Quantum Information Processing With Two-mode Cavity QED And Neutral Rydberg Atom System

In this thesis, we theoretically study the implementation of quantum informationprocessing in the realm of two-mode cavity QED system and neutral Rydberg atomsystem.In quantum optics, the definition of nonclassical light can be given by the Wignerfunction or more generally the Glauber-Sudarshan P function. The typical features ofa nonclassical light, such as squeezing, photon sub-poisson distribution, are describedby physical quantities like Mandel Q factor, second-order correlation function. Bymaking use of them, in the first part of chapter2, we discuss squeezing, photonstatistics, and violation of Cauchy-Schwarz inequality of a two-mode field afterinteracting with a Λ-typed three-level atom. Correlation between the two cavitymodes is also investigated. In the second part of chapter2, we propose a scheme fordirectly measuring the Wigner function of a two-mode field. In this scheme, the twoorthogonal polarized cavity modes dispersively coupling to a strongly drivenmultilevel atom is described by a time-averaged effective Hamiltonian that iscommuted with photon number operator. We show that the probability of detecting theatom in the ground state directly refers to the relevant value of Wigner function in thephase space. It is then generalized for quantum nondemolition measurements ofphotons.A setup with an atom passing through multiple cavities can be used for entanglingquantized cavity fields. The quantum computational logic0and1can be encoded inthe left-circularly polarized state and right-circularly polarized state of a single photon,respectively. Using this kind of setup, in chapter3, we firstly present a scheme forpreparing four-mode entangled state through adiabatic passage. The nice feature ofour scheme is the insensitivity of the fluctuation of atomic position and theimprecision of laser pulses. Then, we propose that a one-dimensional cluster state formultiple cavity fields can be prepared in a way that an atom couples to bothorthogonal polarized cavity modes through two independent Raman transitionchannels assisted by an external driving laser. Decoherence due to atomic spontaneousemission can be reduced with the atom-cavity interaction being in the large-detuningregime. Chapter4discusses the preparation of cluster state and implementation ofquantum logic gate for atoms in two-mode cavity QED system. Long-lived hyperfineground states of the atoms are used for storing quantum information. To prepareatomic cluster state, we use a cavity QED setup where photons leaking out of thecavity are collected by a photon detector. The dissipative dynamics in this system canbe described by a non-Hermitian Hamiltonian if there is not photon detected duringthe preparation process. The photon click incident can be finally used for confirmingthe successful events in the experiment. We show that the fidelity of prepared state isinsensitive to inefficiency of the photon detector. In the second part, based onwell-established entanglement channels, we show that a remote quantum phase gatecan be implemented between two distant qubits via entanglement swapping andentanglement purification. This protocol can be realized in different kinds of physicalsystems. To be specific, we give a brief example for its realization in two-mode cavityQED system, using trapped atoms as material qubits and photons as flying qubits.In chapter5, we focus on implementation of quantum logic gates in neutralRydberg atom system. The long range interaction between distant neutral atoms isinduced by Rydberg blockade mechanism. Combining classical-like dipolarinteraction and F rster interaction we propose three schemes for implementing amulti-qubit quantum phase gate, a two-qubit quantum swap gate, and a two-qubitgeometric phase gate, respectively. Individual atom addressing are not involved in ourproposals. The effect of imperfect Rydberg blockade, perturbation of van der waalsinteraction and atomic spontaneous emission on gate fidelity are discussed in detailed.The scalability of quantum computation based on neutral Rydberg atom system is alsobriefly commented.