Controlled single-photon emission from a single trapped two-level atom

By illuminating an individual rubidium atom stored in a tight optical tweezer with short resonant light pulses, we created an efficient triggered source of single photons with a well-defined polarization. The measured intensity correlation of the emitted light pulses exhibits almost perfect antibunching. Such a source of high-rate, fully controlled single-photon pulses has many potential applications for quantum information processing.     

Paleontology. Feathers, or flight of fancy?

On page 2202, researchers describe the 220-million-year-old Longisquama insignis, a squat, mouse-sized reptile with at least six vanelike appendages up to 12 centimeters long sprouting from its spine. The authors argue that the appendages are feathers much like those of modern birds; outside the paper, they have touted the fossil as "an ideal bird ancestor." That conclusion has infuriated paleontologists--not just because it challenges the prevalent theory that birds evolved from theropod dinosaurs, but because they say it does so in an unscientific way.     

Frequency Dependence of Polarization of Pulsar CP 0328

The circularly polarized emission from the pulsar CP 0328 has an approximately flat spectrum in the 1-megahertz band centered at 113.6 megahertz, whereas the linearly polarized emission varies with frequency and from pulse to pulse. A simple model for the source that has a constant Faraday rotation measure fits some of the linearly polarized spectra observed for individual pulses, but changes in the rotation measure of as much as 30 radians per square meter are required between adjacent pulses. The simple model does not fit the average spectrum of the linearly polarized emission, although the average spectrum had the same form on two nights.     

United time-frequency spectroscopy for dynamics and global structure

Ultrashort laser pulses have thus far been used in two distinct modes. In the time domain, the pulses have allowed probing and manipulation of dynamics on a subpicosecond time scale. More recently, phase stabilization has produced optical frequency combs with absolute frequency reference across a broad bandwidth. Here we combine these two applications in a spectroscopic study of rubidium atoms. A wide-bandwidth, phase-stabilized femtosecond laser is used to monitor the real-time dynamic evolution of population transfer. Coherent pulse accumulation and quantum interference effects are observed and well modeled by theory. At the same time, the narrow linewidth of individual comb lines permits a precise and efficient determination of the global energy-level structure, providing a direct connection among the optical, terahertz, and radio-frequency domains. The mechanical action of the optical frequency comb on the atomic sample is explored and controlled, leading to precision spectroscopy with an appreciable reduction in systematic errors.     

The mother of all HIV challenges

AIDS researchers are finding cheaper and simpler ways to slow the spread of HIV from mother to child, and more pregnant women, even in the poorest countries, have access to anti-HIV drugs and formula--thanks to the largesse of donors, discounts from industry, new trade laws, and the tenacity of individual clinicians. But just as researchers offer ways to clear one enormous hurdle--drug availability--they run smack into other ones, ranging from social stigmas that discourage testing to disinterest on the part of cash-strapped health authorities to a deeply ingrained culture of breast-feeding--often supported by government policy for otherwise sound health reasons.     

基于TLS数据的单木胸径和树高提取研究

利用TLS测量森林参数(树高、胸径和林分密度等)不仅节省人力,而且还能提高工作效率,目前已成为快速获取树木几何参数的有效方法之一。本文利用地基激光雷达数据,提出了一种半自动方式提取单木参数的方法。首先,利用发射脉冲与接收回波脉冲的形状偏差和强度对原始点云数据中的“飞点”和噪点进行滤除;然后,配准各站点云数据;接下来采用不同目标回波过滤方法滤除部分树叶点和其他非树干点;抽稀、合并各站点云数据,对合并后的点云数据水平分层并生成对应的灰度影像;对灰度影像采用Hough变换方法估测单木位置和胸径;对Hough变换检测层数较少的结果采用椭圆拟合方法重新检测;最后,结合Hough变换和椭圆拟合检测结果从点云数据中提取单木位置、胸径和树高。用小兴安岭凉水自然保护区的白桦天然次生林对算法进行了检验,5块多站扫描样地单木识别的平均精度为72.16%,将所提取的单木位置、胸径和树高与外业实际测量数据对比,结果一致性较高。5块多站扫描样地的胸径和树高均方根误差分别为2.38、2.55、3.58、2.21、1.92 cm和4.31、3.87、2.34、5.00、3.47 m。      

Dynamics of magnetic domain walls under their own inertia

The motion of magnetic domain walls induced by spin-polarized current has considerable potential for use in magnetic memory and logic devices. Key to the success of these devices is the precise positioning of individual domain walls along magnetic nanowires, using current pulses. We show that domain walls move surprisingly long distances of several micrometers and relax over several tens of nanoseconds, under their own inertia, when the current stimulus is removed. We also show that the net distance traveled by the domain wall is exactly proportional to the current pulse length because of the lag derived from its acceleration at the onset of the pulse. Thus, independent of its inertia, a domain wall can be accurately positioned using properly timed current pulses.     

