Manipulating atoms with light : cold atoms

The cooling and trapping of atoms by laser light has undergone considerable development over the last thirty years. Initially designed to increase the precision of spectroscopic measurements by reducing - or even eliminating - the Doppler effect, methods of manipulating atoms with light have led to many other applications, including in particular the preparation and study of new quantum phases of ultra-cold gases (Bose Einstein condensates and degenerate Fermion gases). The second lecture of the lecture recalls the principle of laser cooling and trapping of atoms and some of its applications, excluding studies of ultra-cold quantum gases, which are described in the following lecture. In the first part, the nature of radiative forces is reviewed, distinguishing between the dissipative contribution (linked to radiation pressure) and the reactive contribution (linked to light shifts in atomic energies). The mechanisms that use these forces to slow down atoms are then described. Cooling at the so-called Doppler limit (enabling temperatures of a few hundred microdegrees Kelvin to be reached) and the principle of optical molasses are analyzed, as is sub-Doppler cooling by the Sisyphus effect, which correlates the effects of the reactive force and optical pumping on atoms, and enables atomic temperatures to be reduced to the microdegree Kelvin level. The lesson continues with a description of traps for neutral atoms, whether light traps exploiting radiation pressure (magneto-optical MOT traps) or dispersive dipole force (single wells or periodic arrays of wells, optical tweezers for moving atoms, traps in the evanescent wave in the vicinity of transparent microstructures or nanostructures). Magnetic traps for cold paramagnetic atoms in the absence of light are also described. Atom optics experiments are then described, in which cold atoms are reflected or diffracted by light waves: atom mirrors made by evanescent waves on which the atoms bounce, light semi-reflecting blades formed by counter-propagating laser beams... The last part of the lesson shows how these elements of atom optics are combined to make atom interferometers enabling the construction of extremely sensitive gravimeters and gyrometers.