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Habilitation à diriger des recherches

REFROIDISSEMENT PAR EVAPORATION D'UN JET D'ATOMES FROIDS GUIDE MAGNETIQUEMENT. DYNAMIQUE DES GAZ D'ATOMES FROIDS PIEGES.

Abstract : In this manuscript, I present the scientific work I did after my
Ph.D. During this period, my research activity was focussed on two
main topics: (i) an experiment on ultracold atoms aimed at
realizing a continuous source of coherent matter waves, and (ii) a
theoretical investigation of the dynamics of trapped gases.

In our experimental project, a non-degenerate, but already slow
and cold beam of atoms, is injected into a magnetic guide where
transverse evaporation is implemented. In other words, we intend
to replace the time-dependent aspects of the cooling process
involved in a regular Bose Einstein condensation machine, by a
setup where the atoms are progressively cooled as they move along
spatially separated cooling regions. This should allow us to reach
continuous fluxes of condensed atoms. This source, whose
properties will be very different from those of a thermal atomic
beams, will provide a very useful tool for many experiments. Among
those are atomic Rb fountain clocks, for which a continuous
operation would allow for a significant increase of the signal to
noise ratio. It is ideally suited for precision measurement. Also
matter wave interferometers, atom holography and nanolithography
experiments would benefit of such a well-collimated atomic beam.

In the first chapter, I describe the different stages of the
experimental setup and the first results that we obtained. Two
atom sources have been studied to load efficiently an anistropic
magneto-optical trap which serves as an injector of ultracold
atoms into a magnetic guide: (i) a two dimensional magneto-optical
trap with high cooling laser power, and (ii) an optimized Zeeman
slower placed at the exit of a recirculating effusive oven. With
the latter, a loading rate of $4\times 10^{10}$ atoms/s have been
measured for the injector. The confinement of atoms in the
injector is provided by transverse gradient of magnetic fields,
and the launching of atoms relies on the moving molasses
technique. The magnetic guide, whose entrance is placed at few
centimeters from the injector magneto-optical trap center,
provides strong magnetic gradient without affecting the
performance of the magneto-optical trap. We have demonstrated
experimentally, and for the first time, the continuous loading of
a slow and cold atomic beam into a magnetic guide. To optimize the
transfer, we have studied different coupling schemes in the
continuous and pulsed modes. The characteristics of the beam
magnetically guided are the following: a flux of $7\times 10^9$
atoms/s, a temperature of 400 $\mu$K for a strength of the
confinement of 600 Gauss/cm, and a mean velocity of 1 m/s.

The second chapter is devoted to the physics of collisions on our
magnetically guided atomic beam. We first present a new
spectroscopic method to measure the temperature of the beam. By
means of two radio-frequency antennas placed at different location
on the magnetic guide, we were able to set the beam in an
out-of-equilibrium state, and to follow the restoring of
equilibrium with the second antenna. This experiment shows the
occurrence of thermalization in the d-wave dominated regime. Atoms
have also been subsequently slowed down to 60 cm/s using an upward
slope. The relative high collision rate in the beam allows us to
start forced evaporative cooling of the beam, leading to the
reduction of the beam temperature by a factor of 4, and a ten-fold
increase on the on-axis phase-space density.

The last chapter addresses some theoretical questions related to
the dynamics of trapped gases. We have developed two new tools
well-adapted to analyse the dynamics of gases confined by a
harmonic potential. The first one is the method of averages for a
classical gas. It has permitted to study the collective
oscillations, the spinning up by a rotating anisotropy, and the
thermalization of mixtures of ultracold gases. This tool has also
been extended to the case of a Bose Einstein condensate in the
Thomas-Fermi regime. It has been used to propose the investigation
of the scissors mode. Such a pendular oscillation reveals the
effect of superfluidity exhibited by a Bose Einstein condensate.
The second tool that we have developed is based on approximated
solutions of the Boltzmann equation with a scaling ansatz. Within
this framework, we have shown a link between the relaxation times
relevant for the damping of the collective oscillations and for
the expansion of a gas in a time-of-flight experiment. Both
methods can also describe the crossover between the collisionless
and hyrodynamic regime of a trapped gas in the classical regime.
Our theoretical predictions have been confronted to experiments.
The tools we have developed have also been extensively used and
adapted to address related questions of our field.
Document type :
Habilitation à diriger des recherches
Complete list of metadatas

https://tel.archives-ouvertes.fr/tel-00112221
Contributor : David Guery-Odelin <>
Submitted on : Tuesday, November 7, 2006 - 7:10:32 PM
Last modification on : Tuesday, September 22, 2020 - 3:59:15 AM
Long-term archiving on: : Saturday, May 14, 2011 - 12:28:12 AM

Identifiers

  • HAL Id : tel-00112221, version 1

Citation

David Guéry-Odelin. REFROIDISSEMENT PAR EVAPORATION D'UN JET D'ATOMES FROIDS GUIDE MAGNETIQUEMENT. DYNAMIQUE DES GAZ D'ATOMES FROIDS PIEGES.. Physique Atomique [physics.atom-ph]. Université Pierre et Marie Curie - Paris VI, 2005. ⟨tel-00112221⟩

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