Generation Of Squeezed And Entangled Light Via Atomic Ensembles

Entanglement play a vital role for quantum information because of its convenient and efficient operation in quantum processing. The corresponding researches has also been experimentally applied such as quantum communication network, logical gate, dense coding, and telecloning. There are various methods to achieve entanglement such as utilizing optical nondegenerate parametric oscillation, wave-mixing interaction, correlated emission laser, Kerr nonlinearity, atomic coherence effects and so on. Along with the development of the entanglement research, multimode entangled states of light have been theoretically proposed from tripartite, quadripatite to collective fields. On the other hand, two-or multimode squeezing are closely related to the two-mode and multimode entanglement. When the squeezing parameter is large one has Einstein-Podolsky-Rosen (EPR) entanglement. Recently, people use light interactions with atomic ensembles to achieve multimode squeezed and entangled states, i.e., an optical system on the basis of atomic ensemble couples to quantum light. A great deal of interest has been paid to the practical applications of atomic-field systems to quantum communication networks (QCNs). In this paper we have studied the interactions of multiple cavity fields with atomic ensembles and propose same new schemes for obtain multimode squeezed state, then Greenberger-Horne-Zeilinger (GHZ) and cluster entangled states. The main initiative results are presented as follows;1. Parametric interaction of a pair of cavity fields in a near-resonantly driven atomic system is described by a bilinear Hamiltonian that decouples from atomic flip operators and is proportional to the population difference between dressed states. For proper choice of parameters, the parametric interaction is enhanced by at least two orders compared to the dispersively dressed cases. Although spontaneous emission is fed into the cavity fields, destructive interference occurs in the fluctuations of a pair of collective modes. As a result of the two factors, perfect squeezing and Einstein-Podolsky-Rosen entanglement in the output occur when the cavity relaxation rates are much larger than in the dispersive case. The mechanism is applicable to a great variety of multilevel systems and has experimental advantage in the cavity QED generation of squeezed and entangled states of light.2. It has been known that two-mode entangled light can possibly be generated by employing near-resonant interaction with an ensemble of two-level atoms. The respon- sible mechanism is the absorption of two photons from the strong driving field and the emission of two new photons into the cavity field. Here we generalize such a mechanism to three separated atomic ensembles and establish cascade interactions for four nongenerate fields. It is shown that quadripartite cluster and Greenberger-Horne-Zeilinger entangled states happens for continuous variables. The advantage of the present scheme for the multipartite entanglement lies in that the coupling strengths are much larger due to the near resonances than for far-off-resonance based parametric processes.3. We describe the effects of Raman interactions on quantum correlations of the multimode optical fields. It is shown that both quantum beats and four-wave mixing processess exist in the Raman interactions of the multimode fields with atomic reservoirs. These two mechanisms combine to lead to emission in pairs into multiple cavity modes. Exemplify four cavity fields, we find that the four-mode squeezed state can be created at steady state. From this we can obtain the Greenberger-Horne-Zeilinger entangled state, and the linear and square cluster entangled states. This scheme does not need the preparation of the initial states of atoms and cavity modes, and is robust against atomic spontaneous decay because of the absence of excitation.