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1.
1996年 ,《科学美国人》杂志的资深记者约翰·霍根出版了他的一本科普著作———《科学的终结》 .在书中霍根断言 :科学发现的伟大时代已经一去不复返了 .这一令人瞩目的观点 ,加上作者对众多科学家尖刻的讽刺 ,使得该书在上市不久便引发了科学界乃至社会各界的争论 .美国Science周刊曾于 1996年 6月 14日发表了戴维·古德斯坦对该书的书评 .5年后的今天 ,作为对古德斯坦书评的注释 ,在这篇短文中 ,我们将以玻色 -爱因斯坦凝聚 (BEC)的研究进展为例 ,批判霍根的科学悲观主义 .将 4千年前的金字塔与哈勃太空望远镜相比 ,哪一个是更… 相似文献
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众所周知 ,玻色 -爱因斯坦凝聚 (BEC)是指成千上万的原子以稀薄气体的形式在瞬间凝聚到具有相干性的基态上 ,这种状态可以显示许多极有意义的量子效应 .当然要出现BEC状态 ,首要的条件是必须让系统处于临界点 .但长期以来科学家们却很少关注凝聚时的崩塌现象 .最近荷兰原子与分子物理研究所的J .Walraven教授及其研究组对此进行了研究 ,他们细致地观察了这个崩塌现象 ,并发现了在凝聚时出现的奇异的类似于雪茄烟状的凝聚云团 .研究组采用“激波冷却 (shockcooling)”方法来冷却原子 (即利用无线频率波使原子在 1ms内迅速冷却 ,而不是采… 相似文献
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本文介绍了北京大学建立的玻色-爱因斯坦凝聚实验平台,实现了玻色-爱因斯坦凝聚(图1),获得了原子数为五十万个,温度为50纳开尔文的玻色凝聚体。在此基础上我们精密测量了玻色-爱因斯坦凝聚的相变温度,还利用玻色-爱因斯坦凝聚实验平台通过马越让那跃迁获得了可控的多量子态玻色爱因斯坦凝聚体。并利用四种方法获得了原子激光(图2),其中有三种方法是国际上第一次使用。另外,我们提出了将玻色一爱因斯坦凝聚转入Magic光晶格阱,实现精度优于10^-17的新型原子钟的设想。 相似文献
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自1995年首次实现玻色一爱因斯坦凝聚(BEC)以来,相关研究发展迅速.截止到2005年6月,已实现凝聚的碱金属原子气有六种:7^Li,23^Na,41^K,85^Rb,和133^Cs.自旋极化1^H最早被遴选为BEC的对象,但因技术上的难度问题,它的凝聚迟至1998年6月,是由“BEC教父”Daniel Kleppneer领导的小组实现的. 相似文献
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玻色-爱因斯坦凝聚在中国科学院上海光机所实现 总被引:4,自引:0,他引:4
文章报道了在稀薄87Rb原子气体中观测到的玻色 -爱因斯坦凝聚 (BEC)现象 .在四极矩和Ioffe组合磁阱 (QUIC)中装载了 1× 10 8个原子 ,经过 19s蒸发冷却达到了相变条件 .在高原子密度的情况下 ,文章作者观察到了BEC对探测光的衍射光环 .这时降低磁阱势垒和绝热的放开冷原子样品 ,我们拍摄到冷原子和BEC的吸收像 .根据数据拟合满足双高斯分布 ,表明发生了BEC相变 .相变温度约 2 15nK ,凝聚的原子数约为 5× 10 4 . 相似文献
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研究玻色-爱因斯坦凝聚的相变特征,证明了粒子间存在弱排斥相互作用的玻色系统的玻色-爱因斯坦凝聚是二级相变。 相似文献
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1995年6月5日.美国国家标准和技术研究所和科罗拉多大学物理系联合研究所(JILA)的物理学家怀曼(C.wieman)和康耐尔(E.Cornell)等人,采用激光冷却和蒸发冷却的联合技术.首先在原于钩(87Rb)的蒸汽中产生了玻色-爱因斯坦凝聚(BEC),使上千个原子进入同一个量子态[1].这一发现轰动了整个物理界,美国《科学》杂志显要地报道了这个消息[2,3].麻省理工学院的一位在探素BEC方面很有经验的原子物理学家——克理普耐尔(Kleppner)说:“这是一项惊人的发现,简直让我透不过气… 相似文献
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A new artificial neural network that can reject strange patterns is presented. The new network is an improved version of the RCE network. Each cell in the last layer of the present network has two thresholds. The new threshold produces the smallest boundary that encloses all examples associated with the cell in the pattern vector space. The present network can reject strange patterns by using this boundary. The rejection of strange patterns in the present and RCE networks was investigated by practical experiments using digits sampled from X-ray films of the human chest. 相似文献
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We have studied the ground state configurations of a rotating Bose-Einstein condensation in a toroidal trap as the radius of the central Gaussian potential expands adiabatically. Firstly, we observe that the vortices are devoured successively into the central hole of the condensate to form a giant vortex as the radius of the trap expands. When all the pre-existing vortices are absorbed, the angular momentum of the system still increase as the radius of the
gaussian potential enlarges. When increasing the interaction strength, we find that more singly quantized vortices are squeezed into the condensate, but the giant vortex does not change. 相似文献
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XIAO Yun-feng HAN Zheng-fu GUO Guang-can 《量子光学学报》2006,12(B08):11-12
We propose a potential quantum-computer hardware-architecture model on a silicon chip in which the basic cell gate is the atom-photon controlled phase flip gate. This gate can be implemented through a single-photon pulses scattering by a toroidal microcavity trapping a neutral atom, and it does not require very strict strong-coupling regime and can work beyond the Lamb-Dicke limit with high fidelity and success probability under practical noise environments. Especially, good and bad losses of the toroidal cavity are discussed in detail. Finally, a possibly simple experiment based on current experimental technology is proposed to demonstrate our scheme. 相似文献
