61 research outputs found
Approximated center-of-mass motion for systems of interacting particles with space- and velocity-dependent friction and anharmonic potential
We study the center-of-mass motion in systems of trapped interacting
particles with space- and velocity-dependent friction and anharmonic traps. Our
approach, based on a dynamical ansatz assuming a fixed density profile, allows
us to obtain information at once for a wide range of binary interactions and
interaction strengths, at linear and nonlinear levels. Our findings are first
tested on different simple models by comparison with direct numerical
simulations. Then, we apply the method to characterize the motion of the center
of mass of a magneto-optical trap and its dependence on the number of trapped
atoms. Our predictions are compared with experiments performed on a large Rb85
magneto-optical trap.Comment: 9 pages, 8 figure
Light self-trapping in a large cloud of cold atoms
We show that, for a near-resonant propagating beam, a large cloud of cold
87Rb atoms acts as a saturable Kerr medium and produces self-trapping of light.
By side fluorescence imaging we monitor the transverse size of the beam and,
depending on the sign of the laser detuning with respect to the atomic
transition, we observe self-focusing or -defocusing, with the waist remaining
stationary for an appropriate choice of parameters. We analyze our observations
by using numerical simulations based on a simple 2-level atom model.Comment: 3 pages, 4 figures, submitted to Optics Letter
Multiple scattering of light in cold atomic clouds with a magnetic field
Starting from a microscopic theory for atomic scatterers, we describe the
scattering of light by a single atom and study the coherent propagation of
light in a cold atomic cloud in the presence of a magnetic field B in the
mesoscopic regime. Non-pertubative expressions in B are given for the
magneto-optical effects and optical anisotropy. We then consider the multiple
scattering regime and address the fate of the coherent backscattering (CBS)
effect. We show that, for atoms with nonzero spin in their ground state, the
CBS interference contrast can be increased compared to its value when B=0, a
result at variance with classical samples. We validate our theoretical results
by a quantitative comparison with experimental data.Comment: 16 pages, 7 figure
Coherent backscattering of light by cold atoms: theory meets experiment
Coherent backscattering (CBS) of quasi-resonant light by cold atoms presents
some specific features due to the internal structure of the atomic scatterers.
We present the first quantitative comparison between the experimentally
observed CBS cones and Monte-Carlo calculations which take into account the
shape of the atomic cloud as well as the internal atomic structure.Comment: 5 pages, 3 figures, to appear in Eur. Phys. Let
Magnetic Field Enhanced Coherence Length in Cold Atomic Gases
We study the effect of an external magnetic field on coherent backscattering
of light from a cool rubidium vapor. We observe that the backscattering
enhancement factor can be {\it increased} with . This surprising behavior
shows that the coherence length of the system can be increased by applying a
magnetic field, in sharp contrast with ususal situations. This is mainly due to
the lifting of the degeneracy between Zeeman sublevels. We find good agreement
between our experimental data and a full Monte-Carlosimulation, taking into
account the magneto-optical effects and the geometry of the atomic cloud
Fast optimal transition between two equilibrium states
We demonstrate a technique based on invariants of motion for a time-dependent
Hamiltonian, allowing a fast transition to a final state identical in theory to
that obtained through a perfectly adiabatic transformation. This method is
experimentally applied to the fast decompression of an ultracold cloud of
Rubidium 87 atoms held in a harmonic magnetic trap, in the presence of gravity.
We are able to decompress the trap by a factor of 15 within 35 ms with a strong
suppression of the sloshing and breathing modes induced by the large vertical
displacement and curvature reduction of the trap. When compared to a standard
linear decompression, we achieve a gain of a factor of 37 on the transition
time.Comment: 5 pages, 4 figures, an error in Eq. (2) has been correcte
Synchronization of a Self-Sustained Cold Atom Oscillator
Nonlinear oscillations and synchronisation phenomena are ubiquitous in
nature. We study the synchronization of self oscillating magneto-optically
trapped cold atoms to a weak external driving. The oscillations arise from a
dynamical instability due the competition between the screened magneto-optical
trapping force and the inter-atomic repulsion due to multiple scattering of
light. A weak modulation of the trapping force allows the oscillations of the
cloud to synchronize to the driving. The synchronization frequency range
increases with the forcing amplitude. The corresponding Arnold tongue is
experimentally measured and compared to theoretical predictions. Phase-locking
between the oscillator and drive is also observed.Comment: Corrected typo
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