538 research outputs found
A Quantum Degenerate Mixture of Metastable Helium-3 and Helium-4 Atoms
Hogervorst, W. [Promotor]Vassen, W. [Copromotor
Cross inhibition improves activity selection when switching incurs time costs
We consider a behavioural model of an animal choosing between two activities, based on positive feedback, and examine the effect of introducing cross inhibition between the motivations for the two activities. While cross-inhibition has previously been included in models of decision making, the question of what benefit it may provide to an animal's activity selection behaviour has not previously been studied. In neuroscience and in collective behaviour cross-inhibition, and other equivalent means of coupling evidence-accumulating pathways, have been shown to approximate statistically-optimal decision-making and to adaptively break deadlock, thereby improving decision performance. Switching between activities is an ongoing decision process yet here we also find that cross-inhibition robustly improves its efficiency, by reducing the frequency of costly switches between behaviours
Conceptual spatial representations for indoor mobile robots
We present an approach for creating conceptual representations of human-made indoor environments using mobile
robots. The concepts refer to spatial and functional properties of typical indoor environments. Following ďŹndings
in cognitive psychology, our model is composed of layers representing maps at diďŹerent levels of abstraction. The
complete system is integrated in a mobile robot endowed with laser and vision sensors for place and object recognition.
The system also incorporates a linguistic framework that actively supports the map acquisition process, and which
is used for situated dialogue. Finally, we discuss the capabilities of the integrated system
Producing and Detecting Correlated atoms
We discuss experiments to produce and detect atom correlations in a
degenerate or nearly degenerate gas of neutral atoms. First we treat the atomic
analog of the celebrated Hanbury Brown Twiss experiment, in which atom
correlations result simply from interference effects without any atom
interactions.We have performed this experiment for both bosons and fermions.
Next we show how atom interactions produce correlated atoms using the atomic
analog of spontaneous four-wavemixing. Finally, we briefly mention experiments
on a one dimensional gas on an atom chip in which correlation effects due to
both interference and interactions have been observed.Comment: to appear in conference proceedings "Atomic Physics 20
A model for the atomic-scale structure of a dense, nonequilibrium fluid: the homogeneous cooling state of granular fluids
It is shown that the equilibrium Generalized Mean Spherical Model of fluid
structure may be extended to nonequilibrium states with equation of state
information used in equilibrium replaced by an exact condition on the two-body
distribution function. The model is applied to the homogeneous cooling state of
granular fluids and upon comparison to molecular dynamics simulations is found
to provide an accurate picture of the pair distribution function.Comment: 29 pages, 11 figures Revision corrects formatting of the figure
Magneto-optical trapping of bosonic and fermionic neon isotopes and their mixtures: isotope shift of the ^3P_2 to ^3D_3 transition and hyperfine constants of the ^3D_3 state of Ne-21
We have magneto-optically trapped all three stable neon isotopes, including
the rare Ne-21, and all two-isotope combinations. The atoms are prepared in the
metastable ^3P_2 state and manipulated via laser interaction on the ^3P_2 to
^3D_3} transition at 640.2nm. These cold (T = 1mK) and environmentally
decoupled atom samples present ideal objects for precision measurements and the
investigation of interactions between cold and ultracold metastable atoms. In
this work, we present accurate measurements of the isotope shift of the ^3P_2
to ^3D_3 transition and the hyperfine interaction constants of the ^3D_3 state
of Ne-21. The determined isotope shifts are (1625.9\pm0.15)MHz for Ne-20 to
Ne-22, (855.7\pm1.0)MHz for Ne-20 to Ne-21, and (770.3\pm1.0)MHz for Ne-21 to
Ne-22. The obtained magnetic dipole and electric quadrupole hyperfine
interaction constants are A(^3D_3)= (-142.4\pm0.2)MHz and
B(^3D_3)=(-107.7\pm1.1)MHz, respectively. All measurements give a reduction of
uncertainty by about one order of magnitude over previous measurements
Granular fluid thermostatted by a bath of elastic hard spheres
The homogeneous steady state of a fluid of inelastic hard spheres immersed in
