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861.
Paredes B Widera A Murg V Mandel O Fölling S Cirac I Shlyapnikov GV Hänsch TW Bloch I 《Nature》2004,429(6989):277-281
Strongly correlated quantum systems are among the most intriguing and fundamental systems in physics. One such example is the Tonks-Girardeau gas, proposed about 40 years ago, but until now lacking experimental realization; in such a gas, the repulsive interactions between bosonic particles confined to one dimension dominate the physics of the system. In order to minimize their mutual repulsion, the bosons are prevented from occupying the same position in space. This mimics the Pauli exclusion principle for fermions, causing the bosonic particles to exhibit fermionic properties. However, such bosons do not exhibit completely ideal fermionic (or bosonic) quantum behaviour; for example, this is reflected in their characteristic momentum distribution. Here we report the preparation of a Tonks-Girardeau gas of ultracold rubidium atoms held in a two-dimensional optical lattice formed by two orthogonal standing waves. The addition of a third, shallower lattice potential along the long axis of the quantum gases allows us to enter the Tonks-Girardeau regime by increasing the atoms' effective mass and thereby enhancing the role of interactions. We make a theoretical prediction of the momentum distribution based on an approach in which trapped bosons acquire fermionic properties, finding that it agrees closely with the measured distribution. 相似文献
862.
The controlled production of single photons is of fundamental and practical interest; they represent the lowest excited quantum states of the radiation field, and have applications in quantum cryptography and quantum information processing. Common approaches use the fluorescence of single ions, single molecules, colour centres and semiconductor quantum dots. However, the lack of control over such irreversible emission processes precludes the use of these sources in applications (such as quantum networks) that require coherent exchange of quantum states between atoms and photons. The necessary control may be achieved in principle in cavity quantum electrodynamics. Although this approach has been used for the production of single photons from atoms, such experiments are compromised by limited trapping times, fluctuating atom-field coupling and multi-atom effects. Here we demonstrate a single-photon source based on a strongly localized single ion in an optical cavity. The ion is optimally coupled to a well-defined field mode, resulting in the generation of single-photon pulses with precisely defined shape and timing. We have confirmed the suppression of two-photon events up to the limit imposed by fluctuations in the rate of detector dark counts. The stream of emitted photons is uninterrupted over the storage time of the ion, as demonstrated by a measurement of photon correlations over 90 min. 相似文献
863.
Evidence of power-law flow in the Mojave desert mantle 总被引:2,自引:0,他引:2
Studies of the Earth's response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation. Laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power, n (refs 2, 3). Most geodynamic models of flow in the lower crust and upper mantle, however, resort to newtonian (linear) stress-strain rate relations. Here we show that a power-law model of viscous flow in the mantle with n = 3.5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 Landers and 1999 Hector Mine earthquakes in southern California. A power-law rheology implies that viscosity varies spatially with stress causing localization of strain, and varies temporally as stress evolves, rendering newtonian models untenable. Our findings are consistent with laboratory-derived flow law parameters for hot and wet olivine--the most abundant mineral in the upper mantle--and support the contention that, at least beneath the Mojave desert, the upper mantle is weaker than the lower crust. 相似文献
864.
Genome duplication in the teleost fish Tetraodon nigroviridis reveals the early vertebrate proto-karyotype 总被引:2,自引:0,他引:2
Jaillon O Aury JM Brunet F Petit JL Stange-Thomann N Mauceli E Bouneau L Fischer C Ozouf-Costaz C Bernot A Nicaud S Jaffe D Fisher S Lutfalla G Dossat C Segurens B Dasilva C Salanoubat M Levy M Boudet N Castellano S Anthouard V Jubin C Castelli V Katinka M Vacherie B Biémont C Skalli Z Cattolico L Poulain J De Berardinis V Cruaud C Duprat S Brottier P Coutanceau JP Gouzy J Parra G Lardier G Chapple C McKernan KJ McEwan P Bosak S Kellis M Volff JN Guigó R Zody MC Mesirov J Lindblad-Toh K 《Nature》2004,431(7011):946-957
Tetraodon nigroviridis is a freshwater puffer fish with the smallest known vertebrate genome. Here, we report a draft genome sequence with long-range linkage and substantial anchoring to the 21 Tetraodon chromosomes. Genome analysis provides a greatly improved fish gene catalogue, including identifying key genes previously thought to be absent in fish. Comparison with other vertebrates and a urochordate indicates that fish proteins have diverged markedly faster than their mammalian homologues. Comparison with the human genome suggests approximately 900 previously unannotated human genes. Analysis of the Tetraodon and human genomes shows that whole-genome duplication occurred in the teleost fish lineage, subsequent to its divergence from mammals. The analysis also makes it possible to infer the basic structure of the ancestral bony vertebrate genome, which was composed of 12 chromosomes, and to reconstruct much of the evolutionary history of ancient and recent chromosome rearrangements leading to the modern human karyotype. 相似文献
865.
