CONCLUSION & PERSPECTIVES

In this part of my thesis, I described the experiments that took place between November 2006 and October 2008, and concerned the loading of a dipole trap of Cs atoms by various methods.

The main motivation for this line of experiments, was the expectation to provide with ultra-cold Cs atomic samples (and eventually condensates), via a method that would be simpler and faster with comparison to the only successful method reported at that time [Web03b]. The group's special interest on Cs ultra-cold samples and condensates originated from the group's research activity in cold molecules, and in particular in cold Cs dimers. The long term goal for the Cs atomic experiment, was to allow the study of formation and manipulation of cold molecules in these ultra-cold conditions. Such an experiment would permit the study of several methods, explored by members of the group in a different set up, like photoassociation for the production of cold molecules [Fio98], like trapping and accumulating cold molecules in conservative traps [Zahz06, Vanh02] and like cooling their internal degrees of freedom [Vit08c].

The main approach followed in the atomic Cs experiments, in which I participated in the period 2006-2008, was based in a proposal reported in [Comp06], and involved loading of the dipole trap from an atomic reservoir provided initially by a magnetic trap. The experimental realization of this proposal was already the subject of [Stern08]. I continued this line of studies, by trying various types of traps in the role of the atomic reservoir (C-MOT, Dark-SPOT), as well as an additional loading strategy which was similar to the ones followed by [Web03b, Hung08], and which was based in Raman-Sideband cooling.

The results of our studies are discussed in Chapter I4 and summarized in Table 4.1. The number of atoms, obtained in all the different loading approaches, is almost the same (in the order of 10⁵ atoms), while their temperature is always close to 150 µK. The results for the Raman-Sideband cooling were somehow improved, with the number of atoms being ~7 10⁵ and a much lower temperature of ~3 µK. However, this loading strategy, as well as the strategy involving instantaneous molasses loading, are very far from the analysis considered in Chapter I2 and in [Comp06]; so these results serve only to compare the reservoir loading to other loading methods which do not involve an atomic reservoir, in the same set up. The temperature of the trapped atoms seems to be mostly due to the heating during the transfer to the dipole trap, since whenever the trap is localized in ~100 µm the temperature is ~150 µK and it drops to ~3 µK for the case of the much shallower ~700 µm trap loaded with RSC. The application of evaporating cooling on these traps did not result to a significant phase density increase. The reason for this, is that the general characteristics of the dipole traps obtained in our studies (mostly the obtained density, but also the small lifetime of the traps), were such that they could not be considered as a good starting point for evaporation; this is why I chose to focalize on the loading techniques in the body-text of my thesis.

In brief, we considered evaporation in order to improve the characteristics of our dipole traps (decrease temperature/increase density), and to approach the required conditions of the very efficient evaporation process predicted in [Comp06]. Since the density of the atoms in the dipole trap were always smaller than the ones required for evaporation, we attempted to 'compress' our dipole trap with the use of the mechanical 'zoom' described in I.3.4. Despite the partial success of compression, it did not led to a successful evaporation, since it was not accompanied by evaporating cooling which could compensate the heating caused by compression. All our studies on dipole trap lifetime evaporation and compression are discussed in the Annexes I1 and I2. Additionally, in Annex I3, I present a set of experimental studies of the quality of the dipole laser potential and of interferences. Again, the concussion is that the low densities obtained in our set up could not be explained by none of these two experimental parameters.

The results of our experimental realization of the reservoir loading, show striking differences between the theoretical prediction [Comp06] and the experiment. These differences are not only in the final output of the method, i.e. the number of atoms transferred by the reservoir to the dipole trap and their temperature, but also in the dynamics of the loading process. In particular, the models developed in [Comp06], predict a loading process of several hundreds of ms, while in the experiment it was found to be ~10 times smaller, which had as a consequence the poor loading of the dipole trap. In a sense, we could say that our experimental realization of the technique failed in providing the conditions considered in [Comp06]. One possible explanation, the reservoir is considered to have an infinite lifetime with respect to the duration of the loading process, and none of its basic characteristics, as the density and the temperature, are considered to change during the loading process.

In a more general point of view, where in our studies we include the results of all other relevant experiments [Perr98,Boir98,Hung08,Web03b], it seems that the combination Raman-Sideband Cooling and shallow dipole traps, consists of an approach which is difficult to be 'beaten', especially after the improvements reported in [Hung08]. The Raman-Sideband Cooling manages to provide with a sufficiently high number of atoms (~10⁷). Similar number of atoms loaded from a magnetic trap reservoir, is only reported in [Stern08], in the case of loading a single-arm dipole trap. Loading a single-arm dipole trap from an atomic trap reservoir, could perhaps provide with an attractive alternative to the combination of Raman-Sideband Cooling and shallow dipole traps, as it has lead to notable results in the case of Cr [Pfau07, Beau08]. However, even if such an approach was widely discussed among the group, it was not studied tested during my involvement to the dipole trap experiments.

Recent developments in the area of cold molecules, which resulted the form the efforts of members of the Cold Molecule group, have shifted the interest of our team towards this area. The demonstration of a novel method that permitted the cooling of the vibration in Cs dimers [Vit08], had opened the way towards many fruitful applications. The initial, long term goal of my thesis, was the preparation of ultra-cold Cs dimers in an optical dipole trap via photoassociation, as demonstrated by its initial title : “Preparation of cold Cs molecules, in their ground state, without vibration or rotation, in a classical or quantum gas”. These developments in the area of cold molecules opened new perspectives, thus, we have decided to devote the rest of my time in the studies of cold molecules, which will be discussed in the second part of this thesis.