Theoretical Research On Properties And Preparation Of Continuous Variable Entanglement

In addition to being of fundamental interest in quantum mechanics, entanglement as quantum correlations among quantum systems is a key element in rapid developing quantum informatics. Now, continuous variable entanglement has attracted a lot of attention in quantum optics and quantum information due to the experimental realization of quantum information processings in continuous variable regime. Any attempt to exploit the entanglement have to, however, face the corruption of the entanglement by unavoidable decoherence. As a result, how to produce the robust, steady, and high entanglement is an interesting research issue. In this thesis, we focus mainly on the preparation of bright and high continuous variable entanglement.We investigate quantum properties of the cavity mode and exciton in a single quantum dot embedded in a cavity which is coupled to a broadband squeezed vacuum bath. It is found that the exciton can exhibit the squeezing due the linear mixing between the exciton and the cavity mode. The entanglement between the exciton and the cavity mode can be established as a consequence of the injected squeezing to the cavity and the linear mixing between the exciton and the cavity mode. We discuss the entanglement between the excitons in two quantum dots insides two separate single-mode cavity coupled to a broadband two-mode squeezed vacuum, and the entanglement among excitons in multiple quantum dots in a single-mode cavity driven by a squeezed vacuum. It shows that in absence of the dissipations the entanglement is dependent of the initial squeezing of the cavity. When considering the dissipations of the system, the long-time entanglement between the excitons depends only on the property of the injected squeezed vacuum. From our discussions, it concludes that the nonclassicality is a prerequisite for the entanglement of the linear-mixing systems.Next, we discuss how to prepare bright and high entanglement by means of nonlinear optics. We propose to generate entangled laser in a three-level cascade correlated spontaneous emission laser, and find the laser field exhibits entanglement with a lot of photo at short time region. We investigate the enhancement of the intracavity entanglement of a self-phase-locked type-II nondegenerate optical parameter oscillator via homodyne-mediated quantum feedback. It is found that the feedback can effectively enhance the squeezing, entanglement and purity of the two-mode intracavity field. Especially for an ordinary NOPO, our feedback scheme can simulate a standard two-mode squeezed vacuum environment and the pure steady two-mode squeezed vacuum cavity field can be generated. In addition, the generation of macroscopic and spatially separated three-mode entangled light for triply coupled x³ Kerr coupler inside an pumped optical cavity is investigated. With the linearized analysis for small perturbation around the steady state of the pumped cavity modes, the linearized Hamiltonian and the corresponding master equation are obtained. It is found that the bright three-mode squeezing and full-inseparable entanglement can be achieved both inside and outside the cavity.Finally, the preparation of the entangled beams from an intracavity trapped two-level ion is investigated. By means of adiabatical elimination of the excited level with assumption of the large detuning between the pumping and ion, we have one cavity mode couples to the vibrational mode of the ion with a nondegenerate downversion and the linear mixing happens between the second cavity mode and the vibrational mode. It is found that the bright output light with the steady robust entanglement can be always achieved even in the presence of the decoherence involving the vibrational heating and the cavity damping. Next, we introduce the biphoton process of the cavity modes to the master equation discussed above by considering the case of non-adiabatical elimination. We find the the extra biphoton process can enhance the entanglement of the cavity field.