b. The optical dipole and hybrid approaches

Two body inelastic collisions can be avoided if one considers trapping of the F = 3, MF = 3 state of Cs atoms. This is for Cs the absolute ground state, and thus, it cannot collide inelastically, since none of the colliding particles can loose energy in the collision. On the other hand, this state is not longer magnetically trappable since it is a high field seeker and static magnetic field maxima are forbidden [Wing84]. In fact, trapping of high field seekers with means of dynamic magnetic fields is possible and this has been demonstrated for the case of Cs atoms [Corn91]. But these traps, which create a local maximum by magnetically pushing the atoms with fast varying magnetic fields, did not achieve high enough densities for an evaporation process.

Trapping of Cs atoms in their absolute ground state is possible with the use of dipole trap, which as we said, are not state selective and can be used to trap various types of particles, from atoms to molecules [Zahz06]. Trapping the absolute ground state F = 3, MF = 3 means that the external magnetic field can be used to tune the inter-atomic interactions, to avoid for example the hydrodynamic regime, since magnetic field depended collisions are now absent. However, an inherit disadvantage of dipole traps, comes from the fact that these traps are not state-selective and thus RF knifes cannot be used for evaporation. Evaporation can be achieved simply by reducing the power of the trapping laser, but this leads to reduction of the trapping potential’s confinement and thus to a loss in the trap density. Since confinement is a very important parameter in optical dipole traps, these are usually realized in a crossed configuration, and trapping is realized in the intersection of the two beams. Depending on the choice for the wavelength of the dipole trap laser, it is possible that the potential out of the crossed region, the so called ‘arms’ of the trap, is deep enough to trap atoms. It is also possible, that atoms that are removed from the trap during the evaporation process, are guided back to it from the weaker, but non zero potential of the trap’s arms. An optical dipole trap that was realized in ENS nicely confronted this problem by placing the trap’s arms in the vertical plane, thus all atoms that escape the trapping potential are quickly carried away by gravity[Perr98, Boir96]. However, at the times were this experiment was active, the picture

of Cs collision properties in the ultra-cold regime was still haze, and this experiment did not result to a Cs BEC.

A variation of optical dipole traps are the hybrid traps, where magnetic fields are combined to strong, off resonant laser fields. In these traps, magnetic fields are used to compensate gravity, that is to levitate the atoms, while dipole forces confine the atoms in the x-y plane [Bouss04, Hung08]. Dipole forces can compensate gravity, but magnetic levitation provides with some additional advantages. Since the magnetic dipole moment of each spin projection state is different, it can be used to selectively levitate one particular magnetic sub-level and thus ensure the 100%

polarization in the sample. Additionally, one can remove atoms from such a trap by performing RF transitions to other magnetic sub-levels and achieve evaporation without lowering the dipole potential and reducing the trap's confinement. The Innsbruck group took advantage of both these properties, while the collision properties of Cs and especially the notorious three body recombination rate, where studied in detail and controlled to all possible extents with the use of magnetic fields. In their experiment, a sample of Cs atoms was cooled down with the use of Raman sideband cooling technique and used to load ~2 10⁷ atoms in a temperature of ~1 µK inside a magnetically levitated optical trap. The subsequent evaporation enabled them to create samples of 25000 atoms in a temperature of 60 nK, which was still not enough for BEC. Condensation was achieved only after placing an additional, strongly focalized laser field to create a 'dimple' in their trap, in which the cold atoms where loaded from the magnetically levitated dipole trap during the evaporation process, and which could be further cooled down with plain evaporation. With this process, they were able to create a sample of ~6*10⁵ atoms in a temperature of 30 nK, and prepare the first Cs BEC in a process of ~20 seconds [Web03b].

Fig. I.1.12: An optical dipole trap realized in [Perr98].

The vertical configuration of the dipole trap ensures that the atoms that are removed during the evaporation process are carried away by gravity.

A similar, but improved experiment was performed in 2008 by a different group in Chicago [Hung08]. There, Raman sideband cooling was also employed to provide with a very cold, levitated dipole trap, but did not implement the same evaporation neither used the dimple trick. There, the magnetic field gradient that was used for levitation was increased gradually, in order to expel atoms from the trap region and thus perform evaporation. As shown in the left part of Fig. I.1.14, the combined dipole and magnetic levitation potential appears turned in the vertical direction with respect to the dipole trap potential. This method introduces only weak reductions in confinement strength over a large range of potentials and thus speeds up the evaporation process. An additional advantage of this method is that the atoms are expelled towards all directions (3D evaporation) once they are pulled out of the confinement region of the dipole trap. This runaway evaporation process is much more efficient and allowed for the condensation of Cs atoms in ~2 seconds.

Fig. I.1.13: The Innsbruck experiment dipole trap configuration and the dimple. (A) Initially only the levitated dipole trap is present. The dimple is ramped up during the evaporation process (B) while the levitated trap is gradually removed (C). Figure adapted by [Web03b].

Fig. I.1.14: Configuration of the Cs BEC experiment described in [Hung08]. Right: The dipole trap geometry and the direction of gravity and levitating field in the experiment. Left: Potentials felt by the atoms in the vertical direction; the initial dipole potential (dotted line) and the combined potential of the dipole force and the levitating (red line). Figure adapted by [Hung08].