17 Octobre – Thesis defense - Martin Rabault

14 h Amphi - Institut d'Optique d'Aquitaine (Talence)

All-optical Bose-Einstein condensation in microgravity for atom interferometry.

The ICE experiment aims at testing the weak equivalence principle underlying Einstein's theory of general relativity and postulate the equivalence between the inertial mass and the gravitationnal mass. If this principle has always been verified until today, it is of fundamental interest for physics to continue the measurements with greater precision. Indeed, new unifying theories of quantum mechanics and general relativity predict a violation of this principle. To carry out a test of the weak equivalence principle, it suffices to compare the accelerations of two objects in free fall in the same gravitationnal field. This is what the ICE experiment, on the quantum scale, achieves (unlike the Microscope mission, which to date has been able to verify the principle of equivalence with macroscopic objects at 2.10-14). Thus the experiment consists in performing, by an interferometric method, the measurement of the acceleration of two atomic species of different mass and composition (namely 39K and 87Rb) in free fall in a vacuum chamber. The measurement sensitivity of the inertial effects to which the atoms are sensitive (accelerations and rotations) is all the greater as the free fall time of the atoms is high and their temperature is low. But on Earth in the laboratory, the atoms eventually fall to the bottom of the vacuum chamber containing them under the effect of gravity, which greatly limits the measurement sensitivity achievable. This is why it is interesting to place the experiment in a microgravity environment in which the atoms stay in the center of the vacuum chamber in order to reach much longer interrogation times. As such, several times a year, the experiment is put aboard the aircraft ZERO-G of the Novespace company. The available microgravity durations make it possible to reach theoretical interrogation times of the order of one second, which should raise the sensitivity level to 10-11. However, we are today very strongly limited by the high level of vibrations of the aircraft as well as its rotations : the loss of contrast of the interference fringes caused, does not allow us to exceed 5ms of interrogation times in zero-g. Since the coherence of an atomic source is directly related to its temperature, the use of ultra-cold clouds is of great interest to improve the contrast of the interference fringes, especially for the long interrogation times targeted. In parallel, the laboratory is now equipped with a microgravity simulator on which is mounted the experiment, giving access to interrogation times of more than 250ms with parabolic trajectories of a very good repeatability (of the order of 3 mg). This manuscript synthesizes the work that produced the first 87Rb Bose-Einstein condensate in microgravity by all-optical methods, with a repetition rate of 12 seconds. We demonstrate the efficiency of our dipole trap loading method based on the association of a gray molasses cooling and a spatial modulation of the dipole beams. These results pave the way for future highly sensitive interferometric measurements with a large scale factor.

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