41 research outputs found
Frequencies and Damping rates of a 2D Deformed Trapped Bose gas above the Critical Temperature
We derive the equation of motion for the velocity fluctuations of a 2D
deformed trapped Bose gas above the critical temperature in the hydrodynamical
regime. From this equation, we calculate the eigenfrequencies for a few
low-lying excitation modes. Using the method of averages, we derive a
dispersion relation in a deformed trap that interpolates between the
collisionless and hydrodynamic regimes. We make use of this dispersion relation
to calculate the frequencies and the damping rates for monopole and quadrupole
mode in both the regimes. We also discuss the time evolution of the wave packet
width of a Bose gas in a time dependent as well as time independent trap.Comment: 13 pages, latex fil
Cold atom gas at very high densities in an optical surface microtrap
An optical microtrap is realized on a dielectric surface by crossing a
tightly focused laser beam with an horizontal evanescent-wave atom mirror. The
nondissipative trap is loaded with cesium atoms through elastic
collisions from a cold reservoir provided by a large-volume optical surface
trap. With an observed 300-fold local increase of the atomic number density
approaching , unprecedented conditions of cold atoms
close to a surface are realized
Multi Mode Interferometer for Guided Matter Waves
We describe the fundamental features of an interferometer for guided matter
waves based on Y-beam splitters and show that, in a quasi two-dimensional
regime, such a device exhibits high contrast fringes even in a multi mode
regime and fed from a thermal source.Comment: Final version (accepted to PRL
Observation of radiation pressure exerted by evanescent waves
We report a direct observation of radiation pressure, exerted on cold
rubidium atoms while bouncing on an evanescent-wave atom mirror. We analyze the
radiation pressure by imaging the motion of the atoms after the bounce. The
number of absorbed photons is measured for laser detunings ranging from {190
MHz} to {1.4 GHz} and for angles from {0.9 mrad} to {24 mrad} above the
critical angle of total internal reflection. Depending on these settings, we
find velocity changes parallel with the mirror surface, ranging from 1 to {18
cm/s}. This corresponds to 2 to 31 photon recoils per atom. These results are
independent of the evanescent-wave optical power.Comment: 6 pages, 4 figure
Creating a low-dimensional quantum gas using dark states in an inelastic evanescent-wave mirror
We discuss an experimental scheme to create a low-dimensional gas of
ultracold atoms, based on inelastic bouncing on an evanescent-wave mirror.
Close to the turning point of the mirror, the atoms are transferred into an
optical dipole trap. This scheme can compress the phase-space density and can
ultimately yield an optically-driven atom laser. An important issue is the
suppression of photon scattering due to ``cross-talk'' between the mirror
potential and the trapping potential. We propose that for alkali atoms the
photon scattering rate can be suppressed by several orders of magnitude if the
atoms are decoupled from the evanescent-wave light. We discuss how such dark
states can be achieved by making use of circularly-polarized evanescent waves.Comment: 8 pages, 4 figure
Bose-Einstein condensation in quasi2D trapped gases
We discuss BEC in (quasi)2D trapped gases and find that well below the
transition temperature the equilibrium state is a true condensate,
whereas at intermediate temperatures one has a quasicondensate
(condensate with fluctuating phase). The mean-field interaction in a quasi2D
gas is sensitive to the frequency of the (tight) confinement in the
"frozen" direction, and one can switch the sign of the interaction by changing
. Variation of can also reduce the rates of inelastic
processes, which opens prospects for tunable BEC in trapped quasi2D gases.Comment: 4 revtex pages, 1 figure, text is revised, figure improve
Pairing in two-dimensional boson-fermion mixtures
The possibilities of pairing in two-dimensional boson-fermion mixtures are
carefully analyzed. It is shown that the boson-induced attraction between two
identical fermions dominates the p-wave pairing at low density. For a given
fermion density, the pairing gap becomes maximal at a certain optimal boson
concentration. The conditions for observing pairing in current experiments are
discussedComment: 10 pages, 5 figs, revtex
Distributed Dendritic Processing Facilitates Object Detection: A Computational Analysis on the Visual System of the Fly
Hennig P, Möller R, Egelhaaf M. Distributed Dendritic Processing Facilitates Object Detection: A Computational Analysis on the Visual System of the Fly. PLoS ONE. 2008;3(8): e3092.Background: Detecting objects is an important task when moving through a natural environment. Flies, for example, may land on salient objects or may avoid collisions with them. The neuronal ensemble of Figure Detection cells (FD-cells) in the visual system of the fly is likely to be involved in controlling these behaviours, as these cells are more sensitive to objects than to extended background structures. Until now the computations in the presynaptic neuronal network of FD-cells and, in particular, the functional significance of the experimentally established distributed dendritic processing of excitatory and inhibitory inputs is not understood. Methodology/Principal Findings: We use model simulations to analyse the neuronal computations responsible for the preference of FD-cells for small objects. We employed a new modelling approach which allowed us to account for the spatial spread of electrical signals in the dendrites while avoiding detailed compartmental modelling. The models are based on available physiological and anatomical data. Three models were tested each implementing an inhibitory neural circuit, but differing by the spatial arrangement of the inhibitory interaction. Parameter optimisation with an evolutionary algorithm revealed that only distributed dendritic processing satisfies the constraints arising from electrophysiological experiments. In contrast to a direct dendro-dendritic inhibition of the FD-cell (Direct Distributed Inhibition model), an inhibition of its presynaptic retinotopic elements (Indirect Distributed Inhibition model) requires smaller changes in input resistance in the inhibited neurons during visual stimulation. Conclusions/Significance: Distributed dendritic inhibition of retinotopic elements as implemented in our Indirect Distributed Inhibition model is the most plausible wiring scheme for the neuronal circuit of FD-cells. This microcircuit is computationally similar to lateral inhibition between the retinotopic elements. Hence, distributed inhibition might be an alternative explanation of perceptual phenomena currently explained by lateral inhibition networks
Determinants of synaptic integration and heterogeneity in rebound firing explored with data-driven models of deep cerebellar nucleus cells
Significant inroads have been made to understand cerebellar cortical processing but neural coding at the output stage of the cerebellum in the deep cerebellar nuclei (DCN) remains poorly understood. The DCN are unlikely to just present a relay nucleus because Purkinje cell inhibition has to be turned into an excitatory output signal, and DCN neurons exhibit complex intrinsic properties. In particular, DCN neurons exhibit a range of rebound spiking properties following hyperpolarizing current injection, raising the question how this could contribute to signal processing in behaving animals. Computer modeling presents an ideal tool to investigate how intrinsic voltage-gated conductances in DCN neurons could generate the heterogeneous firing behavior observed, and what input conditions could result in rebound responses. To enable such an investigation we built a compartmental DCN neuron model with a full dendritic morphology and appropriate active conductances. We generated a good match of our simulations with DCN current clamp data we recorded in acute slices, including the heterogeneity in the rebound responses. We then examined how inhibitory and excitatory synaptic input interacted with these intrinsic conductances to control DCN firing. We found that the output spiking of the model reflected the ongoing balance of excitatory and inhibitory input rates and that changing the level of inhibition performed an additive operation. Rebound firing following strong Purkinje cell input bursts was also possible, but only if the chloride reversal potential was more negative than −70 mV to allow de-inactivation of rebound currents. Fast rebound bursts due to T-type calcium current and slow rebounds due to persistent sodium current could be differentially regulated by synaptic input, and the pattern of these rebounds was further influenced by HCN current. Our findings suggest that active properties of DCN neurons could play a crucial role for signal processing in the cerebellum