337 research outputs found
Quantum dynamics in ultra-cold atomic physics
We review recent developments in the theory of quantum dynamics in ultra-cold
atomic physics, including exact techniques, but focusing on methods based on
phase-space mappings that are appli- cable when the complexity becomes
exponentially large. These phase-space representations include the truncated
Wigner, positive-P and general Gaussian operator representations which can
treat both bosons and fermions. These phase-space methods include both
traditional approaches using a phase-space of classical dimension, and more
recent methods that use a non-classical phase-space of increased
dimensionality. Examples used include quantum EPR entanglement of a four-mode
BEC, time-reversal tests of dephasing in single-mode traps, BEC quantum
collisions with up to 106 modes and 105 interacting particles, quantum
interferometry in a multi-mode trap with nonlinear absorp- tion, and the theory
of quantum entropy in phase-space. We also treat the approach of variational
optimization of the sampling error, giving an elementary example of a nonlinear
oscillator
Realization of Bose-Einstein condensates in lower dimensions
Bose-Einstein condensates of sodium atoms have been prepared in optical and
magnetic traps in which the energy-level spacing in one or two dimensions
exceeds the interaction energy between atoms, realizing condensates of lower
dimensionality. The cross-over into two-dimensional and one-dimensional
condensates was observed by a change in aspect ratio and saturation of the
release energy when the number of trapped atoms was reduced
Numerical Solution of Differential Equations by the Parker-Sochacki Method
A tutorial is presented which demonstrates the theory and usage of the
Parker-Sochacki method of numerically solving systems of differential
equations. Solutions are demonstrated for the case of projectile motion in air,
and for the classical Newtonian N-body problem with mutual gravitational
attraction.Comment: Added in July 2010: This tutorial has been posted since 1998 on a
university web site, but has now been cited and praised in one or more
refereed journals. I am therefore submitting it to the Cornell arXiv so that
it may be read in response to its citations. See "Spiking neural network
simulation: numerical integration with the Parker-Sochacki method:" J. Comput
Neurosci, Robert D. Stewart & Wyeth Bair and
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2717378
Low energy atomic collision with dipole interactions
We apply quantum defect theory to study low energy ground state atomic
collisions including aligned dipole interactions such as those induced by an
electric field. Our results show that coupled even () relative orbital
angular momentum partial wave channels exhibit shape resonance structures while
odd () channels do not. We analyze and interpret these resonances within the
framework of multichannel quantum defect theory (MQDT).Comment: 27 pages, 17 figures, an inadvertent typo correcte
The compositional and evolutionary logic of metabolism
Metabolism displays striking and robust regularities in the forms of
modularity and hierarchy, whose composition may be compactly described. This
renders metabolic architecture comprehensible as a system, and suggests the
order in which layers of that system emerged. Metabolism also serves as the
foundation in other hierarchies, at least up to cellular integration including
bioenergetics and molecular replication, and trophic ecology. The
recapitulation of patterns first seen in metabolism, in these higher levels,
suggests metabolism as a source of causation or constraint on many forms of
organization in the biosphere.
We identify as modules widely reused subsets of chemicals, reactions, or
functions, each with a conserved internal structure. At the small molecule
substrate level, module boundaries are generally associated with the most
complex reaction mechanisms and the most conserved enzymes. Cofactors form a
structurally and functionally distinctive control layer over the small-molecule
substrate. Complex cofactors are often used at module boundaries of the
substrate level, while simpler ones participate in widely used reactions.
Cofactor functions thus act as "keys" that incorporate classes of organic
reactions within biochemistry.
The same modules that organize the compositional diversity of metabolism are
argued to have governed long-term evolution. Early evolution of core
metabolism, especially carbon-fixation, appears to have required few
innovations among a small number of conserved modules, to produce adaptations
to simple biogeochemical changes of environment. We demonstrate these features
of metabolism at several levels of hierarchy, beginning with the small-molecule
substrate and network architecture, continuing with cofactors and key conserved
reactions, and culminating in the aggregation of multiple diverse physical and
biochemical processes in cells.Comment: 56 pages, 28 figure
Signal Propagation in Feedforward Neuronal Networks with Unreliable Synapses
In this paper, we systematically investigate both the synfire propagation and
firing rate propagation in feedforward neuronal network coupled in an
all-to-all fashion. In contrast to most earlier work, where only reliable
synaptic connections are considered, we mainly examine the effects of
unreliable synapses on both types of neural activity propagation in this work.
