Measuring fluorescence to track a quantum emitter's state: a theory review

We review the continuous monitoring of a qubit through its spontaneous emission, at an introductory level. Contemporary experiments have been able to collect the fluorescence of an artificial atom in a cavity and transmission line, and then make measurements of that emission to obtain diffusive quantum trajectories in the qubit's state. We give a straightforward theoretical overview of such scenarios, using a framework based on Kraus operators derived from a Bayesian update concept; we apply this flexible framework across common types of measurements including photodetection, homodyne, and heterodyne monitoring and illustrate its equivalence to the stochastic master equation formalism throughout. Special emphasis is given to homodyne (phase-sensitive) monitoring of fluorescence. The examples we develop are used to illustrate basic methods in quantum trajectories, but also to introduce some more advanced topics of contemporary interest, including the arrow of time in quantum measurement, and trajectories following optimal measurement records derived from a variational principle. The derivations we perform lead directly from the development of a simple model to an understanding of recent experimental results.     

沙丘背风侧不同铁路结构形式对风沙环境的适应性分析

当铁路穿越大风沙漠地区时,风沙灾害会对铁路工程及其正常运营产生严重威胁,而设计一种合理的铁路结构形式能够减小风沙沉积对铁路工程的危害. 在本文中,以敦煌至格尔木铁路沙山沟段落为研究对象,采用多相流的方法对越过沙丘的风沙运动过程进行数值模拟,分别讨论了风沙运动对位于沙丘背风坡的铁路路基工程和桥梁工程的影响. 主要的模拟结果显示:路基工程明显降低了风速并且将沙丘后的回流区分成了两部分,而桥梁工程的导流效应则压缩了沙丘背风坡的回流区;轨道间的道碴增大了铁路表面的粗糙度,在轨道间有少量沙粒沉积,而路基工程两侧则有大量积沙;铁路表面的积沙量与摩阻风速呈现出非线性关系,随着摩阻风速的增大,路基工程沙粒沉积的增加速度大于风蚀能力的增加速度,而桥梁工程则正好相反. 在防止风沙危害铁路方面,设置桥梁工程明显优于路基工程. 本研究为风沙运动对铁路工程的影响提供了理论支持,也为今后的铁路工程设计提供了新的思路与研究工具.      

垂直旋转流场中单颗粒运动状态判别及受力分析

为研究单颗粒在旋转流场中的运动状态及受力情况,以毫米级球形颗粒为例,利用旋转流场颗粒运动装置,通过使用摄像机记录颗粒在流场中的运动轨迹以获取其运动参数,分析了不同转速和颗粒直径条件下颗粒的运动轨迹,拟合得到了颗粒运动状态判别公式以及颗粒运动轨迹公式,分析了颗粒在旋转流场中的受力情况。结果表明,颗粒在旋转流场平衡状态下运动状态主要分为两类,一类是未离开壁面保持静止,另一类是离开壁面保持稳定周向运动;颗粒进行周向运动的轨迹为椭圆形,并且圆心随着转速的增大靠近旋转中心,而随着粒径的增大靠近壁面;颗粒在旋转流场的运动过程中主要受到离心力和旋转科式力作用。     

Experimental study on age and gender differences in microscopic movement characteristics of students

Campus security has aroused many concerns from the whole society. Stampede is one of the most frequent and influential accidents in campus. Studies on pedestrian dynamics especially focusing on students are essential for campus security, which are helpful to improve facility design and emergency evacuation strategy. In this paper, primary and middle school students were recruited to participate in the single-file experiments. The microscopic movement characteristics,including walking speed, headway, gait characteristics(step length, step frequency and swaying amplitude) and their relations were investigated. Age and gender differences in the headway-speed diagram and space requirements were analyzed by statistical tests. The results indicated that the impacts of age and gender were significant. There were three stages for the influence of gender on the headway-speed diagram for both age groups. The impacts on students' space requirements were consistent for different age and gender groups. But the impacts of age and gender on free-flow speed were affected by each other. Due to the connection of walking speed and gait characteristics, the comparisons of gait characteristics between different ages and genders were performed to understand the corresponding differences in speed more deeply. The results showed that differences in step length and swaying amplitude between males and females were significant for both age groups. The effect of gender on step frequency was significant for primary students. But for middle school students,whether gender had significant impact on step frequency was not clear here because of the large P-value. Besides, the influence of age on gait characteristics changed with gender.     

Experimental analysis of sand particles'' lift-off and incident velocities in wind-blown sand flux

The probability distributions of sand particles' lift-off and incident velocities in a wind-blown sand flux play very important roles in the simulation of the wind-blown sand movement. In this paper, the vertical and the horizontal speeds of sand particles located at 1.0 mm above a sand-bed in a wind-blown sand flux are observed with the aid of Phase Doppler Anemometry (PDA) in a wind tunnel. Based on the experimental data, the probability distributions of not only the vertical lift-off speed but also the lift-off velocity as well as its horizontal component and the incident velocity as well as its vertical and horizontal components can be obtained by the equal distance histogram method. It is found, according to the results of the χ²-test for these probability distributions, that the probability density functions (pdf's) of the sand particles' lift-off and incident velocities as well as their vertical components are described by the Gamma density function with different peak values and shapes and the downwind incident and lift-off horizontal speeds, respectively, can be described by the lognormal and the Gamma density functions. These pdf's depend on not only the sand particle diameter but also the wind speed. The project supported by the National Natural Science Foundation of China (10532040) and the Hundred Talents Project, the Knowledge Innovation Project of Chinese Academy of Sciences (KZCX2-304). The English text was polished by Keren Wang.     

