【作者】 杨柳；

【导师】 吴金辉；

【作者基本信息】 吉林大学 ， 光学， 2016， 博士

【摘要】 光与原子相互作用,已成为量子光学研究中最重要的课题之一,在现代物理学和量子信息科学领域中展现出强劲的发展势头和独特优势,而基于相干光场与原子介质相互作用的产物——原子相干效应,更是受到人们的广泛研究及应用。在量子水平探究光与原子相互作用不仅对量子光学的发展有着重要意义,还促进了量子光学和信息学科的交叉。本论文中,我们主要在多能级冷原子模型中利用原子相干效应研究量子光学系统中一些有趣的现象和重要的应用。本文将对原子相干效应进行回顾,例如电磁诱导透明和受激拉曼绝热过程,重点介绍了单频光场在传播过程中的任意Berry相位的积累、双色不互惠性质以及存储和提取等相关研究工作的物理方法、研究背景及近况。为进一步在实验上实现有效的相位门、光学二极管和光学开关等提供有意义的指导,也为量子信息的实际应用打下坚实基础。第三章中,我们在三能级(43)型相干冷原子系统中提供了有效的部分受激拉曼绝热过程来精确地积累任意Berry几何相位的方案。不精确的量子门操作给实验上实现量子信息过程带来了一定的影响,而绝热技术可以提供有效而且稳定的方法来克服这种不确定性。部分受激拉曼绝热过程不同于标准的受激拉曼绝热过程,它可以实现初始态和目标态的任意粒子数布局进而实现两个态的任意相干叠加态,并可以实现任意相位的积累。但是Berry几何相位的积累需要经历一个循环演化路径,所以我们需要精确安排泵浦和斯托克斯脉冲的时间序列来构成两个对称的部分受激拉曼绝热过程。随后,我们可以通过由泵浦脉冲和斯托克斯脉冲决定的混合角来进行调节所获得的几何相位的大小。通过数值计算表明,由于脉冲宽度和时间延迟对各个能级的粒子数布局并不敏感,故可以用来更精确地积累Berry几何相位。精确积累任意Berry相位可以用来构建特殊的量子门,并在量子计算和快速产生相位门上做出一定贡献。我们还可以利用更复杂的脉冲序列将这个方案还拓展到其他多能级系统中。第四章中,我们将Tripod型冷原子装载进一维光学晶格中,并在运动的光晶格系统中提出一个获得双色不互惠反射和透射效应的方案。与先前的工作相比,我们选择一个实验上更加合理的高斯型的原子密度分布和一个低至3 m/s的原子运动速度。由于多普勒效应,运动原子的光学响应将会被改变。这主要表现在探测场的极化率沿着传播方向会出现一个频移。当原子晶格低速运动,探测场的反射和透射系数会被改变,则我们可以分别在两个频率区域获得高效率的反射和透射不互惠效应。经过计算发现,我们在可调光子带隙的陡峭边缘附近观察到对比度高为92%(85%)的透射(反射)不互惠效应。并且,不互惠效应的效率和频率区域可以通过耦合场的失谐、探测脉冲的空间长度以及原子晶格速度等参数进行调控。这个方案得到的高效透射不互惠效应可以用来实现全光二极管、光隔离器以及单向传输等;反射不互惠效应可以用来实现隐形材料。这项技术还可以轻易地拓展到其他多能级系统中用来实现多色不互惠效应。第五章中,我们在相干超冷里德堡原子系综中,研究一对单光子脉冲的动态存储和提取过程,并提供光脉冲的动力学演化和光子-光子相互作用的更真实图像。先前关于里德堡EIT方面的工作主要是基于光的稳态传播图像,我们则采用数值模拟其动力学公式的方法来研究相互作用脉冲的动态吸收和传播的整个量子过程。脉冲之间不断变化的相互作用会使这个过程变得十分复杂,即使在微扰情况下也会显著地改变它们的吸收和色散性质。通过数值计算我们发现,在里德堡介质中,当两个光子靠近时带有正、负失谐的光子在传播过程中会展现出不同的图像,而不总是由里德堡阻塞导致二能级系统的现象。这个包含光子存储的过程可以用来实现光开关和光子-光子门。此外,我们也对其他相关问题进行详细地探究,例如单光子脉冲在不均匀相互作用下转化的自旋波图像、存储光子过程中的绝热和非绝热条件带来的影响,以及在不同情况下存储和提取光子的最优方案等。深入了解这个过程会对相关实验提供有利的指导,并为全光量子器件和非经典光调控等方面工作的进一步研究打下良好基础。更多还原

