1,676 research outputs found
Coherent Collisions between Bose-Einstein Condensates
We study the non-degenerate parametric amplifier for matter waves,
implemented by colliding two Bose-Einstein condensates. The coherence of the
amplified waves is shown by observing high contrast interference with a
reference wave and by reversing the amplification process. Since our
experiments also place limits on all known sources of decoherence, we infer
that relative number squeezing is most likely present between the amplified
modes. Finally, we suggest that reversal of the amplification process may be
used to detect relative number squeezing without requiring single-particle
detection.Comment: 4.2 pages, 4 figures, please take postscript version for best quality
of picture
Effective one-component description of two-component Bose-Einstein condensate dynamics
We investigate dynamics in two-component Bose-Einstein condensates in the
context of coupled Gross-Pitaevskii equations and derive results for the
evolution of the total density fluctuations. Using these results, we show how,
in many cases of interest, the dynamics can be accurately described with an
effective one-component Gross-Pitaevskii equation for one of the components,
with the trap and interaction coefficients determined by the relative
differences in the scattering lengths. We discuss the model in various regimes,
where it predicts breathing excitations, and the formation of vector solitons.
An effective nonlinear evolution is predicted for some cases of current
experimental interest. We then apply the model to construct quasi-stationary
states of two-component condensates.Comment: 8 pages, 4 figure
Enhanced heat flow in the hydrodynamic-collisionless regime
We study the heat conduction of a cold, thermal cloud in a highly asymmetric
trap. The cloud is axially hydrodynamic, but due to the asymmetric trap
radially collisionless. By locally heating the cloud we excite a thermal dipole
mode and measure its oscillation frequency and damping rate. We find an
unexpectedly large heat conduction compared to the homogeneous case. The
enhanced heat conduction in this regime is partially caused by atoms with a
high angular momentum spiraling in trajectories around the core of the cloud.
Since atoms in these trajectories are almost collisionless they strongly
contribute to the heat transfer. We observe a second, oscillating hydrodynamic
mode, which we identify as a standing wave sound mode.Comment: Sumitted to Phys. Rev. Letters, 4 pages, 4 figure
Where to place inaccessible subjects in Dutch: The role of definiteness and animacy
Cross-linguistically, both subjects and topical information tend to be placed at the beginning of a sentence. Subjects are generally highly topical, causing both tendencies to converge on the same word order. However, subjects that lack prototypical topic properties may give rise to an incongruence between the preference to start a sentence with the subject and the preference to start a sentence with the most accessible information. We present a corpus study in which we investigate in what syntactic position (preverbal or postverbal) such low-accessible subjects are typically found in Dutch natural language. We examine the effects of both discourse accessibility (definiteness) and inherent accessibility (animacy). Our results show that definiteness and animacy interact in determining subject position in Dutch. Non-referential (bare) subjects are less likely to occur in preverbal position than definite subjects, and this tendency is reinforced when the subject is inanimate. This suggests that these two properties that make the subject less accessible together can âgang upâ against the subject first preference. The results support a probabilistic multifactorial account of syntactic variation
Reptielen in de heide : verslag veldwerkplaats Droog zandlandschap Hoge Veluwe, 23 juni 2009
Reptielen staan in Nederland onder druk. Een belangrijk habitat wordt gevormd door heide. Slangen en hagedissen stellen echter specifieke eisen aan hun leefgebied. Voor zes van de zeven hagedissen soorten die in Nederland voorkomen, is heide een belangrijk of het belangrijkste habitat. Wat maakt een heideveld wel geschikt als habitat? â We maaien, branden, plaggen, zorgen voor paarse heide. Dat kan bij ons, doordat we zoân grote oppervlakte hebben. Wel gaan we nu naar wat kleinschaliger beheer. Maar bovenal voeren we een consistent beheer, niet afhankelijk van waar toevallig subsidie voor is. Ik denk dat we het daarom zo goed doen wat reptielen betreft.â aldus Leidekker, hoofdbeheerder bij de Hoge Veluw
When a stone tries to climb up a slope:The interplay between lexical and perceptual animacy in referential choices
Several studies suggest that referential choices are influenced by animacy. On the one hand, animate referents are more likely to be mentioned as subjects than inanimate referents. On the other hand, animate referents are more frequently pronominalized than inanimate referents. These effects have been analyzed as effects of conceptual accessibility. In this paper, we raise the question whether these effects are driven only by lexical concepts, such that referents described by animate lexical items (e.g., âtoddlerâ) are more accessible than referents described by inanimate lexical items (e.g., âshoeâ), or can also be influenced by context-derived conceptualizations, such that referents that are perceived as animate in a particular context are more accessible than referents that are not. In two animation-retelling experiments, conducted in Dutch, we investigated the influence of lexical and perceptual animacy on the choice of referent and the choice of referring expression. If the effects of animacy are context-dependent, entities that are perceived as animate should yield more subject references and more pronouns than entities that are perceived as inanimate, irrespective of their lexical animacy. If the effects are tied to lexical concepts, entities described with animate lexical items should be mentioned as the subject and pronominalized more frequently than entities described with inanimate lexical items, irrespective of their perceptual animacy. The results show that while only lexical animacy appears to affect the choice of subject referent, perceptual animacy may overrule lexical animacy in the choice of referring expression. These findings suggest that referential choices can be influenced by conceptualizations based on the perceptual context
Co-dependent excitatory and inhibitory plasticity accounts for quick, stable and long-lasting memories in biological networks
The brainâs functionality is developed and maintained through synaptic plasticity. As synapses undergo plasticity, they also affect each other. The nature of such âco-dependencyâ is difficult to disentangle experimentally, because multiple synapses must be monitored simultaneously. To help understand the experimentally observed phenomena, we introduce a framework that formalizes synaptic co-dependency between different connection types. The resulting model explains how inhibition can gate excitatory plasticity while neighboring excitatoryâexcitatory interactions determine the strength of long-term potentiation. Furthermore, we show how the interplay between excitatory and inhibitory synapses can account for the quick rise and long-term stability of a variety of synaptic weight profiles, such as orientation tuning and dendritic clustering of co-active synapses. In recurrent neuronal networks, co-dependent plasticity produces rich and stable motor cortex-like dynamics with high input sensitivity. Our results suggest an essential role for the neighborly synaptic interaction during learning, connecting micro-level physiology with network-wide phenomena
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