Theoretical Study On Generation Of Continuous-variable Entanglement

With a more comprehensive understanding on the microscopic world as well as the grow-ing demand for information processing, quantum information has emerged. As an interdisci-plinary field between quantum mechanics and information, the birth of quantum information not only enriches quantum mechanics, but more importantly takes a new look different from classical information. Entanglement is the basis of quantum information and important re-source.Preparation and application of continuous-variable entanglement(CV) in quantum infor-mation, as an advanced subject in the field of quantum mechanics and quantum optics, not only testify some basic principles but also provide the major resources. CV can be acted as information memory in constructing quantum network. In addition, the primary advantages of abilities in storage for CV, are infinite degree of freedom over single bit. Regardless of which scheme used should pay attention to producing strong CV.This dissertation is mainly focused on the preparation of the strong antijamming, stable and high CV. The thesis consists of nine chapters and our main contributions are given in Chapter 3 through 9. Quantum information, entanglement and especially CV are introduced in Chapter 1. In Chapter 2, firstly, basic knowledge on interaction between atom(s) and field are presented, including quantization of fields, the interaction Hamiltonian between atom(s) and fields and Weisskopf-Wigner approximate theory. Secondly, a squeezed state is defined and the measure methods of CV are given. Lastly, the quantum correlation function is derived.The quantum correlation between a pair of photons is studied in Chapter 3. The cor-relation shows anti-bunching property, and the second correlation function dependents on the classical fieldΩ. So we can control the magnitude of the joint detection probability by adjusting the driving field.In Chapter 4, a driven three-level atom system in free-space is investigated. The quantum entropy between the three-level atom and its spontaneous field is calculated. In addition, the entanglement of the k photons and q photons is studied, which shows that the two fields are entangled for short time evolution. In chapter 5, the generation and evolution of entanglement in a four-level laser with a nondegenerate parametric oscillator are discussed. Used Duan criterion, the entanglement properties of the two-mode light generated by this scheme is studied. It is shown that the high intensity of the entangled light between the two-mode cavity can be achieved with the help of the parametric oscillatorIn chapter 6, a three-level cascade laser with a self-phase locked type-Ⅱnondegener-ate optical parameter oscillator via a homodyne-mediate quantum feedback is investigated. The entanglement properties and the mean photon numbers between the two-mode light are studied. The influence of the feedback action for SPL NOPO on entanglement and the mean photon numbers are considered. Due to the presence of the feedback loops for the SPL NOPO, the squeezing, entanglement and mean photon number in the steady region can be enhanced efficiently.In Chapter 7, the generation of entanglement in a double A system is studied. The available entanglement criterion for the double A system and the condition for the existence of entanglement are analysed. The results show that, under proper parameters, the two-mode field can be entangled and amplifiedmean photon number in the steady region can be enhanced efficiently.In Chapter 8, a single two-level atom and exciton mode in a microcavity system are stud-ied. The second-order correlation function and the continuous-variable entanglement between the cavity and exciton mode are investigated. It is found that nonclassical (antibunching) effect and continuous-variable entangled light can be generated. More importantly, the gen-eration of continuous-variable entanglement can be controlled by tuning the frequency of the pumping field.In Chapter 9, a quantum well in a microcavity driven by an external field is studied. Applying a linearized fluctuation analysis, the single mode quadrature squeezing spectra as well as CV entanglement between the cavity field and exciton are calculated. It is found that maximum squeezing can be achieved by choosing appropriate quadrature angles for different nonlinearity. More importantly, CV entangled light can be generated in this scheme by tuning the frequency of the laser field and changing the nonlinear coefficient.In chapter 10, a scheme for entangled state generation in quantum nodes is presented, which is immune to the collective noise in a channel. It is shown how to implement quantum communication by those states prepared in the present scheme. Furthermore, the scheme can be generalized to generate entanglement in N nodes. Finally, the results are summarized.