【Abstract】 The interaction of the light with atoms, as a very important research content of the quantum optics, has thriving development in recent decades. Based on the generation of the interaction of coherent light with atomic medium, atomic coherence effects have received increasing attention. Studies on the coherent photon-atom interaction at quantum level have enhanced the development of quantum mechanics, and given birth to a new interdisciplinary field bridging quantum optics and information technologies. In this thesis, we find several exciting and unexcepted results in quantum optical systems by using the atomic coherence.This thesis mainly focuses on atomic coherence effects related phenomena in various systems involving multilevel atoms. Based on atomic coherence effects, such as electromagnetically induced transparency(EIT) and stimulated Raman adiabatic passage(STIRAP), this thesis mainly introduces the physical methods, research background and recent developments related to the accumulation of arbitrary Berry phase, two-color nonreciprocity, storage and retrieval of photons, and so on. All of these may provide a more reliable theory for experimently constructing the effective phase gate, the optical diodes and the optical switch, and lay a solid foundation for the practical application of quantum information.In the third chapter, we propose and analyze a feasible scheme for achieving geometric Berry phases via the efficient technique of fractional stimulated Raman adiabatic passage(f-STIRAP) in the typical Λ coherent cold atomic system. Experimental implementations of QI algorithms suffer from inaccuracies in the gate operations, which reduce the fidelity of QI processing. Unlike standard STIRAP,f-STIRAP allows the creation of any preselected coherent superposition of the intial and final states. Note, As accumulation of a Berry phase needs in a cyclic evolution path, we carefully design two reversed f-STIRAP processes. Then, we can control the Berry phase by the accurately designed mixing angle of pump and Stocks. And because the evolution paths of population and the ultimate values of Berry phase are not sensitive to the pulse duration and the pulse delay, we can pulse duration and pulse delay may also be modulated to accumulate a desired Berry phase by controlling the intermediate dark states. The technique can be easily extended to other multi-level systems with a little more complicated pulse sequences applied to bring convenience to the quantum computer and rapidly generate some quantum gates.In the fourth chapter, We study the two-color nonreciprocal effects of transmission and reflection in cold atoms driven into the tripod configuration and confined in a moving optical lattice. Compared with previous work, we choose a more reasonable atomic density distribution in experiments and a lower atomic speed of 3 m/s in the optical lattice. As expected from the Doppler effect, the optical response of a moving atom will be changed. The main manifestation is that the probe susceptibility will experience a frequency shift along the direction of motion. The probe transmissivity and absorption coefficient of the moving atomic ensemble will be changed due to the Doppler frequency shift experienced by the probe susceptibility. Our numerical results show that a very high contrast of the forward-backward transmission up to around 92%(reflection up to around 85%) is observable near the sharp edges of two tunable PBGs. And the such two-color optical nonreciprocitycan be dynamically manipulated by varying the driving and coupling field detunings, the probe pulse length, the atomic lattice velocity, etc. The transmission nonreciprocity can be used to produce optical diodes or optical isolators, while the reflection nonreciprocity can be used to design invisible metamaterials. The technique shown here could be easily extended to other multilevel systems with multicolor nonreciprocal effects.In the fifth chapter, we study how to stop and regenerate a pair of single-photon pulses at adjacent locations and provide a realistic picture for photon-photon interactions in coherently prepared Rydberg atomic ensemble. A majority of the previous works about Rydberg EIT are based on the picture of steady-state propagation of a light. However, we provide an efficient approach to such complicated quantum dynamical processes in Rydberg medium. Such dynamical process can become rather complicated due to the ever-changing interaction between photon pulses, which is found to be capable of changing their absorption and dispersion significantly even in the perturbative regime. Our numerical results show that when two photons get close to each other in Rydberg EIT medium, there will exist different scenarios, not always the commonly known Rydberg blockade leading to a two-level medium. The similar processes involving stopped photons can be applied to implementing photon switchers and photon-photon gates. The other related problems, such as the converted spinwave profiles under inhomogeneous interaction, the difference between adiabatic and non-adiabatic passages for stopping photons, as well as the appropriate ways to stop and regenerate photons under different circumstances, are also investigated in our approach. Providing the optimal strategies for implementing photon transistors and photonic gates, the understanding of the processes could be valuable guide for the relevant experimental researches.更多还原