Calculating parameters for infiltration equations from soil hydraulic functions

Simple equations for predicting infiltration of water into soil are valuable both for hydrological application and for investigating soil hydraulic properties. Their value is greatly enhanced if they involve parameters that can be related to more basic soil hydraulic properties. In this paper we extend infiltration equations developed previously for positive surface heads to negative heads. The equations are then used to calculate infiltration into a sand and a clay for a range of initial and surface conditions. Results show errors of less than three percent compared with accurate numerical solutions. Analytical approximations to parameters in the equations are developed for a Brooks and Corey power law hydraulic conductivity-water content relation combined with either a Brooks and Corey or a van Genuchten water retention function. These are compared with accurate numerical values for a range of hydraulic parameters encompassing the majority of soil types and a range of initial and boundary conditions. The approximations are excellent for a wide range of soil parameters.An important attribute of the infiltration equations is their use of dimensionless parameters that can be calculated from normalised water retention and hydraulic conductivity functions. These normalised functions involve only parameters that it may be possible to estimate from surrogate data such as soil particle size distribution. Application of the equations for predicting infiltration, or their use in inferring hydraulic properties, then involves only simple scaling parameters.     

Survey study and experimental investigation on the local behavior of pedestrian groups

Current research on pedestrian dynamics mainly focuses on the interactions among isolated individuals, the impact of the presence of groups is not fully considered. In recent 5 years, researchers have started to investigate pedestrian group movement. The aim of this work is to explore the local behavior of pedestrian groups by questionnaires and field observation. Survey study focused on pedestrians' psychology when walking in groups, which included five parts: group size, interpersonal distance, spatial relationship among group members, speed adjustment of group members, information transmission, and acid action among group members. Meantime a field observation was carried out to study group movement characteristics, which contained speed, step frequency, offset angle and interpersonal distance. The survey results show that group members have a closer interpersonal distance, faster information transmission and plenty of acid action. Conversely, group walking has a negative influence on pedestrian's speed, step frequency by comparing with the way isolated pedestrian walks. In addition, it is found that for a certain group, the group members are able to keep movement consistent. Also there exists obvious movement diversity among different group types (male dyads, female dyads, couple groups, and ordinary‐friend groups) because of different gender and social relationship. Ultimately the results will be more promising for helping to model the movement of pedestrian groups. © 2014 Wiley Periodicals, Inc. Complexity 20: 87–97, 2015     

路径约束条件下车辆行为的时空演化模型

复杂地理环境下车辆运行线路具有空间三维特征, 它对车辆行为的约束是显然的, 非一维空间和二维空间所能描述. 将复杂地理环境下车辆运行线路抽象为空间曲线, 引入微分几何理论, 利用弧长、曲率和挠率等几何不变量参数建立沿空间曲线运动的Serret-Frenet活动标架; 然后, 对空间曲线上任意一点处Serret-Frenet标架具有时变属性的动态行为进行数学描述, 进而建立路径约束条件下车辆行为的时空演化模型, 并在数学上严格证明了所建时空演化模型适用于车辆(Serret-Frenet标架)直线运行和做匀速圆周运动的特殊情形. 为后续复杂地理环境中交通线路上的车辆跟驰、变道等微观行为和交通流宏观行为研究, 奠定了理论基础.     

利用高阶拉盖尔-高斯横模精确测量法布里-珀罗腔内原子的运动轨迹

研究了冷原子与法布里-珀罗腔内拉盖尔-高斯横模强耦合相互作用体系的透射光谱, 分析了透射光谱与原子在腔中运动轨迹的关系. 结果表明, 与厄米特-高斯横模相比, 拉盖尔-高斯横模的腔场与原子的最大耦合系数几乎不随阶数的增加而变化, 使得探测光谱的对比度受模式阶数的影响较小. 在拉盖尔-高斯横模场分布的圆环边缘附近, 原子运动轨迹的微小偏移会引起透射光谱的很大变化, 因此在这些位置可以实现原子运动轨迹的高精度探测.     

Ca2+ and cAMP regulations of microtubule sliding in hyperactivated motility of bull spermatozoa

To reach and fertilize the egg, mammalian spermatozoa change their flagellar movement in the female reproductive tract, named hyperactivation. The biochemical analyses of the hyperactivated movement using demembranated spermatozoa defined the factors inducing this peculiar movement; namely, large asymmetrical flagellar movement observed in the early stage of the hyperactivation was induced with a high Ca²⁺ concentration while large symmetrical flagellar movement in the late stage of the hyperactivation was generated with low Ca²⁺ and high cAMP concentrations. Under these conditions, the microtubule sliding of bull sperm flagella was investigated by disintegrating the sperm flagella with MgATP²⁻ after extracting their plasma membrane and mitochondria. The large asymmetrical flagellar movement was caused by a long sliding displacement of a fiber of the doublet microtubules. On the other hand, the large symmetrical flagellar movement was generated by a large amount of microtubule sliding by many doublet microtubules.