The Study Of Frozen Quantum Discord In Open Quantum Systems

It is one of the crucial issues in quantum engineering field to evaluate the detrimental impacts of decoherence on the schemes of the single-quantum-state control and exploring its efficient suppression strategy. The rapid development of the experimental technique in this field also asks for the establishment of the general description to the decoherence going beyond the widely-used Born-Markovian approximation and the pursuit of the new physics induced by the non-Markovian effect in explicit physical systems. On the other hand, it is found that quantum entanglement cannot exhaust all the characters of quantum correlation, and quantum correlation can be characterized by the so-called quantum discord in a more general way than the quantum entanglement. Therefore, the study on exploring the quantitative or even qualitative difference between quantum discord and quantum entanglement in the protocols of quantum information processing is a hot topics in quantum information science recently. Motivated by these two aspects, we study in this thesis the non-Markovian correlation dynamics of the following several systems influenced by different type of decoherence channels:Firstly, we study the non-Markovian correlation dynamics of two initially quantum corre-lated discrete-variable systems immersed in two independent dephasing noises. It is found that much different from the behavior of the monotonic decay to zero of quantum entanglement, the behavior of quantum discord and classical correlation shows a transition behavior at certain time from classical decoherence to quantum decoherence. Before this time, the system is in classical decoherence regime, where the quantum discord is frozen to its initial value, while the classical correlation decays. After this time, the system is in quantum decoherence regime, where the classical correlation is frozen, while the quantum discord begins to decay to zero eventually. In this model, the non-Markovian takes no qualitative impact on the frozen behavior of quantum discord.Secondly, we study the non-Markovian correlation dynamics of two initially quantum cor-related continuous-variable systems immersed in two independent dissipative noises. It is found that the non-Markovian effect makes partial of the initial quantum discord frozen to its long-time steady state, which is qualitatively different from the monotonic decay behavior under the Born-Markovian approximation. It indicates that the Born-Markovian appximation fails to capture the physics in this parameter regime. Further study reveals that such forever frozen of the quantum discord is caused by the formation of a localized mode between each system and its coupled environments. According to this mechanism, we give a quantitative criterion to judge the physical condition in the presence of the quantum-discord frozen behavior, which can give a useful guideline to experiment in suppressing the detrimental impacts of decoherence.Thirdly, we study the correlation dynamics of two initially quantum dis-corrclated discrcte-variable systems with one of them immersed in a dissipative noise. It is found that the single local noise can not only induce transient quantum discord between the two systems, but also can make the induced quantum discord preserved in the long-time steady state. This is dramatically different from the previous results in the literature, where the transiently induced quantum discord decays to zero eventually. Our result reveals that we can generate steady quantum discord without resorting to interaction, neither direct interaction nor indirect interaction, and a single local dissipative noise can efficiently induce a steady quantum discord between the two systems. Further study shows that, same as the continuous-variable system, such steady quantum-discord generation is also dependent on the formation of a bound state between the system and its local noise. Our result indicates a new way to generate steady quantum discord for experiment without resorting to interaction but a local dissipative environment.All of our studies reveal the profound impacts of the non-Markovian effect in quantum information processing and the distinctive difference between quantum discord and quantum entanglement in characterizing quantum correlation. The mechanism of the formation of a bound state or a coupled localized mode in suppressing decoherence supplies a new method to experiment in beating the decoherence in practise.