共查询到20条相似文献，搜索用时 31 毫秒
1.
Emerson J Silva M Moussa O Ryan C Laforest M Baugh J Cory DG Laflamme R 《Science (New York, N.Y.)》2007,317(5846):1893-1896
A major goal of developing high-precision control of many-body quantum systems is to realize their potential as quantum computers. A substantial obstacle to this is the extreme fragility of quantum systems to "decoherence" from environmental noise and other control limitations. Although quantum computation is possible if the noise affecting the quantum system satisfies certain conditions, existing methods for noise characterization are intractable for present multibody systems. We introduce a technique based on symmetrization that enables direct experimental measurement of some key properties of the decoherence affecting a quantum system. Our method reduces the number of experiments required from exponential to polynomial in the number of subsystems. The technique is demonstrated for the optimization of control over nuclear spins in the solid state. 相似文献
2.
Khaykovich L Schreck F Ferrari G Bourdel T Cubizolles J Carr LD Castin Y Salomon C 《Science (New York, N.Y.)》2002,296(5571):1290-1293
We report the production of matter-wave solitons in an ultracold lithium-7 gas. The effective interaction between atoms in a Bose-Einstein condensate is tuned with a Feshbach resonance from repulsive to attractive before release in a one-dimensional optical waveguide. Propagation of the soliton without dispersion over a macroscopic distance of 1.1 millimeter is observed. A simple theoretical model explains the stability region of the soliton. These matter-wave solitons open possibilities for future applications in coherent atom optics, atom interferometry, and atom transport. 相似文献
3.
Greilich A Shabaev A Yakovlev DR Efros AL Yugova IA Reuter D Wieck AD Bayer M 《Science (New York, N.Y.)》2007,317(5846):1896-1899
The hyperfine interaction of an electron with the nuclei is considered as the primary obstacle to coherent control of the electron spin in semiconductor quantum dots. We show, however, that the nuclei in singly charged quantum dots act constructively by focusing the electron spin precession about a magnetic field into well-defined modes synchronized with a laser pulse protocol. In a dot with a synchronized electron, the light-stimulated fluctuations of the hyperfine nuclear field acting on the electron are suppressed. The information about electron spin precession is imprinted in the nuclei and thereby can be stored for tens of minutes in darkness. The frequency focusing drives an electron spin ensemble into dephasing-free subspaces with the potential to realize single frequency precession of the entire ensemble. 相似文献
4.
Topinka MA LeRoy BJ Shaw SE Heller EJ Westervelt RM Maranowski KD Gossard AC 《Science (New York, N.Y.)》2000,289(5488):2323-2326
Scanning a charged tip above the two-dimensional electron gas inside a gallium arsenide/aluminum gallium arsenide nanostructure allows the coherent electron flow from the lowest quantized modes of a quantum point contact at liquid helium temperatures to be imaged. As the width of the quantum point contact is increased, its electrical conductance increases in quantized steps of 2 e(2)/h, where e is the electron charge and h is Planck's constant. The angular dependence of the electron flow on each step agrees with theory, and fringes separated by half the electron wavelength are observed. Placing the tip so that it interrupts the flow from particular modes of the quantum point contact causes a reduction in the conductance of those particular conduction channels below 2 e(2)/h without affecting other channels. 相似文献
5.
A scanning probe technique was used to obtain a high-resolution map of the random electrostatic potential inside the quantum Hall liquid. A sharp metal tip, scanned above a semiconductor surface, sensed charges in an embedded two-dimensional (2D) electron gas. Under quantum Hall effect conditions, applying a positive voltage to the tip locally enhanced the 2D electron density and created a "bubble" of electrons in an otherwise unoccupied Landau level. As the tip scanned along the sample surface, the bubble followed underneath. The tip sensed the motions of single electrons entering or leaving the bubble in response to changes in the local 2D electrostatic potential. 相似文献
6.
基于土地流转下农村人力资本流动的障碍及对策 总被引：1，自引：0，他引：1
农村人力资本的合理流动是土地流转的前提。土地流转能够有效提高土地资源配置效率，进一步激活农村剩余劳动力的转移。但农村人力资本流动却受到了观念障碍、信息障碍、成本障碍、技能障碍、制度障碍、职位障碍、收益障碍和公平障碍八大障碍的影响，严重制约了农村人力资本的流动，阻碍了土地流转的进一步实施。因此，主要就如何消除这些障碍进行探讨。 相似文献
7.
