Manipulating atoms with photons*

Electromagnetic interactions play a central role in low energy physics. They are responsible for the cohesion of atoms and molecules and they are at the origin of the emission and absorption of light by such systems. This light is not only a source of information on the structure of atoms. It can also be used to act on atoms, to manipulate them, to control their various degrees of freedom. With the development of laser sources, this research field has considerably expanded during the last few years. Methods have been developed to trap atoms and to cool them to very low temperatures. This has opened the way to a wealth of new investigations and applications. Two types of degrees of freedom can be considered for an atom: the internal degrees of freedom, such as the electronic configuration or the spin polarization, in the center of mass system; the external degrees of freedom, which are essentially the position and the momentum of the center of mass. The manipulation of internal degrees of freedom goes back to optical pumping (Kastler, 1950), which uses resonant exchanges of angular momentum between atoms and circularly polarized light for polarizing the spins of these atoms. These experiments predate the use of lasers in atomic physics. The manipulation of external degrees of freedom uses the concept of radiative forces resulting from the exchanges of linear momentum between atoms and light. Radiative forces exerted by the light coming from the sun were already invoked by J. Kepler to explain the tails of the comets. Although they are very small when one uses ordinary light sources, these forces were also investigated experimentally in the beginning of this century by P. Lebedev, E. F. Nichols and G. F. Hull, R. Frisch. For a historical survey of this research field, we refer the reader to review papers (Ashkin, 1980; Letokhov and Minogin, 1981; Stenholm, 1986; Adams and Riis, 1997) which also include a discussion of early theoretical work dealing with these problems by the groups of A. P. Kazantsev, V. S. Letokhov, in Russia, A. Ashkin at Bell Labs, and S. Stenholm in Helsinki. It turns out that there is a strong interplay between the dynamics of internal and external degrees of freedom. This is at the origin of efficient laser cooling mechanisms, such as ‘‘Sisyphus cooling’’ or ‘‘Velocity