Quantum Information Processing In Cavity QED

Quantum information science is a new inter-discipline generallycovering quantum cryptogram, quantum computation and quantumcommunication. Quantum secret sharing, quantum teleportation, quantumdense coding, quantum key distribution, and (remote) generation ofquantum state all belong to the quantum communication. Quantumentanglement is a special resource of quantum information and plays a keyrole in the realization of quantum communication.Considering some matters, such as the experimental problems inrealizing the joint measurement, wide applications and importantsignificances of quantum cloning and its difficult realization, based onunderstanding basic conceptions and properties of quantum communication,we pay main attention to finding more perfect and simpler physics schemesof quantum information processing. We have been done some researches onquantum communication processing and quantum cloning machine as follows:1. Realization of controlled quantum dense coding in cavity-QED.The model we adopt in the scheme is that two atoms simultaneouslyinteract with a single mode cavity, meanwhile are driven by aclassic pulse. At the appropriate time, the Bell measurement canrealize by single-atom measurement, so the method largelyreduces the experiment difficulty. Under ideal situation, we canrealize the controlled quantum dense coding using GHZ state andW state respectively, both of which have advantages and specialcharacteristics. It is very important that all of operators inthe scheme are feasible under the present experimental techniqueand the result of experiment would be very perfect.2. Realization of quantum dense coding via adiabatic passage andlinear optics. The scheme adopts adiabatic passage to generate entanglement between atom and photon that emits with thetransition driven by a classical laser pulse which is collinearwith the cavity axis. Then a maximal entangle state of two atomscan be obtained by a measurement on two photons. Finally, quantumdense coding on the basis of the two atoms can be realized. Theadvantage of the scheme is that atomic quhits ideally serve asstationary qubits and photonic qubits as fly quhits, so it ispropitious to realize long distant quantum communication.Moreover, the scheme is also robust against certain types ofpractical noise like atomic spontaneous emission, outputcoupling and the inefficiency of the detectors.3. Implementation of quantum cloning restoring machine in cavityQED. In the scheme, two atoms (clones) simultaneously interactwith a single-mode cavity, at the same time, are driven by aclassic field. Then an arbitrary unknown input state can berestored in the ancilla through applying appropriate unitarylocal, operation. Furthermore, the quantum restoring machine canbe obtained determinately and perfectly.4. Implementation of economical phase-covariant quantum cloningmachine in cavity QED. In our scheme, two SQUID qubits are fixedinto a microwave cavity by adiabatic passage method for theirmanipulation. The optimal economical phase-covariant quantumcloning machine that the input states locate at the northern andsouthern hemisphere and x-y plane of the Bloch sphere can berealized with high fidelity. The advantage of the scheme is thatthe experiment operation just requires one step to complete onthe two SQUID qubits coupled with a single-mode microwave cavityvia adiabatic passage.