885 research outputs found
Experimental study of the flight envelope and research of safety requirements for hang-gliders
The flight mechanic computations were computed, providing both the flight envelopes with all sorts of limits and a fairly precise idea of the influence of several parameters, such as pilot's weight, wing settings, aeroelasticity, etc... The particular problem of luffing dives was thoroughly analyzed, and two kinds of causes were exhibited in both the rules of luffing and aeroelastic effects. The general analysis of longitudinal stability showed a strong link with fabric tension, as expected through Nielsen's and Twaites' theory. Fabric tension strongly depending upon aeroelasticity, that parameter was found to be the most effective design one for positive stability. Lateral stability was found to be very similar in all gliders except perhaps the cylindro-conical. The loss of stability happens in roll at low angle of attack, whereas it happens in yaw at high angle. Turning performance was a bit suprising, with a common maximum value of approximately 55 deg of bank angle for a steady turn
A magnetic lens for cold atoms controlled by a rf field
We report on a new type of magnetic lens that focuses atomic clouds using a
static inhomogeneous magnetic field in combination with a radio-frequency
field. The experimental study is performed with a cloud of cold cesium atoms.
The rf field adiabatically deforms the magnetic potential of a coil and
therefore changes its focusing properties. The focal length can be tuned
precisely by changing the rf frequency value. Depending on the rf antenna
position relative to the DC magnetic profile, the focal length of the atomic
lens can be either decreased or increased by the rf field
Optimized production of large Bose Einstein Condensates
We suggest different simple schemes to efficiently load and evaporate a
''dimple'' crossed dipolar trap. The collisional processes between atoms which
are trapped in a reservoir load in a non adiabatic way the dimple. The
reservoir trap can be provided either by a dark SPOT Magneto Optical Trap, the
(aberrated) laser beam itself or by a quadrupolar or quadratic magnetic trap.
Optimal parameters for the dimple are derived from thermodynamical equations
and from loading time, including possible inelastic and Majorana losses. We
suggest to load at relatively high temperature a tight optical trap. Simple
evaporative cooling equations, taking into account gravity, the possible
occurrence of hydrodynamical regime, Feshbach resonance processes and three
body recombination events are given. To have an efficient evaporation the
elastic collisional rate (in s) is found to be on the order of the
trapping frequency and lower than one hundred times the temperature in
micro-Kelvin. Bose Einstein condensates with more than atoms should be
obtained in much less than one second starting from an usual MOT setup.Comment: 14 page
Spontaneous demagnetization of a dipolar spinor Bose gas at ultra-low magnetic field
Quantum degenerate Bose gases with an internal degree of freedom, known as
spinor condensates, are natural candidates to study the interplay between
magnetism and superfluidity. In the spinor condensates made of alkali atoms
studied so far, the spinor properties are set by contact interactions, while
magnetization is dynamically frozen, due to small magnetic dipole-dipole
interactions. Here, we study the spinor properties of S=3 Cr atoms, in
which relatively strong dipole-dipole interactions allow changes in
magnetization. We observe a phase transition between a ferromagnetic phase and
an unpolarized phase when the magnetic field is quenched to an extremely low
value, below which interactions overwhelm the linear Zeeman effect. The BEC
magnetization changes due to magnetic dipole-dipole interactions that set the
dynamics. Our work opens up the experimental study of quantum magnetism with
free magnetization using ultra-cold atoms.Comment: 6 pages, 4 figures, 2 appendice
Évaluer la difficulté d'une grille de sudoku à l'aide d'un modèle contraintes
Le sudoku est un jeu de logique qui est devenu en quelques mois un phénomène de société en France. Il envahit les métros, les trains, les bus, les salles de cours et même le journal Le Monde. Grâce à ce jeu, le grand public est devenu le M. Jourdain de la Programmation Par Contraintes. En effet, l'intérêt de ce jeu pour montrer très rapidement et très simplement les principes premiers de la programmation par contraintes n'est plus à démontrer. De plus, la technologie contraintes est très performante pour modéliser à l'aide de quelques contraintes globales ce problème et le résoudre quasiment simplement par propagation. Par contre, la mesure de la difficulté d'une grille – qui laisse à désirer pour de nombreuses instances publiées actuellement – n'a pas encore été capturée de manière satisfaisante par un modèle contraintes. Une raison est qu'une telle mesure est totalement subjective car elle dépend de la façon dont un joueur aborde son instance. Dans cet article, nous montrons qu'il est possible de définir des modèles contraintes permettant de retrouver des combinaisons de règles utilisées par les joueurs. Ces modèles ouvrent la porte à une évaluation de la difficulté d'une instance par une approche purement contraintes et même de fournir des systèmes d'aide eux-aussi basés sur un telle approche
