新型部分相干光束的产生及其相干特性

利用相干理论,研究了新型部分相干光束的相干特性.用激光光束透过旋转的特殊毛玻璃,产生一种相干性分布特殊的新型部分相干光束.实验上经双孔干涉,记录了该光束经不同小孔间距双孔产生的干涉条纹,计算得出所对应的相干度,并获得了该光束在不同传输距离下的相干度分布情况,发现该光束的相干度分布与高斯-谢尔模型光束的相干度分布不相同,其相干度随着传输距离的增大而变强.     

相干合成子光束一致性检测研究

分析了激光相干合成装置及其光路调整要求,提出利用剪切干涉检测合成光路中子光束一致性的方法。分析剪切干涉原理,建立了剪切板干涉物理模型,以相干合成中两路矩形光束拼接组束为例,数值模拟并研究了子光束的整体倾斜、离焦所对应的剪切干涉条纹,得到了条纹图像与子光束光轴误差、离焦误差之间的对应关系,用于相干合成光路中子光束光轴和发散角误差的判断。光轴倾斜大于5μrad时,可以直接观察互干涉和自干涉条纹变化,发散角在大于10μrad时,干涉条纹变化明显。此外,剪切干涉的自干涉条纹平移可用来检测子光束间的活塞误差,可作为光束拼接相干合成中闭环相位检测和控制的手段。     

一种基于空间光程差调制的条纹位置测量方法

提出了一种基于空间光程差调制的条纹位置测量方法,用于恒星干涉仪条纹搜寻和条纹追踪.来自基线两端的两光束合束时,通过合束器在两光束之间引入一个倾角,用以实现静态的空间光程差调制.使用成像透镜将静态光程差调制得到的白光干涉条纹成像到CCD探测器上.白光条纹位置的偏移与两光束之间的光程差大小相关,使用获得的干涉条纹实时计算白光条纹位置,测量出两束光之间的光程差,用于延迟线的实时光程差补偿,从而可以稳定干涉条纹.数值模拟和实验结果表明,采用该方法获得的最大光程差测量误差为0.159μm,小于数值模拟和实验所用宽带光的平均波长0.555μm,测量精度满足条纹相干的要求.与时间调制方法相比,该方法原理和算法简单,且对于大气扰动更不敏感.     

光折变光子晶格中空间二次谐波的产生

用干涉法在自散焦光折变晶体 LiNbO₃：Fe中写入光子晶格的动态过程中，发现了双光束干涉条纹一分为二，四光束干涉点阵一分为四的分裂现象. 研究证明：这是干涉条纹空间频率的倍频现象，是入射的干涉光场与写入的光子晶格之间相互作用的结果. 本实验说明利用光折变效应可以容易地实现干涉光场空间频率的倍频和空间高次谐波的产生，并可利用产生的空间谐波感应出二倍频和高倍频的光折变光子晶格. 关键词： 光折变晶体 光子晶格 空间谐波     

多光束激光干涉光刻图样

利用电磁理论,研究了多光束激光干涉图样的产生原理,结合计算机数值模拟和相关实验结果,分析了干涉图样的影响因素.研究结果表明:多光束激光干涉图样可以看成是多组余弦分布的平行线条纹的叠加;相干光束的偏振方向、入射方向、光束间相位差是干涉图样的重要影响因素,改变这些因素,余弦分布的平行线条纹的振幅、位置、周期和方向发生变化,...     

基于光束的相干特性拓展双缝实验教学实验内容

对部分相干光束的双缝实验干涉条纹进行了研究。利用转动的毛玻璃以及两个透镜构成了一个简单的产生部分相干光束的光学系统,通过调整毛玻璃与两个透镜的共焦点之间的距离,可以定量地控制光束的相干性。对不同相干性的光束经过双缝干涉之后的光强进行了实验观测,发现光束的相干性会对双缝干涉条纹的衬比度产生影响,光束的相干性越低,条纹衬比度也越小。并对实验结果进行了理论模拟,理论数值模拟的结果与实验观测结果基本一致。     

部分相干厄米-高斯光束杨氏实验中的光强分布

将杨氏干涉实验作为双缝衍射现象处理,以部分相干厄米-高斯(H-G)光束为例,研究分析了多模部分相干光的杨氏双缝干涉光强和干涉条纹可见度的空间分布。数值计算说明,多模部分相干光束的空间相干参数、模阶数和缝遮拦比对衍射场中光强的空间分布(干涉花样)都有影响。当多模部分相干光的阶数为奇数时,轴上光强与光束的相干性会出现相消干涉;当多模部分相干光的阶数为偶数时,轴上光强与光束的相干性会出现相长干涉等。     

