Entanglement Study Based On Quantum Feedback Control

The related research of quantum entanglement has attracted much attention with the development of quantum optics and quantum information theory. Ideal quantum systems are closed, so that they can keep their quantum nature. However, real quantum systems are open to the outside environment and are therefore susceptible to the phenomenon of decoherence, resulting in the loss of entanglement. For the effective quantum informa-tion tasks, disentanglement phenomenon is a main obstacle. Therefore, it is significant to study entanglement dynamics evolution properties of open quantum systems and to master the methods of controlling entanglement decay. Recently, quantum feedback con-trol is regarded as a promising approach to deal with the decoherence problem and pre-vent entanglement degradation with the development of experiments and theories about real-time monitoring and manipulating individual quantum system. Therefore, we study disentanglement properties and dynamic behaviors of spin systems and cavity quantum electrodynamics systems, focus on production and controlling of steady entangled states using quantum feedback control method.We investigate the dynamics of quantum entanglement and quantum correlation of a qutrit-qubit mixed spin open system coupled to two bosonic reservoirs at different tem-peratures by using logarithm negativity and measurement-induced disturbance. It is dis-cussed in detail that the influences of initial states of the spin system, temperatures of the reservoirs, the ferromagnetic coupling, the antiferromagnetic coupling between the spin particles and anisotropic parameters on the evolution of quantum entanglement and quantum correlation. It is found that quantum entanglement is largely influenced by the initial states of the system and temperatures of the reservoirs, and there is entanglement sudden death phenomenon, while the quantum correlation is different. This also shows that entanglement is a kind of special quantum correlation and separable states may also have quantum correlation. The quantum entanglement and quantum correlation can be enhanced by appropriately modulating the coupling constants and anisotropy parameters. The antiferromagnetic spin chain has stronger robustness.As the number of qubits grows, the entanglement dynamics becomes more complex, and the entanglement is very fragile in the practical environment. Therefore, developing the methods to control multipartite disentanglement is still a challenge. We investigate entanglement dynamics of three driven A-type three-level atoms off-resonantly coupled with a single-mode cavity under quantum-jump-based feedback control. The entangle-ment evolution properties of three atoms are discussed in the cases of three-atom sepa-rable initial states and three-atom entangled initial states. The results demonstrate that the tripartite entanglement can be effectively enhanced by setting the Rabi frequencies of classical fields and choosing the local quantum feedback control when the three atoms are initially in separable states. It is important that stable W-class states with arbitrary coefficients can be obtained by modulating the Rabi frequency of only a classical field and choosing proper local feedback, including important tripartite entangled states, such as W state|W>, W-class states|W’±> and three-qubit decoherence-free state for collective amplitude dampin｜一(?)>. Furthermore, the decoherence-free condition of the tripartite W-class state｜φ> is found. The asymmetric W states and W state can be converted into each other by properly adjusting the Rabi frequencies of classical fields and choosing feedback control. The multiqubit W state can be generated and stabilized by using our scheme in theory. We hope that our work may be useful for the experimental realization of a steady multiqubit W state in the near future.Compared to the two-dimensional systems, the efficiency of quantum communica-tion encoding in high-dimensional space will be improved greatly. Quantum key distribu-tion and quantum cryptography encoding in qudit systems are more secure against certain types of eavesdropping attacks. A robust and scalable theoretical scheme to generate a steady three-dimensional entangled state of two atoms is proposed by feedback control based on quantum-jump detection. The specific system is consisted of a V-type atom and a A-type atom trapped in a strongly dissipative bimodal cavity in the scheme, which are driven by two classical fields. The robustness of this scheme reflects in the insensitivi-ty to detection inefficiencies and the strong ability against the parameter fluctuations in the feedback, classical field driving, and atom-cavity coupling strengths. The influence of atomic spontaneous emission can be suppressed by using the local feedback control. The scalability is ensured that N-dimensional entangled states of two atoms can be deter-ministically generated. Finally, the feasibility of the scheme is discussed in experiments.