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V. E. Asadchikov V. V. Volkov Yu. O. Volkov K. A. Dembo I. V. Kozhevnikov B. S. Roshchin D. A. Frolov A. M. Tikhonov 《JETP Letters》2011,94(7):585-587
The structure of the transient layer at the interface between air and the aqueous solution of silica nanoparticles with the
size distribution of particles that has been determined from small-angle scattering has been studied by the X-ray reflectometry
method. The reconstructed depth profile of the polarizability of the substance indicates the presence of a structure consisting
of several layers of nanoparticles with the thickness that is more than twice as large as the thickness of the previously
described structure. The adsorption of 1,2-distearoyl-sn-glycero-3-phosphocholine molecules at the hydrosol/air interface
is accompanied by the condensation of anion silica nanoparticles at the interface. This phenomenon can be qualitatively explained
by the formation of the positive surface potential due to the penetration and accumulation of Na+ cations in the phospholipid membrane. 相似文献
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太赫兹(T Hz)波在物质检测方面发挥着巨大的作用,是一种非常有潜力的生化传感工具.但是传统的太赫兹时域光谱系统(TDS)结构复杂,系统的集成度低,占用空间较大.所以,如何对T Hz波进行有效引导、实现集成化传输并得到高质量光谱就成为太赫兹光谱系统的研究热点.太赫兹片上系统是将T Hz的产生、传输以及探测都集成到同一芯... 相似文献
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Condensation may occur in an open-flow thermoacoustic cooler with stack temperatures below the saturation temperature of the flowing gas. In the experimental device described here the flowing gas, which is also the acoustic medium, is humid air, so the device acts as a flow-through dehumidifier. The humid air stream flows through an acoustic resonator. Sound energy generated by electrodynamic drivers produces a high-amplitude standing wave inside of the resonator, which causes cooling on a thermoacoustic stack. Condensation of water occurs as the humid air passes through the stack and is cooled below its dew point, with the condensate appearing on the walls of the stack. The dry, cool air passes out of the resonator, while the condensate is wicked away from the end of the stack. Thermoacoustic heat pumping is strongly affected by the form of the condensate inside of the stack, whether condensed mostly on the stack plates, or largely in the form of droplets in the gas stream. Two simple models of the effect of the condensate are matched to a measured stack temperature profile; the results suggest that the thermoacoustic effect of droplets inside the stack is small. 相似文献
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Kurt Johansson 《Communications in Mathematical Physics》1991,141(1):41-61
We consider a one-dimensional lattice gas in the canonical ensemble with interaction energy 1/r
, 1<2. Using an energy-entropy argument we show that the gas condenses at sufficiently low temperatures meaning that the gas has a non-uniform density in the thermodynamic limit.Research supported by the Swedish research councils NFR and STUF 相似文献
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A new expression for the expansion coefficient of a flexible chain polymer molecule can be given in such a way that its first term corresponds to the case with only a hard-sphere repulsion and the other terms represent the effects of attraction between the segments. For low and high temperatures, more convenient expressions are used to find a possible condensation into a liquid-like condensed state from a gas-like state. It is found that in the condensed state, the square average end-to-end distance varies as N?, where N is the number of segments and the exponent ? is around 0.6 in contrast to 1.2 for the high temperature case. 相似文献