Preparation Of Quantum Entanglement Via The Dissipative Process

Quantum entanglement, which is predicated by quantum mechanism and has been demonstrated by experiment, is the unique property of quantum physics. Quantum entangled state is the basic resource for quantum information processing tasks; on the other hand, quantum entangled state can be used to text the basic principle of quantum mechanics, which can help people to understand deeply the quantum world. Therefore, how to effectively prepare and manipulate entanglement is crucial for many quantum information processing tasks. However, in practical situations, quantum system would inevitably couple with the bath or environment around it, which induces the decoherence. For the cavity QED system, the typical dissipative factors that induce the decoherence including atomic spontaneous emission process, cavity leakage process and so on. Different with the traditional opinion, a number of recent researches demonstrate that the above mentioned dissipative process in cavity QED system can be used to prepare rather than destroy the entangled state. The relevant process is the so-called dissipative dynamics. The main purpose of this dissertation is to design schemes which utilize the dissipative processes in the cavity QED system to prepare entanglement. We hope that the theoretical consideration can offer some information for relevant experiments.With the optical cavity that traps two Λ-type atom, we propose a scheme to prepare entanglement in which the atomic spontaneous emission and cavity decay are used as necessary resources. We can see the dynamics process clearly through re-expanding the Hamiltonian under the dressed state basis. Then we can choose the suitable system parameters to pump the undesired state in the ground state subspace to the well-defined state in excited state subspace which can be converted to the desired state through dissipation. Compared with the traditional unitary dynamics based schemes, the dissipative factors are utilized as necessary resources to prepare entanglement in our schemes. Besides, our scheme does not require specifying the initial states and controlling evolution time accurately.In the coupled two-mode cavity model, we propose a scheme to prepare threedimensional entanglement via the atomic spontaneous emission and cavity decay. The basic method is the effective operator method, which relies on the weak driving condition. Based on this method, we adiabatically eliminate the excited states and thus achieve the effective coupling process and the effective Hamiltonian that only contain interactions among ground states. Using the effective process of the system, one can easily choose suitable parameters to make some desired process being stronger and some undesired process being weaker. On that basis, the desired state would be the steady state of the system.To clearly get the influence of each dissipative process, we consider the atomic spontaneous emission process and cavity mode leakage process separately. The results show that the performance of the cavity leakage based scheme is worse than that of the atomic spontaneous emission based scheme. We then use the quantum-jump-based quantum feedback control to improve the fidelity of the cavity leakage based scheme. Interestingly,since the conditions of the cavity leakage based scheme are almost the same as that of the atomic spontaneous emission based scheme, we thus can use both of these two dissipative factors to achieve the desired three-dimensional entanglement. Compared with the unitary-dynamics-based schemes, our scheme does not require controlling evolution and specifying the initial state. Numerical simulations show that the scheme has high fidelity.The long distance and strong interactions between Rydberg atoms always make the Rydberg energy level shifted, which would induce the blockade mechanism. Until now,there are many quantum information protocols based on the blockade mechanism. Differently, some researches show that through adjusting the frequency of the driving field, one can achieve the anti-blockade mechanism. The basic principle of the anti-blockade mechanism is to compensate or cancel the Rydberg-Rydberg interaction strength. Based on this mechanism, one can realize the resonant coupling between ground state and bi-excitation Rydberg states. The bi-excitation Rydberg state would be converted into single-excitation subspace by atomic spontaneous emission and further be converted to ground states. With the help of microwave field and classical field, the undesired state in the ground state subspace would be pumped to Rydberg states again. The whole process would be repeated until the target state is prepared. Besides, under the similar physical thoughts, the threedimensional entanglement would also be realized. Numerical simulations show that the scheme has higher fidelity and robust on the fluctuation of parameters.With the cavity–fiber–cavity system, we propose scheme to prepare distributed entanglement. For the convenience of research, we use the eigenvectors of atom-cavity Hamiltonian as dressed states. Then we find out the dressed state in the single-excitation subspace which can be transformed to the desired state through atomic spontaneous emission, cavity mode leakage, and fiber mode leakage. The next step is to realize the resonate coupling between undesired state and the well-defined dressed state in single-excitation subspace. At the same time, microwave field should be designed to translate all of the undesired states. By doing so, the scheme is independent of initial state. Since the distributed entanglement is critical for many remote quantum information processing tasks,our research may provide theoretical supports for some relevant experiments.