a bath of elastic hard spheres kept at equilibrium is analyzed by means of the
first Sonine approximation to the (spatially homogeneous) Enskog--Boltzmann
equation. The temperature of the granular fluid relative to the bath
temperature and the kurtosis of the granular distribution function are obtained
as functions of the coefficient of restitution, the mass ratio, and a
dimensionless parameter measuring the cooling rate relative to the
friction constant. Comparison with recent results obtained from an iterative
numerical solution of the Enskog--Boltzmann equation [Biben et al., Physica A
310, 308 (202)] shows an excellent agreement. Several limiting cases are also
considered. In particular, when the granular particles are much heavier than
the bath particles (but have a comparable size and number density), it is shown
that the bath acts as a white noise external driving. In the general case, the
Sonine approximation predicts the lack of a steady state if the control
parameter is larger than a certain critical value that
depends on the coefficient of restitution and the mass ratio. However, this
phenomenon appears outside the expected domain of applicability of the
approximation.Comment: 16 pages, 7 figures; minor changes; to be published in Phys. Rev.
Non-linear instability of Kerr-type Cauchy horizons
Using the general solution to the Einstein equations on intersecting null
surfaces developed by Hayward, we investigate the non-linear instability of the
Cauchy horizon inside a realistic black hole. Making a minimal assumption about
the free gravitational data allows us to solve the field equations along a null
surface crossing the Cauchy Horizon. As in the spherical case, the results
indicate that a diverging influx of gravitational energy, in concert with an
outflux across the CH, is responsible for the singularity. The spacetime is
asymptotically Petrov type N, the same algebraic type as a gravitational shock
wave. Implications for the continuation of spacetime through the singularity
are briefly discussed.Comment: 11 pages RevTeX, two postscript figures included using epsf.st
Life lessons from and for distributed MPC â Part 1: Dynamics of cooperation
This paper and a second accompanying paper (Olaru et al., 2018) explore the potential of Distributed Predictive Control (DMPC) literature to provide valuable insights into social behaviour. In particular this first paper focuses on the mechanisms of group regulation in social systems. It will be noted that there are major differences between the way in which DMPC algorithms and Social Human Participants (SHPs) form decisions. DMPC can make optimal decisions but these are only optimal with respect to a given objective and model, both of which must be explicit. SHPs operate, by and large, with only vague, implicit objectives and models â which can be surprisingly accurate â but often make sub-optimal decisions both individually (because of irrationality or poor anticipation and due to a short horizon, bad model or misjudgement of objectives) and in a group sense (for the previous reasons plus selfishness). Thus while SHPsâ decisions would typically be suboptimal, with respect to their desired goals, for the aforementioned reasons, it can be expected that SHPsâ decision making would evolve towards an optimal solution as groups of SHPs develop more experience within the system theyâre operating in
Molecular dynamics simulations of vibrated granular gases
We present molecular dynamics simulations of mono- or bidisperse inelastic
granular gases driven by vibrating walls, in two dimensions (without gravity).
Because of the energy injection at the boundaries, a situation often met
experimentally, density and temperature fields display heterogeneous profiles
in the direction perpendicular to the walls. A general equation of state for an
arbitrary mixture of fluidized inelastic hard spheres is derived and
successfully tested against numerical data. Single-particle velocity
distribution functions with non-Gaussian features are also obtained, and the
influence of various parameters (inelasticity coefficients, density...)
analyzed. The validity of a recently proposed Random Restitution Coefficient
model is assessed through the study of projected collisions onto the direction
perpendicular to that of energy injection. For the binary mixture, the
non-equipartition of translational kinetic energy is studied and compared both
to experimental data and to the case of homogeneous energy injection
(``stochastic thermostat''). The rescaled velocity distribution functions are
found to be very similar for both species
- âŚ