Lebrun F Terrier R Bazzano A Bélanger G Bird A Bouchet L Dean A Del Santo M Goldwurm A Lund N Morand H Parmar A Paul J Roques JP Schönfelder V Strong AW Ubertini P Walter R Winkler C 《Nature》2004,428(6980):293-296
The Milky Way is known to be an abundant source of gamma-ray photons, now determined to be mainly diffuse in nature and resulting from interstellar processes. In the soft gamma-ray domain, point sources are expected to dominate, but the lack of sensitive high-resolution observations did not allow for a clear estimate of the contribution from such sources. Even the best imaging experiment revealed only a few point sources, accounting for about 50% of the total Galactic flux. Theoretical studies were unable to explain the remaining intense diffuse emission. Investigating the origin of the soft gamma-rays is therefore necessary to determine the dominant particle acceleration processes and to gain insights into the physical and chemical equilibrium of the interstellar medium. Here we report observations in the soft gamma-ray domain that reveal numerous compact sources. We show that these sources account for the entirety of the Milky Way's emission in soft gamma-rays, leaving at most a minor role for diffuse processes. 相似文献
866.
Perennial water ice identified in the south polar cap of Mars 总被引:1,自引:0,他引:1
Bibring JP Langevin Y Poulet F Gendrin A Gondet B Berthé M Soufflot A Drossart P Combes M Bellucci G Moroz V Mangold N Schmitt B;OMEGA Team 《Nature》2004,428(6983):627-630
The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet. From the early mapping observation of the permanent ice caps on the martian poles, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures, altimetry profiles, underlying buried hydrogen, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2 km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap. 相似文献
867.
868.
The Microprocessor complex mediates the genesis of microRNAs 总被引:5,自引:0,他引:5
Gregory RI Yan KP Amuthan G Chendrimada T Doratotaj B Cooch N Shiekhattar R 《Nature》2004,432(7014):235-240
869.
西昆仑"库地蛇绿岩"的解体及有关问题的讨论 总被引:5,自引:0,他引:5
路线地质填图和构造变形、变质作用研究表明,构成"库地蛇绿岩"的超镁铁质岩石和火山-复理石建造在产状和变形、变质等地质特征上存在着截然的差异,古生物和同位素年代学资料也显示着它们可能属于2个不同时代的岩石组合,因此建议将其解体成两部分:前一部分是在早古生代早或中期发生构造侵位,构成加里东期古昆仑造山带的一个构造单元,被早古生代末期和晚古生代早期形成在拉张构造背景的碱性花岗岩和煌斑岩脉侵入,显示已经作为新一期构造旋回基底的一部分;后一部分形成在中泥盆世至石炭纪,是新一期大陆裂解作用的产物. 相似文献
870.
Factor XIII subunit A as an intracellular transglutaminase 总被引:2,自引:0,他引:2
Over the last 2 decades there has been increasing evidence that the role of factor XIII (FXIII) is not restricted to the area of hemostasis and that its subunit A functions as an intracellular enzyme in platelets and monocytes/macrophages. FXIII is already expressed during compartmentalisation of the precursors of megakaryocyte/platelet and monocyte/macrophage cell lines in the bone marrow. FXIII-A, produced by megakaryocytes, is packaged into budding platelets and is present in huge quantity in circulating ones. It seems very likely that it plays an important role in the cytoskeletal remodelling associated with the activation stages of platelets. FXIII-A can also be detected in blood monocytes and in all subsets of monocyte-derived macrophages throughout the body. FXIII-A is mainly localised in the cytoplasm, in association with cytoskeletal filaments, but at a relatively early stage of macrophage differentiation it also appears transiently in the nucleus. Cytoplasmic expression has a very close relationship with phagocytic activities. Further research is needed to understand the biological significance of its nuclear presentation. 相似文献