【作者】 平云霞；

【导师】 成泽；

【作者基本信息】 华中科技大学 ， 光学， 2009， 博士

【摘要】 量子信息学作为新兴学科在国际上兴起是在1994年之后，虽然时间很短，但已取得一系列的重要突破。量子信息学主要包括了量子信息论、量子通信、量子光通信和量子算法与量子计算等等。近年来，量子信息在理论上和实验上都取得了重大进展，已经引起各国政府、科学界和信息产业界的高度重视。量子纠缠是量子信息学中最基础、最核心的部分，其作为几乎所有量子信息处理任务中都不可缺少的物理资源，吸引了众多物理学家的巨大兴趣。近来，有关连续变量纠缠态的制备及其在量子信息处理中的应用成为量子光学和量子信息科学的前沿研究领域。连续变量纠缠态性质的研究不仅可以用于验证量子力学的基本原理，而且还是量子信息处理的基本资源。因此，如何制备出连续变量纠缠态引起了人们的广泛关注。本论文主要呈现的是几种连续变量系统纠缠性质的理论研究，研究内容分三大部分：第一部分，利用连续变量系统的双模纠缠判据，分别研究了三能级梯形原子系统的稳态与瞬态的量子纠缠性质。通过在原子的两个能级之间施加一个相干外驱动场，使原子能级之间跃迁的两个不同频率的光场发生纠缠。研究发现泵浦强度、原子与光场之间的耦合系数等对系统的纠缠性质起着积极作用，而失谐量、原子的衰减系数及腔的衰减系数对系统的纠缠性质起着抑制作用。在此基础上，还研究了三能级梯形原子系统中量子纠缠与Bell不等式的违背之间的关系，发现该系统中存在有些量子纠缠态并没有违背Bell不等式。第二部分，分别研究了反铁磁体与铁磁体中磁振子的纠缠特性。通过求解时间演化算符，得到产生湮灭算符的时间演化关系，然后得到自旋分量的量子涨落表达式。依据量子纠缠判据，研究发现反铁磁体中两束向不同方向传播的自旋波之间会产生周期性的纠缠，铁磁体中沿同方向传播的两束不同能量的自旋波之间也会产生周期性的纠缠。第三部分，分别研究了压缩相干态与叠加相干态的量子纠缠性质。对于压缩相干态，量子纠缠仅与压缩参数与压缩角有关，与平移参数无关；对于叠加相干态，量子纠缠会受到相干振幅、相位角及相对相位的取值影响。更多还原

【Abstract】 Quantum informatics as a newly subject didn’t rise in the world until 1994. Thoughthe time is short, a series of important breakthroughs have been gained. Quantuminformatics mainly includes quantum information, quantum communication, quantumoptical communication, quantum computation and so on. In recent years, a surprisingprogress on quantum information has been made at the theoretic and experimental fields,which have attracted much attention of the states of all the countries and people who workin the field of science or in the information industries. Quantum entanglement, the core ofthe quantum information theory, attracts much attention from many researchers due to thefact that it is the indispensable physical resource in quantum information processing. Now,continuous variable entanglement has attracted a lot of attention in quantum optics andquantum information due to the experimental realization of quantum informationprocessings in continuous variable regime. Therefore, the preparation of continuousvariable entanglement attracts much attention now.Entanglement propertities of several continuous variable systems are studied in thisthesis, and the main research contents are as follows:First, entanglement properties of the steady states and the transient states in acascade three-level atomic system are investigated respectively according to the two-modecontinuous variable criterion. The top and bottom levels of the system are coupled by astrong external pump field. Two different frequency fields that may be entangled can beproduced by the transition from the top to the bottom level via the intermediate level. Wefind that the density of pump field and atom-field coupling constants play the positive rolein entanglement properties, while detuning, damping constant and atom decay rates playthe negative role in entanglement properties. Furthermore, we study the relation betweenquantum entanglement and violation of Bell’s inequality in another three-level cascade atomic system. We find there are some states which are entangled do not violate the Bell’sinequality.Second, the dynamics of entanglement for two-mode magnons in antiferromagnetand ferromagnet is investigated respectively according to the two-mode continuousvariable criterion. We can obtain the time-dependent operators of magnons by the timeevolution operator. The quantum fluctuation expressions of the spin components can alsobe obtained. It is shown that entanglement between the two spin-waves along the oppositedirection can be generated and occurs periodically with time in the antiferromagnet, andentanglement between the two spin-waves along the same direction which has differentenergies can also be generated and occurs periodically with time in the ferromagnet.Third, entanglement properties of two-mode squeezed coherent states andsuperposition states from several two-mode coherent states in radiation field areinvestigated according to the continuous variable entanglement criteria. It shows that thesqueeze angle and squeeze parameter play an essential role in the study of entanglementproperties for two-mode squeezed coherent states. On the other hand, for superpositionstates from several two-mode coherent states, it is found that the entanglement dependsstrongly on coherent amplitude of each mode, phase angle and relative phase angle.更多还原