Study On The Feedback-based Control Of Quantum Decoherence And Disentanglement

Quantum information processing (QIP) may be implemented with principles of quantum physics. Compared with the classical information processing, QIP is genuine parallel processing, which is based on quantum coherence and quantum entanglement. But, quantum coherence and entanglement will decrease because of environment noise, which will largely decrease the efficiency of quantum information processing. So, it is of high importance to maintain the coherence and entanglement of the quantum states. In the thesis, based on the analysis of the relationship between quantum feedback control and suppression of quantum decoherence, we will design some strategies to keep coherence and entanglement in several typical open quantum systemsand emphases are placed on the specific decoherence factors such as the dissipation, the drift and the decay of the systems.The thesis discusses the decoherence suppression and the corresponding entanglement control problems with the help of quantum-jump-based feedback and numerical simulation. The main contributions are as follows:(1) The decoherence suppression of two qubits Markovian system is investigated by the optimal quantum-jump-based feedback control. Based on quantum feedback techniques and numerical simulation methods, we studied the quantum coherence and entanglement dynamics of two coupling qubits (one of them is subject to damping while the other qubit is not, and the coupling will induce the dcoherence of the non-damping qubit). Here, we focused on the effects of the mixed entangled initial states and the feedback Hamiltonian on the coherence and entanglement dynamics. This study is of great importance for overcoming decoherence and disentanglement of two-qubit quantum systems. The results show that the quantum-jump-based feedback on the damping qubit can effectively protect the coherence of the non-damping qubit and the entanglement between the two qubits.(2) We study the entanglement dynamics of three identical atoms resonantly coupled to a sing-mode cavity with heavy damping. To modify this fast decaying entanglement dynamics, we introduce quantum feedback control and focus on the entanglement dynamics of two atoms (concurrence) and three atoms (negativity). The possibility to greatly improve the degree of three-atom entanglement by means of Markovian feedback is shown. With appropriate feedbacks, the entanglement of the final stable states of two atoms or three atoms can be greatly larger than zero. We compare the effects of different feedback controls and show that the quantum-jump-based feedback can stabilize and protect highly entangled tri-atom states against decoherence caused by spontaneous emission. The most obvious difference for the GHZ and W state cases is the bipartite entanglement dynamics without feedback. For the W state case, the bipartite entanglement will decay to zero and will not revive thereafter. But, in the GHZ state case, the bipartite entanglement will revive from zero at some time point and then the bipartite entanglement will decay to zero after some time, and cannot revive anymore. The results show that, the feedbacks on one, two or three atoms all can weaken the bipartite and tripartite entanglement sudden death for GHZ state and W state. But, the patterns of the weakening process for bipartite entanglement are different for GHZ state and W state, which is caused by the difference in the entanglement features between GHZ state and W state. Furthermore, all these feedback schemes can drive the three-atom system in a stable partially entangled bipartite state and tripartite state for both GHZ and W state cases.