Enhancement Of Two-mode Entanglement With Spontaneously Generated Coherence

Quantum information science is a cross-disciplinary field of computer science, in-formation science and quantum physics and also of information network, superconduc-tive science and nano science, which is an important direction of science and technology in the future. Quantum information science not only provides new theories and meth-ods for the development of information science but also helps to understand quantum mechanics in depth. Entanglement is the foundation and important resource of quan-tum information, which has a variety of potential applications in quantum computing, quantum communication and quantum cryptography. In recent years, preparation and manipulation of continuous variable(CV) entanglement as well as its application, as frontier of quantum physics and information science, not only testify some fundamental principle of quantum mechanics but also provide the basic resources. Moreover, CV has infinite degree of freedom so as to possess a bigger ability in storage over single bit and can be generated and controlled with nonlinear optical device in high precision. However, the system in practice will interact with environment inevitably, which results in decay of coherence, i.e., decoherence. On the other hand, with the development of nanotechnology, spontaneously generated coherence(SGC) has been demonstrated ex-perimentally, which can modify the features of many important quantum effects. Based on these, by using SGC in a cavity quantum electrodynamics(QED), how to produce the robust, stable, and high CV entanglement is investigated in this dissertation. The main contents are as follows:Firstly, in independent atomic system, we propose a scheme for preparation of high degree of two-mode CV entangled light in a laser-driven three-levelⅤ-type atom inside a cavity by taking into account SGC. The scheme relies strongly on the coupling configurations of the cavity modes to the transitions. Under suitable conditions, the system can reduce to a nondegenerate parametric amplifier which is responsible for the entanglement between the two cavity modes. It is shown that the crucial condition for the generation of entanglement between the modes is to creat population inversion between the dressed states, which has a very close relationship with SGC. Based on our scheme, it is demonstrated that the presence of SGC modifies considerably the steady dressed-stated populations and can lead to complete population inversion, which brings about an increase of magnitude of nonlinear process such that the entanglement of the cavity field can be significantly enhanced. Next, the effect of dark state on the generation of CV entanglement in a laser-driven A-type three-level atom with SGC is investigated. For a "dark state geometry" interacting with a laser field, the response depends critically on whether or not SGC is present. When SGC is maximal, the dark state is decoupled from environment and the steady values of density matrix elements are determined by their initial values. The results show, compared with the case without dark state, the dark state makes the time evolution of entanglement have a bigger magnitude and maintain a longer time. Meanwhile, the CV entanglement can be manipulated by changing the initial values of dark state, Rabi frequency, and squeezed parameter.Secondly, in collective atomic system, we propose a theoretical scheme for prepa-ration of the CV entanglement between two modes in a laser-driven V-type three-level atomic system by taking advantage of SGC. Based on the master equation for cavity field in the dressed states of driven atoms, the effect of SGC and the collectivity of the atoms on field entanglement are discussed in detail. It is indicated that the condition for enhancement of entanglement is to achieve the maximal population difference be-tween dressed states. The results show that the population difference becomes larger compared to the case of independent atoms or without SGC. Thus, under some condi-tions, the entanglement between the two cavity modes can be significantly enhanced by the collectivity of the atoms and SGC and attain its maximum when the relative phase△φ=π. Moreover, the evolution of entanglement versus relative phase△φis influ-enced by the strength of SGC, the number of the atoms, and the ratio of two spontaneous emission rates of atomic transitions.