Ni KK Ospelkaus S de Miranda MH Pe'er A Neyenhuis B Zirbel JJ Kotochigova S Julienne PS Jin DS Ye J 《Science (New York, N.Y.)》2008,322(5899):231-235
A quantum gas of ultracold polar molecules, with long-range and anisotropic interactions, not only would enable explorations of a large class of many-body physics phenomena but also could be used for quantum information processing. We report on the creation of an ultracold dense gas of potassium-rubidium (40K87Rb) polar molecules. Using a single step of STIRAP (stimulated Raman adiabatic passage) with two-frequency laser irradiation, we coherently transfer extremely weakly bound KRb molecules to the rovibrational ground state of either the triplet or the singlet electronic ground molecular potential. The polar molecular gas has a peak density of 10(12) per cubic centimeter and an expansion-determined translational temperature of 350 nanokelvin. The polar molecules have a permanent electric dipole moment, which we measure with Stark spectroscopy to be 0.052(2) Debye (1 Debye = 3.336 x 10(-30) coulomb-meters) for the triplet rovibrational ground state and 0.566(17) Debye for the singlet rovibrational ground state. 相似文献
8.
Denschlag J Simsarian JE Feder DL Clark CW Collins LA Cubizolles J Deng L Hagley EW Helmerson K Reinhardt WP Rolston SL Schneider BI Phillips WD 《Science (New York, N.Y.)》2000,287(5450):97-101
Quantum phase engineering is demonstrated with two techniques that allow the spatial phase distribution of a Bose-Einstein condensate (BEC) to be written and read out. A quantum state was designed and produced by optically imprinting a phase pattern onto a BEC of sodium atoms, and matter-wave interferometry with spatially resolved imaging was used to analyze the resultant phase distribution. An appropriate phase imprint created solitons, the first experimental realization of this nonlinear phenomenon in a BEC. The subsequent evolution of these excitations was investigated both experimentally and theoretically. 相似文献
9.
Truscott AG Strecker KE McAlexander WI Partridge GB Hulet RG 《Science (New York, N.Y.)》2001,291(5513):2570-2572
We report the attainment of simultaneous quantum degeneracy in a mixed gas of bosons (lithium-7) and fermions (lithium-6). The Fermi gas has been cooled to a temperature of 0.25 times the Fermi temperature by thermal collisions with the evaporatively cooled bosons. At this temperature, the spatial size of the gas is strongly affected by the Fermi pressure resulting from the Pauli exclusion principle and gives clear experimental evidence for quantum degeneracy. 相似文献
10.
人工势场法以其快捷的环境描述和算法形式,在机器人实时避障中得到了广泛的应用。但传统的人工势场法在目标点与障碍太近时,存在机器人不能达到目标点或在目标点附近振荡的缺陷,动态避障一直是避障的难点,本文提出了将时变势场法与角度势场法相融合的策略,此方法适用于动态复杂环境下的避障,仿真结果证明了上述方法的有效性。 相似文献
11.
复杂技术创新是发展中国家进行产业结构升级的一个重要努力方向，也是我国实施自主创新战略的重要方面。高度不确定性是复杂技术创新的主要特征，如何从创新模式及组织形式等方面减少创新过程的不确定性，成为复杂技术创新管理的关键所在。复杂技术创新过程中需要多种知识和技能并且需要创造新知识，创新主体对知识资源的获取成为复杂技术创新不确定性的主要来源。自组织网络能够有效降低这种不确定性，从而促进复杂技术创新。 相似文献
12.
Observations of tidal flow over a sill show that upstream solitary wave generation occurred in conjunction with the downstream movement of a streamline bifurcation over the obstacle crest. Solitary waves appeared upstream of the bifurcation, riding on the surface stratification. Shear flow instabilities on the plunging interface were also evident and may contribute to wave generation through subharmonic interaction. Nonlinear waves generated downstream of an internal control became trapped in the supercritical flow. 相似文献
13.
Fukui Y Yamamoto H Fujishita M Kudo N Torii K Nozawa S Takahashi K Matsumoto R Machida M Kawamura A Yonekura Y Mizuno N Onishi T Mizuno A 《Science (New York, N.Y.)》2006,314(5796):106-109
The central few hundred parsecs of the Milky Way host a massive black hole and exhibit very violent gas motion and high temperatures in molecular gas. The origin of these properties has been a mystery for the past four decades. Wide-field imaging of the (12)CO (rotational quantum number J = 1 to 0) 2.6-millimeter spectrum has revealed huge loops of dense molecular gas with strong velocity dispersions in the galactic center. We present a magnetic flotation model to explain that the formation of the loops is due to magnetic buoyancy caused by the Parker instability. The model has the potential to offer a coherent explanation for the origin of the violent motion and extensive heating of the molecular gas in the galactic center. 相似文献
14.
