348 research outputs found
Stimulated Raman adiabatic passage from an atomic to a molecular Bose-Einstein condensate
The process of stimulated Raman adiabatic passage (STIRAP) provides a
possible route for the generation of a coherent molecular Bose-Einstein
condensate (BEC) from an atomic BEC. We analyze this process in a
three-dimensional mean-field theory, including atom-atom interactions and
non-resonant intermediate levels. We find that the process is feasible, but at
larger Rabi frequencies than anticipated from a crude single-mode lossless
analysis, due to two-photon dephasing caused by the atomic interactions. We
then identify optimal strategies in STIRAP allowing one to maintain high
conversion efficiencies with smaller Rabi frequencies and under experimentally
less demanding conditions.Comment: Final published versio
Observation of Superfluid Flow in a Bose-Einstein Condensed Gas
We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred
by a macroscopic object, a blue detuned laser beam, using nondestructive {\em
in situ} phase contrast imaging. A critical velocity for the onset of a
pressure gradient has been observed, and shown to be density dependent. The
technique has been compared to a calorimetric method used previously to measure
the heating induced by the motion of the laser beam.Comment: 4 pages, 5 figure
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
Dynamical response of a Bose-Einstein condensate to a discontinuous change in internal state
A two-photon transition is used to convert an arbitrary fraction of the 87Rb
atoms in a |F=1,m_f=-1> condensate to the |F=2,m_f=1> state. Transferring the
entire population imposes a discontinuous change on the condensate's mean-field
repulsion, which leaves a residual ringing in the condensate width. A
calculation based on Gross-Pitaevskii theory agrees well with the observed
behavior, and from the comparison we obtain the ratio of the intraspecies
scattering lengths for the two states, a_|1,-1> / a_|2,1> = 1.062(12).Comment: 4 pages, 3 figure
Superchemistry: dynamics of coupled atomic and molecular Bose-Einstein condensates
We analyze the dynamics of a dilute, trapped Bose-condensed atomic gas
coupled to a diatomic molecular Bose gas by coherent Raman transitions. This
system is shown to result in a new type of `superchemistry', in which giant
collective oscillations between the atomic and molecular gas can occur. The
phenomenon is caused by stimulated emission of bosonic atoms or molecules into
their condensate phases
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
Electrophysiological correlates of high-level perception during spatial navigation
We studied the electrophysiological basis of object recognition by recording scalp\ud
electroencephalograms while participants played a virtual-reality taxi driver game.\ud
Participants searched for passengers and stores during virtual navigation in simulated\ud
towns. We compared oscillatory brain activity in response to store views that were targets or\ud
nontargets (during store search) or neutral (during passenger search). Even though store\ud
category was solely defined by task context (rather than by sensory cues), frontal ...\ud
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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
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
A Neural Spiking Approach Compared to Deep Feedforward Networks on Stepwise Pixel Erasement
In real world scenarios, objects are often partially occluded. This requires
a robustness for object recognition against these perturbations. Convolutional
networks have shown good performances in classification tasks. The learned
convolutional filters seem similar to receptive fields of simple cells found in
the primary visual cortex. Alternatively, spiking neural networks are more
biological plausible. We developed a two layer spiking network, trained on
natural scenes with a biologically plausible learning rule. It is compared to
two deep convolutional neural networks using a classification task of stepwise
pixel erasement on MNIST. In comparison to these networks the spiking approach
achieves good accuracy and robustness.Comment: Published in ICANN 2018: Artificial Neural Networks and Machine
Learning - ICANN 2018
https://link.springer.com/chapter/10.1007/978-3-030-01418-6_25 The final
authenticated publication is available online at
https://doi.org/10.1007/978-3-030-01418-6_2
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