从平面波的线性干涉到涡旋光的角向干涉

依托中学生“英才计划”课题，以光场调控技术为选题，开展了从平面波线性干涉到涡旋光角向干涉演化过程的实验教学.从理论上模拟了涡旋光束与平面波的相干叠加，通过数值分析确认了拓扑荷数与干涉条纹的关系.实验上利用螺旋相位板制备涡旋光束，并采用马赫-曾德尔干涉法观测光强分布图.结果表明：涡旋光束与平面波的相干叠加，可以视为在线性干涉图样中移入与拓扑荷数相关的叉型条纹；而具有相反拓扑荷数的涡旋光束的角向干涉图样，则呈现与拓扑荷数相关的角向干涉条纹.     

平行光束入射双棱镜的干涉研究

本文理论推导出了两相干平行光束在交叉区域的干涉条纹计算公式,测量两相干平行光束的夹角和干涉条纹的间距可以得到入射光波波长的结论.利用多光束光纤激光器作为光源,在分光计上放置双棱镜获得两相干平行光束进行实验.结合电荷耦合器件(CCD)和液晶显示器,用望远镜的自准直目镜测量两相干平行光束的夹角,用测微目镜测量干涉条纹的间距.提高了实验数据的精确度,降低了实验的难度,实现了对传统双棱镜干涉实验的拓展创新.     

两个独立全光纤多通道光子纠缠源的Hong-Ou-Mandel干涉

独立光子源的干涉是实现复杂量子体系应用(比如多光子纠缠态产生和量子隐形传态等)的核心技术.利用100 GHz密集波分复用技术,实现了1.55μm全光纤多通道独立纠缠光子源的Hong-Ou-Mandel干涉,在不去除暗符合(随机符合计数)的情况下,可见度为53.2%±8.4%,去除暗符合可见度可达到82.9%±5.3%.给出了关于色散位移光纤中基于自发四波混频过程产生的单光子光谱纯度严格的理论描述,模拟了抽运脉冲宽度和滤波器带宽对单光子光谱纯度的影响,并给出了理论上的最佳条件(最佳的抽运脉冲宽度为8 ps,高斯滤波器带宽为40 GHz及以下).在测量Hong-Ou-Mandel干涉之前,先测量了液氮冷却状态下的色散位移光纤关联光子源的符合和随机符合比率,在抽运功率为23μW的情况下,最大比率可以达到131.Hong-Ou-Mandel干涉在高精度光学测量、测量装置无关的量子密钥分配等应用中扮演着极为重要的角色.     

Coincidence Imaging and interference with coherent Gaussian beams

we present a theoretical study of coincidence imaging and interference with coherent Gaussian beams. The equations for the coincidence image formation and interference fringes are derived, from which it is clear that the imaging is due to the corresponding focusing in the two paths. The quality and visibility of the images and fringes can be high simultaneously. The nature of the coincidence imaging and interference between quantum entangled photon pairs and coherent Gaussian beams are different. The coincidence image with coherent Gaussian beams is due to intensity-intensity correspondence, a classical nature, while that with entangled photon pairs is due to the amplitude correlation a quantum nature. Selected from Acta Sinica Quantum Optica, 2005, 11(4)(in Chinese)     

Ghost interference with partially coherent radiation

Ghost interference with partially coherent radiation sources is studied using optical coherence theory. The visibility of the ghost interference fringes is strongly influenced by the transverse size and transverse coherence width of the source. An increase of the transverse source size leads to a decrease of the fringes' visibility. An increase of the transverse coherence results in an increase of the visibility. The difference between ghost interference formed with entangled photon pairs and with partially coherent light is discussed.     

Experimental observation of coincidence fractional Fourier transform with entanglement photon pairs

Experiment of coincidence fractional Fourier transform with spontaneous parametric down-conversion photon pairs is described in this work. Results agree well with theoretical calculations with Gaussian beams. Visibilities of the results are much greater than those obtained with partially coherent beam.     