博客应用于高校图书馆竞争情报服务的探索

简要介绍了博客的特点和发展现状,指出博客是一种表达个人思想,内容,按时间顺序排列,并且不断更新的出版方式。它的出现给图书馆竞争情报服务带来了机遇和挑战,为竞争情报研究人员提供了便利的工具,具有极大的情报价值,成为竞争情报研究和服务的新热点。     

英语词汇知识深度及其学习策略研究

本文依据词汇知识框架理论和层面分析方法探讨了英语词汇知识所涉及的内容。根据国内一些学者实证研究的结果和教学实践,确定了英语词汇深度习得的主要对象为英语核心词汇,主要内容为多义词不同词义之间的联系,半开放型搭配和词与词之间的各种关系。提出了如下学习策略：围绕核心词义构建多义词家谱,用组块法学习半开放型搭配,学习者应参与构建词汇语义网络。     

Research,extension, and user partnerships: Models for collaboration and strategies for change

Increasing pragmatic and ethical concerns have been raised about the inadequacies of conventional approaches to agricultural research and extension worldwide and the lack of integrated efforts among researchers, extension educators, and users. This paper examines three models of these relationships: the diffusion or supply model; the induced innovation or demand model; and the synthesis triangular or supply/demand model. The triangular model builds and improves upon the previous models by focusing on the role of clients or users in the broadest sense in creating a demand for science and extension education, as well as on the role of scientists and extension educators in creating a supply. The triangular model views the relationship as an interactive partnership in which research and extension education are conducted in response to client needs and demands expressed through negotiation, persuasion, and coercion involving all partners. To implement this model will require significant organizational and managerial changes as well as reorientation in the values and attitudes of researchers, extension educators, clients, and their organizations. The remainder of the paper presents a brief discussion of suggested strategies at the individual, institutional, regional, national, and international levels for enhancing this partnership in order to more effectively meet the future needs of our food system.     

Isolated single-cycle attosecond pulses

We generated single-cycle isolated attosecond pulses around approximately 36 electron volts using phase-stabilized 5-femtosecond driving pulses with a modulated polarization state. Using a complete temporal characterization technique, we demonstrated the compression of the generated pulses for as low as 130 attoseconds, corresponding to less than 1.2 optical cycles. Numerical simulations of the generation process show that the carrier-envelope phase of the attosecond pulses is stable. The availability of single-cycle isolated attosecond pulses opens the way to a new regime in ultrafast physics, in which the strong-field electron dynamics in atoms and molecules is driven by the electric field of the attosecond pulses rather than by their intensity profile.     

Enhanced optical emission during Crab giant radio pulses

We detected a correlation between optical and giant radio pulse emission from the Crab pulsar. Optical pulses coincident with the giant radio pulses were on average 3% brighter than those coincident with normal radio pulses. Combined with the lack of any other pulse profile changes, this result indicates that both the giant radio pulses and the increased optical emission are linked to an increase in the electron-positron plasma density.     

Electrochemical micromachining

The application of ultrashort voltage pulses between a tool electrode and a workpiece in an electrochemical environment allows the three-dimensional machining of conducting materials with submicrometer precision. The principle is based on the finite time constant for double-layer charging, which varies linearly with the local separation between the electrodes. During nanosecond pulses, the electrochemical reactions are confined to electrode regions in close proximity. This technique was used for local etching of copper and silicon as well as for local copper deposition.     

Attosecond synchronization of high-harmonic soft x-rays

Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.     

Laser-selective demagnetization: a new technique in paleomagnetism and rock magnetism

Laser-selective demagnetization (LSD) enables the determination of the magnetic moment associated with individual mineral grains in thin sections of rock. Small volumes can be demagnetize with laser pulses directed through the optics of a microscope, permitting resolution of remanence components in individual mineral grains. LSD of mafic granulite samples revealed two paleomagnetic directional components of opposite polarity: one resided in coarse magnetite, the other in ilmenohematite-hemoilmenite exsolution intergrowths and fine magnetite indusions in clinopyroxene. These directions are consistent with those inferred from bulk demagnetization techniques, but LSD permits direct identification of the remanence carriers. The ability to discriminate magnetization components in different generations of a single mineral and to define intergrain magnetic moment distributions are significant advantages of LSD.     

A quantum dot single-photon turnstile device

Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.     

农村籍女大学生教育个人直接成本实证研究——以西北农林科技大学为例

以西北农林科技大学为例,进行了农村女大学生教育直接成本的实证研究,并得出高等教育对于农民仍是不轻的负担这一结论.     

华北油松人工林单株林木的生物量

本文根据山西太岳林区和河北承德地区的31块标准地共220株标准木的资料分析了华北油松人工林单株林木的生物量与胸径、树高及林分密度之间的数量关系。分别编制了两地区油松单株林木的生物量表。     

Molecular biology. New way found to study closely related proteins

Rather than designing specific inhibitors for closely related proteins, researchers are remodeling the proteins to make them uniquely susceptible to inhibition. As described in the 21 September issue of Nature, the technique involves enlarging the active site of an enzyme so that it can bind an inhibitor that won't fit into the active sites of related--but unaltered--enzymes. Researchers can then insert the gene that encodes the modified enzyme into cells or living animals and turn off that enzyme by feeding them the inhibitor--without affecting other, very similar, enzymes. The technique may have some advantages over other approaches to studying the functions of individual proteins, such as mutating or knocking out the genes that encode them, which may disrupt embryonic development, producing abnormal animals or no animals at all.