The Study On The Preparation Of Two-Atom Entangled States In Cavity QED System

Two-level atom has been regarded as an important principal candidate for quantum bit (Qubit), recently. The atomic entangled states are a part of the vital resources in quantum information processes such as quantum teleportation, quantum information storage, etc. Cavity quantum electrodynamics (CQED), as one of the most conventional system in preparing entangled atoms, plays an important role in quantum information processing. Many schemes about quantum information studyings focus on using CQED to generate atomic entangled states. The motivation of this dissertation is just based on the exciting present situation.In this dissertation, possible schemes for generating two-atom entangled states in CQED system are poposed. The characteristics and the reliability of the prepared entanglement are discussed and evaluated. In chapter 1, some basic quantum theories about coherent collective excitation in a multi-atom system and master equation are introduced. The discussions in chapter 1 are the initial scratch line of this dissertation. From chapter 2 to chapter 6, some representative possible schemes for preparing two-atom entangled states in CQED system are put forward and discussed, respectively. In detail, dissipative entangling dynamics in two-photon Jaynes (會卑) ummings model with degenerate atomic levels is studied in chapter 2. The discussion for this model may extend the overall understandings about decoherence of the Jaynes-Cummings model. The linear entropy of the system and the atom are found to exceed 0.5. In chapter 3, the influence of thermal environment on two-Qubit entanglement in a non-isolated CQED system is studied. In this chapter, the possible positive contribution of the thermal noise to two-atom entanglement is confirmed. Two-atom entangled state can be generated for initial atomic state \ee) when two atoms differently couple to the cavity field.In chapter 4, the collective cooperative interaction between atoms under stimulated emission is clarified to be critically important in inducing two-atom entanglement. Also, this interaction can generate atomic entangled state for initial atomic state \ee). In chapter 5, under areal-time CQED experimental condition, the dependence of two-atom entanglement on atomic spatial position is presented. Furthermore, the ratio of atom-atom dipole-dipole interaction to atom-field coupling strength is proved to be dominant in the generation of two-atom entanglement. Finally, in chapter 6, an interesting scheme for entangling two distant atoms is extended to multi-atom cases. The local laser fields are found to act as tuning switches in generating and modulating the entanglement of distant atoms. Under appropriatecontrolling conditions, Bell state of distant atoms is feasibly prepared in a N atoms system.It can be concluded that special advantages and challenges coexist in using CQED system to prepare two-atom entangled states. On one hand, in view of the intrinsic characters of CQED, the quantized laser field can be conveniently utilized in finely tuning the effective interaction between atoms, thus the atomic entanglement can be generated. On the other hand, inevitable atomic spontaneous emission and cavity leakage depress the reliability of the entanglement, and even lead to thorough decoherence and disentanglement (just as the descriptions in chapter 2). The communication between a quantum system and its thermal environment results in some enhanced dissipations. However, the results in this dissertation indicate that, in addition to weakening and restraining the dissipations by using enhancing stimulated emission (chapter 4) and dipole-dipole interaction (chapter 5), thermal noise (chapter 3) and cavity leakage (chapter 3 and chapter 6) are, at least seemingly, possible substitutions in generating two-atom entanglement. All the conclusions ensure the reliability and the feasibility in the processing of preparing two-atom entangled states in CQED system.