【作者】 方彬；

【导师】 黄运锋； 郭光灿；

【作者基本信息】 中国科学技术大学 ， 光学， 2010， 硕士

【摘要】 从20世纪初量子力学诞生以来,人们对于物质结构及其相互作用的见解发生了革命性的变化。通过量子力学,很多人们无法理解的现象得到了解释。这些现象也都被一些实验所精确验证。随着科学的不断发展,学科间的交叉越来越明显,也越来越紧密。20世纪90年代,量子力学与信息科学的结合将信息科学带入了一个新的发展空间量子信息科学。量子信息的基础则是量子态的制备、操控、传输、存储以及测量。目前,人们已经可以在线性光学、腔量子电动力学、核磁共振、离子阱和量子点等系统中实现纠缠态的制备。光学体系利用光子作为信息的载体,因此具有易操纵、传输性质良好和环境消相干小等优点。纠缠光子对在很多量子通讯领域都有着重要的应用,例如量子密钥分配、量子隐形传态、量子中继和验证量子力学基本理论。大部分光子源都是利用自发参量下转换过程来实现光子对的制备,在很多的实验方案和不同的晶体中都可以实现。通常,该过程都是利用了相位匹配原理,然而传统的块状晶体的产生效率较低,因此,由于光纤的中损耗,不利于长距离传输。最近,很多关于利用周期性极化晶体或者波导来产生光子对的实验被相继报道。它利用准相位匹配的原理,从而可以获得高的产生效率。本文从理论和实验上对基于周期性极化的铌酸锂晶体产生的参量下转换光谱进行了一系列研究,特别是光子的波长和谱宽。由于周期性极化铌酸锂晶体有较宽的工作温度和较大的热膨胀系数,非常适于在较大范围内实现波长可调谐,通过改变晶体的温度,参量下转换光子的波长也线性的变化。另外,在温度不变的情况下,改变泵浦光的波长也能够线性的改变参量光的波长。由于光纤中的色散,光子对的谱宽是一个必须考虑的很重要的因素。通过不同长度晶体和不同脉冲宽度泵浦的实验对比,我们发现下转换光子的谱宽随着泵浦光宽度的增加而增加,随着晶体长度的增加而减小;但是晶体长度的影响将会随着晶体长度的增加和泵浦光宽度的增加而减弱。特别地,在我们的实验中,当用飞秒激光泵浦时,下转换光子的谱宽随着晶体长度的增加变化很小。我们希望这对于使用脉冲激光泵浦此类晶体有一定的帮助。更多还原

【Abstract】 Since quantum mechanics was born at the beginning of the twentieth century, people’s view on the structure of matter and its interaction has changed dramatically. Many phenomena which cannot be understood by people have been explained by quantum mechanics. They were also be exactly verified by many experiments. The interdisciplinary research becomes more and more apparent and close along with development of science and technology. The combination of quantum mechanics and information science has brought information science to a new area—quantum information science. The foundation of quantum information is preparation, operation, transmission, storage and measurement of quantum state. Now, people can prepare quantum state in many systems such as linear optics, cavity QED, NMR, iron trap and quantum dot systems. Since optical system use photons as carrier of information, it has many advantages such as easy to manipulate, good quality of transmission and weak decoherence of environment.Entangled photon pairs have many applications in quantum information field, such as quantum key distribution (QKD), quantum teleportation, quantum repeater and test of basic theory of quantum mechanics. Most of the sources used to create photon pairs utilize the process of spontaneous parametric down-conversion (SPDC), which can be realized in several configurations and by different crystals. Usually, in SPDC process, phase matching is required, but the generation rate of traditional bulk crystals is low, so it limits the long distance transmission due to the attenuation in optical fiber. In recent years, many experiments about the generation of photon pairs based on periodically poled crystals or waveguides were reported, because they have higher generation rate by utilizing quasi-phase matching (QPM).In this paper, we present a theoretic and experimental study on the spectrum of parametric down-converted photons generated from PPLN crystal, especially the wavelength and bandwidth. Because PPLN crystal has large working temperature and coefficient of thermal expansion, it is suitable to tune temperature of wavelength in a wide range. The wavelength of parametric down-converted photons can be changed linearly by changing the temperature of crystal. Besides, when the temperature of crystal is fixed, changing the wavelength of pump laser can also lead to the same result. Due to the chromatic dispersion in the fiber, bandwidth of photon pairs is an important factor needed consideration. Through comparison among crystal with different lengths and pump beams with different bandwidths, we demonstrate that the bandwidth of down-converted photons will increase for boarder bandwidth of pump pulse and decrease for longer crystal, but the influence of crystal length will become weaker along with increase of both crystal length and pump pulse bandwidth. Especially under the condition of our experiment, the bandwidth almost does not change for longer crystals when pumped by femtosecond laser. We wish this would be helpful for schemes in which such crystals are used.更多还原