284 research outputs found
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
Claude Tardits (1921-2007)
Le professeur Claude Tardits vient de s’éteindre à l’âge de quatre-vingt-six ans, au terme d’une existence exceptionnelle due à sa forte personnalité, et abandonne à la mélancolie des souvenirs ses amis et élèves, parmi lesquels l’auteur de ces lignes s’honore de compter. Claude Tardits naît à Paris le 21 avril 1921. Ses parents tiennent un salon de coiffure renommé dans le milieu artiste et cultivé. Élevé en Normandie par ses grands-parents et en internat, le lycéen Claude est envoyé chaque ..
Intentions missionnaires et perception africaine : quelques données camerounaises
The intention of this article is to study variations in attitudes of mind. This can be especially observed in events, which may be qualified as odd, such as attempts at religious conversion, namely the coming about of a new way of thinking and the willingness to change forged in a society which up that point in time had lived in a different manner. Some events concerning the meeting of Catholic missionaries with the Beti of the southern Cameroons will be described. This unusual meeting led to an effective change in the way that both the Beti and the missionaries saw the world
Pairault, Claude & Benoist, Jean. – Portrait d’un jésuite en anthropologue. Entretiens
Ce « portrait » renvoie à tout un pan de l’histoire des sciences d’outre-mer en France (tout juste effleuré par la revue Gradhiva), celui de l’étude des chercheurs qui les ont constituées et dont les individualités remarquables et les intérêts particuliers n’ont pas été sans influencer la manière dont la recherche s’est faite : administrateurs, médecins, missionnaires, journalistes, riches amateurs et, plutôt sur le tard, universitaires, même s’il reste bien entendu que dans chaque catégorie ..
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
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
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
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