基于CFD模拟的输气管道阀门流噪声仿真 总被引：1，自引：0，他引：1
将输气管道中的气体流经阀门时的流场在CFD (Computational Fluid Dynamics)软件中进行三维仿真模拟,建立了输气管道阀门流噪声的产生模型.对气体流经阀门的流场进行稳态模拟和瞬态模拟,结果表明:湍流中的流体脉动(如压力、速度脉动)是流体流噪声产生的根本原因.将模拟仿真得到的阀门流场分布通过Lighthill波动方程转换得到偶极子声源,从而对阀门管段进行流体噪声分析,得到阀门流噪声产生、传播、衰减规律.通过试验对仿真模拟得到的阀门声场进行验证,表明了利用Fluent软件对气体流经阀门的流场和声场进行模拟仿真的可行性.研究成果可为输气管道音波法泄漏检测提供技术支持,为输气管道阀门噪声控制方法的制定提供理论依据. 相似文献
15.
Determining the detailed photoreactivity of radicals that are of importance in atmospheric processes requires information from both laboratory and field measurements and theoretical calculations. Laboratory experiments and quantum calculations have been used to develop a comprehensive understanding of the photoreactivity of chlorine dioxide (OCIO). The photoreactivity is strongly dependent on the medium (gas phase, liquid solution, or cryogenic matrix). These data reveal details of the complex chemistry of OCIO. The potential role of this radical in stratospheric ozone depletion is discussed in accord with these laboratory measurements. 相似文献
16.
天然气汽车储气方式的技术经济性分析 总被引：4，自引：0，他引：4
天然气可用液化,压缩和吸附三种方式来存储。对于车用天然气而言,液化方法实用性有限。而压缩天然气需要高压及昂贵的多级压缩设施,同时压缩天然气（CNG）加气站高昂的造价也限制了CNG汽车进入北美市场。实验室水平的吸附式天然气（ANG）储气只需用CNG约1／6的压力即可达到其3／4的体积能量存储密度。目前,探寻一种新的,更适合车用的吸附剂以进一步提高ANG存储的体积能量密度并降低成本的研究已成为一个十分活跃的研究领域。针对天然气储气方式的经济性进行了探讨,比较了吸附,液化及压缩等方法的存储数据,同时提出了一些建议。 相似文献
17.
Linke H Humphrey TE Lofgren A Sushkov AO Newbury R Taylor RP Omling P 《Science (New York, N.Y.)》1999,286(5448):2314-2317
Adiabatically rocked electron ratchets, defined by quantum confinement in semiconductor heterostructures, were experimentally studied in a regime where tunneling contributed to the particle flow. The rocking-induced electron flow reverses direction as a function of temperature. This result confirms a recent prediction of fundamentally different behavior of classical versus quantum ratchets. A wave-mechanical model reproduced the temperature-induced current reversal and provides an intuitive explanation. 相似文献
18.
Maurer PC Kucsko G Latta C Jiang L Yao NY Bennett SD Pastawski F Hunger D Chisholm N Markham M Twitchen DJ Cirac JI Lukin MD 《Science (New York, N.Y.)》2012,336(6086):1283-1286
Stable quantum bits, capable both of storing quantum information for macroscopic time scales and of integration inside small portable devices, are an essential building block for an array of potential applications. We demonstrate high-fidelity control of a solid-state qubit, which preserves its polarization for several minutes and features coherence lifetimes exceeding 1 second at room temperature. The qubit consists of a single (13)C nuclear spin in the vicinity of a nitrogen-vacancy color center within an isotopically purified diamond crystal. The long qubit memory time was achieved via a technique involving dissipative decoupling of the single nuclear spin from its local environment. The versatility, robustness, and potential scalability of this system may allow for new applications in quantum information science. 相似文献
19.
20.
Sachdev S 《Science (New York, N.Y.)》2000,288(5465):475-480
Small changes in an external parameter can often lead to dramatic qualitative changes in the lowest energy quantum mechanical ground state of a correlated electron system. In anisotropic crystals, such as the high-temperature superconductors where electron motion occurs primarily on a two-dimensional square lattice, the quantum critical point between two such lowest energy states has nontrivial emergent excitations that control the physics over a significant portion of the phase diagram. Nonzero temperature dynamic properties near quantum critical points are described, using simple theoretical models. Possible quantum phases and transitions in the two-dimensional electron gas on a square lattice are discussed. 相似文献