We first study networks composed of purely excitatory neurons. Our results show
that both the successful transmission probability and excitatory synaptic
strength largely influence the propagation of these two types of neural
activities, and better tuning of these synaptic parameters makes the considered
network support stable signal propagation. It is also found that noise has
significant but different impacts on these two types of propagation. The
additive Gaussian white noise has the tendency to reduce the precision of the
synfire activity, whereas noise with appropriate intensity can enhance the
performance of firing rate propagation. Further simulations indicate that the
propagation dynamics of the considered neuronal network is not simply
determined by the average amount of received neurotransmitter for each neuron
in a time instant, but also largely influenced by the stochastic effect of
neurotransmitter release. Second, we compare our results with those obtained in
corresponding feedforward neuronal networks connected with reliable synapses
but in a random coupling fashion. We confirm that some differences can be
observed in these two different feedforward neuronal network models. Finally,
we study the signal propagation in feedforward neuronal networks consisting of
both excitatory and inhibitory neurons, and demonstrate that inhibition also
plays an important role in signal propagation in the considered networks.Comment: 33pages, 16 figures; Journal of Computational Neuroscience
(published
The dynamics of component separation in a binary mixture of Bose-Einstein condensates
We present studies of the time-evolution of a two-component system of
Bose-Einstein condensates (BEC) in the |F=1, mf=-1> and |2,1> spin states of
Rb-87. The two condensates are created with a well-defined relative phase and
complete spatial overlap. In subsequent evolution they undergo complex relative
motions which tend to preserve the total density profile. the motions quickly
damp out, leaving the condensates in a steady state with a nonnegligible (and
adjustable) overlap region.Comment: 4 pages, 5 figure
Uncommon genetic syndromes and narrative production - Case Studies with Williams, Smith-Magenis and Prader- Willi Syndromes
This study compares narrative production among three syndromes with
genetic microdeletions: Williams syndrome (WS), Smith-Magenis syndrome
(SMS), and Prader-Willi syndrome (PWS), characterized by intellectual
disabilities and relatively spared language abilities. Our objective is to study
the quality of narrative production in the context of a common intellectual
disability. To elicit a narrative production, the task Frog! Where Are You was
used. Then, structure, process, and content of the narrative process were
analysed in the three genetic disorders:WS (n52), SMS (n52), and PWS (n52).
Data show evidence of an overall low narrative quality in these syndromes,
despite a high variability within different measures of narrative production.
Results support the hypothesis that narrative is a highly complex cognitive
process and that, in a context of intellectual disability, there is no evidence of
particular âhypernarrativityâ in these syndromes.This research was supported by the grants FEDER â
Primary Production, an Index of Climate Change in the Ocean: Satellite-Based Estimates over Two Decades
Primary production by marine phytoplankton is one of the largest ďŹuxes of carbon on our planet. In the past few decades, considerable progress has been made in estimating global primary production at high spatial and temporal scales by combining in situ measurements of primary production with remote-sensing observations of phytoplankton biomass. One of the major challengesinthisapproachliesintheassignmentoftheappropriatemodelparametersthatdeďŹnethe photosynthetic response of phytoplankton to the light ďŹeld. In the present study, a global database of in situ measurements of photosynthesis versus irradiance (P-I) parameters and a 20-year record of climatequalitysatelliteobservationswereusedtoassessglobalprimaryproductionanditsvariability with seasons and locations as well as between years. In addition, the sensitivity of the computed primaryproductiontopotentialchangesinthephotosyntheticresponseofphytoplanktoncellsunder changing environmental conditions was investigated. Global annual primary production varied from 38.8 to 42.1 Gt C yrâ1 over the period of 1998â2018. Inter-annual changes in global primary production did not follow a linear trend, and regional differences in the magnitude and direction of change in primary production were observed. Trends in primary production followed directly from changes in chlorophyll-a and were related to changes in the physico-chemical conditions of the water column due to inter-annual and multidecadal climate oscillations. Moreover, the sensitivity analysis in which P-I parameters were adjusted byÂą1 standard deviation showed the importance of accurately assigning photosynthetic parameters in global and regional calculations of primary production. TheassimilationnumberoftheP-Icurveshowedstrongrelationshipswithenvironmental variables such as temperature and had a practically one-to-one relationship with the magnitude of change in primary production. In the future, such empirical relationships could potentially be used for a more dynamic assignment of photosynthetic rates in the estimation of global primary production. RelationshipsbetweentheinitialslopeoftheP-Icurveandenvironmentalvariableswere more elusive
An accurate and efficient identification of children with psychosocial problems by means of computerized adaptive testing
Background: Questionnaires used by health services to identify children with psychosocial problems are often rather short. The psychometric properties of such short questionnaires are mostly less than needed for an accurate distinction between children with and without problems. We aimed to assess whether a short Computerized Adaptive Test (CAT) can overcome the weaknesses of short written questionnaires when identifying children with psychosocial problems. Method: We used a Dutch national data set obtained from parents of children invited for a routine health examination by Preventive Child Healthcare with 205 items on behavioral and emotional problems (n = 2,041, response 84%). In a random subsample we determined which items met the requirements of an Item Response Theory (IRT) model to a sufficient degree. Using those items, item parameters necessary for a CAT were calculated and a cut-off point was defined. In the remaining subsample we determined the validity and efficiency of a Computerized Adaptive Test using simulation techniques, with current treatment status and a clinical score on the Total Problem Scale (TPS) of the Child Behavior Checklist as criteria. Results: Out of 205 items available 190 sufficiently met the criteria of the underlying IRT model. For 90% of the children a score above or below cut-off point could be determined with 95% accuracy. The mean number of items needed to achieve this was 12. Sensitivity and specificity with the TPS as a criterion were 0.89 and 0.91, respectively. Conclusion: An IRT-based CAT is a very promising option for the identification of psychosocial problems in children, as it can lead to an efficient, yet high-quality identification. The results of our simulation study need to be replicated in a real-life administration of this CAT
- âŚ