Competition between Bose Einstein Condensation and spin dynamics
We study the impact of spin-exchange collisions on the dynamics of
Bose-Einstein condensation, by rapidly cooling a chromium multi-component Bose
gas. Despite relatively strong spin-dependent interactions, the critical
temperature for Bose-Einstein condensation is reached before the spin-degrees
of freedom fully thermalize. The increase in density due to Bose-Einstein
condensation then triggers spin dynamics, hampering the formation of
condensates in spin excited states. Small metastable spinor condensates are
nevertheless produced, and manifest strong spin fluctuations.Comment: 5 pages, 4 figure
A method to discriminate between localized and chaotic quantum systems
We derive a criterion that distinguishes whether a generic isolated quantum
system initially set out of equilibrium can be considered as localized close to
its initial state, or chaotic. Our approach considers the time evolution in the
Lanczos basis, which maps the system's dynamics onto that of a particle moving
in a one-dimensional lattice where both the energy in the lattice sites and the
tunneling from one lattice site to the next are inhomogeneous. We infer a
criterion that allows distinguishing localized from chaotic systems. This
criterion involves the coupling strengths between Lanczos states and their
expectation energy fluctuations. We verify its validity by inspecting three
cases, corresponding to Anderson localization as a function of dimension, the
out-of-equilibrium dynamics of a many-body dipolar spin system, and integrable
systems. We finally show that our approach provides a justification for the
Wigner surmise and the eigenstate thermalization hypothesis, which have both
been proposed to characterize quantum chaotic systems. Indeed, our criterion
for a system to be chaotic implies the level repulsion (also known as spectral
rigidity) of eigenenergies, which is characteristic of the Wigner-Dyson
distribution; and we also demonstrate that in the chaotic regime, the
expectation value of any local observable only weakly varies as a function of
eigenstates. Our demonstration allows to define the class of operators to which
the eigenstate thermalization applies, as the ones that connect states that are
coupled at weak order by the Hamiltonian.Comment: 15 pages, 6 figure
Cooling of a Bose-Einstein Condensate by spin distillation
We propose and experimentally demonstrate a new cooling mechanism leading to
purification of a spinor Bose-Einstein Condensate (BEC). Our scheme starts with
a BEC polarized in the lowest energy spin state. Spin excited states are
thermally populated by lowering the single particle energy gap set by the
magnetic field. Then these spin-excited thermal components are filtered out,
which leads to an increase of the BEC fraction. We experimentally demonstrate
such cooling for a spin 3 52Cr dipolar BEC. Our scheme should be applicable to
Na or Rb, with perspective to reach temperatures below 1 nK.Comment: 4 figure
Anisotropic excitation spectrum of a dipolar quantum Bose gas
We measure the excitation spectrum of a dipolar Chromium Bose Einstein
Condensate with Raman-Bragg spectroscopy. The energy spectrum depends on the
orientation of the dipoles with respect to the excitation momentum,
demonstrating an anisotropy which originates from the dipole-dipole
interactions between the atoms. We compare our results with the Bogoliubov
theory based on the local density approximation, and, at large excitation
wavelengths, with numerical simulations of the time dependent Gross-Pitaevskii
equation. Our results show an anisotropy of the speed of soundComment: 3 figure
Application of lasers to ultracold atoms and molecules
In this review, we discuss the impact of the development of lasers on
ultracold atoms and molecules and their applications. After a brief historical
review of laser cooling and Bose-Einstein condensation, we present important
applications of ultra cold atoms, including time and frequency metrology, atom
interferometry and inertial sensors, atom lasers, simulation of condensed
matter systems, production and study of strongly correlated systems, and
production of ultracold molecules.Comment: Review paper written in the name of IFRAF to celebrate 50 years of
lasers and their applications to cold atom physics; 15 pages, 2 figures; to
appear in Comptes Rendus de l'Academie des Sciences, Pari
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