Experimental observation of quantum Talbot effects

We report the first experimental observation of quantum Talbot effects with single photons and entangled photon pairs. Both the first- and second-order quantum Talbot self-images are observed experimentally. They exhibit unique properties, which are different from those produced by coherent and incoherent classical light sources. In particular, our experiments show that the revival distance of two-photon Talbot imaging is twice the usual classical Talbot length and there is no net improvement in the resolution, due to the near-field effect of Fresnel diffraction, which is different from the case of previous proof-of-principle quantum lithography experiments in the far field.     

用于符合测量的多通道符合计数器

符合测量在量子光学实验中有着重要的应用,它可以用于测量纠缠光子对和单光子干涉等实验。在使用过程中,经常用到三通道或更多通道的符合测量,这种情况下简单的门电路无法满足使用要求,而现场可编程门阵列(FPGA)为实现低成本、小体积的多通道符合计数系统提供了解决方案。同时,基于FPGA的多通道符合计数系统还可以提供ns精度的符合分辨时间,避免本底光噪声对探测器的影响,从而有效抑制偶然符合,进一步提高了多通道符合计数系统的性能。本文介绍了基于FPGA的多通道符合计数器,并实验测量了单光子探测器信号的符合计数。     

Quantum Discord and Entanglement of Quasi-Werner States Based on Bipartite Entangled Coherent States

Present work is an attempt to compare quantum discord and quantum entanglement of quasi-Werner states formed with the four bipartite entangled coherent states (ECS) used recently for quantum teleportation of a qubit encoded in superposed coherent state. Out of these, the quasi-Werner states based on maximally ECS due to its invariant nature under local operation is independent of measurement basis and mean photon numbers, while for quasi-Werner states based on non-maximally ECS, it depends upon measurement basis as well as on mean photon number. However, for large mean photon numbers since non-maximally ECS becomes almost maximally entangled therefore dependence of quantum discord for non-maximally ECS based quasi-Werner states on the measurement basis disappears.     

Random delayed-choice quantum eraser via two-photon imaging

We report on a delayed-choice quantum eraser experiment based on a two-photon imaging scheme using entangled photon pairs. After the detection of a photon which passed through a double-slit, a random delayed choice is made to erase or not erase the which-path information by the measurement of its distant entangled twin; the particle-like and wave-like behavior of the photon are then recorded simultaneously and respectively by only one set of joint detection devices. The present eraser takes advantage of two-photon imaging. The complete which-path information of a photon is transferred to its distant entangled twin through a “ghost" image. The choice is made on the Fourier transform plane of the ghost image between reading “complete information" or “partial information" of the double-path.     

Effects of multiple pairs on visibility measurements of entangled photons generated by spontaneous parametric processes

Entangled photon-pair sources based on spontaneous parametric processes are widely used in photonic quantum information experiments. In this paper, we clarify the relationship between average photon-pair number and the visibility of two-photon interference (TPI) using those entanglement sources. We consider sources that generate distinguishable and indistinguishable entangled photon pairs, assuming coincidence measurements that use threshold detectors. We present formulas for the TPI visibility of a polarization entanglement that take account of all the high-order multi-pair emission events. Moreover, we show that the formulas can be approximated with simple functions of the average pair number when the photon collection efficiency is small. As a result, we reveal that an indistinguishable entangled pair provides better visibility than a distinguishable one.     

Large-alphabet quantum key distribution using energy-time entangled bipartite States

We present a protocol for large-alphabet quantum key distribution (QKD) using energy-time entangled biphotons. Binned, high-resolution timing measurements are used to generate a large-alphabet key with over 10 bits of information per photon pair, albeit with large noise. QKD with 5% bit error rate is demonstrated with 4 bits of information per photon pair, where the security of the quantum channel is determined by the visibility of Franson interference fringes. The protocol is easily generalizable to even larger alphabets, and utilizes energy-time entanglement which is robust to transmission over large distances in fiber.     

Constructing Robust Entangled Coherent GHZ and W States via a Cavity QED System

Using a system of three distant cavities, we propose a method for constructing tripartite entangled coherent GHZ and W states which are robust due to the photon losses in the cavities. Each of cavities is doped with a semiconductor quantum dot. By the dynamics, the excitonic modes of quantum dots are enabled to exhibit entangled coherent GHZ and W states. Apart from the exciton losses, the master equation approach shows that when the populations of the field modes in the cavities are negligible the destruction of entanglement due to dissipation arises from photon losses, is effectively suppressed.