for any quantum computation in a one-way quantum computer.Here, we propose two proposals to prepare four-photon cluster state through cross-Kerr nonlinearity.Two coherent fields interact when they enter a nonlinear Kerr medium.If the interaction time is chosen appropriately in each Kerr medium,four-photon cluster state can be generated based on the results of two homodyne detectors in the first scheme.Both schemes only use Kerr medium and homodyne measurements on coherent light fields which can be efficiently in quantum optical laboratories.In addition,weak cross-Kerr nonlinearity is sufficient.All of which make this scheme feasible in the experiment.3.Teleportation of Quantum States with Nonmaximally Entangled Quantum ChannelThe coupling between quantum system and the environment is unavoidable and induces decoherence.When we keep and send entangled qubits,the entanglement of the qubits decreases sharply.Quantum channel is not always maximal entangled state but nonmaximal entangled state.Thus,we discusses a probability teleportation scheme for an unknown single-atom state and an unknown two-atom entangled state via a tripartite nonmaximally entangled GHZ state in cavity QED.In the process of teleportation of quantum states we have to introduce an ancillary qubit to solve quantum state distortion problems caused by using nonmaximally entangled quantum channel.In this scheme,the two identical two-level atoms simultaneously interact with a single-mode cavity and are driven by a strongly classical field.The atomic state evolution is independent of the cavity field state and is not affected by the cavity decay and the thermal field.4.Teleportation of an arbitrary mixture of diagonal states of multiquditWe propose a scheme to teleport an arbitrary mixture of diagonal states of multiqutrit via classical correlation and classical communication.To teleport an arbitrary mixture of diagonal states of N qutrits,N classically correlated pairs of two qutrits are used as channel.The sender(Alice)makes Fourier transform and conditional gate(i.e.,XOR⁽³⁾ gate)on her qutrits and does measurement in appropriate computation bases.Then she sends N ctrits to the receiver(Bob).Based on the received information,Bob performs the corresponding unitary transformation on his qutrits,and obtains the teleported state. Teleportation of an arbitrary mixture of diagonal states of multiqudit is also discussed.