ATOMIC PHYSICS 10 H. Narumi, 1. Shimamura (editors) © Elsevier Science Publishers S.V., 1987 365 LASER-ATOM INTERACTIONS : RECENT THEORETICAL DEVELOPMENTS Jean DALIBARD and Claude COHEN-TANNOUDJI Laboratoire de Spectroscopi e Hertzienne de l'Ecole Normale Supérieure* et Collège de France, 24 rue Lhomond, F 75231 Paris Cedex 05 France ln intense laser beams, when perturbative treatments are no longer valid, the dressed atom approach provides a quantitative understanding of the main features of dipole or intensity gradient forces (mean value, fluctu- ations, velocity dependence). ln this lecture, we present such an approach and we apply it to atomic motion in an intense standing wave. New efficient laser cooling schemes taking advantage of stimulated processes are proposed. They work for a blue detuning and do not saturate at high intensity. 1. INTRODUCTION During the last few years, several experiments have demonstrated that laser- atom interactions provide the possibility to control the velocity1,2 and the position3 of an atom. A new exciting field of research is emerging which is called laser cooling and trapping4. ln order to introduce the subject of this talk, we consider first the simplest possible example of an atom irradiated by a laser plane wave with wave vector k. When the atom absorbs a laser photon, the momentum gain is fik. If the atom falls back to the ground state by stimulated emission, the atomic momentum returns to its initial value. But, if the emission process is a spontaneous one, the momentum loss during such a process is zero by symmetry, because spontaneous emission can occur with equal probabilities in opposite directions. It follows that the net atomic momentum gain in a fluorescence cycle (absorption + spontaneous emission) is fik. Consequently, the mean force <F> experienced by the atom is given by <F> = fik r Gee ( 1) where r Gee is the mean number of fluorescence cycles per unit time, equal to the product of the population Gee of the excited atomic state e by the sponta- neous emission rate r , which is also the natural width of e. Such a force is nothing but the well known radiation pressure5-7 which has been used for slowing down atomic beams1. At high intensities, Gee tends to 1/2 and <F> saturates to the value fik r/2. Suppose now that the atom is irradiated by two counterpropagating laser * Unité associée au CNRS (UA 18)