Abstract
The second lecture was devoted to Doppler cooling with laser sources. Sixty years after Einstein's argument studied in the first lecture, two papers by Hänsch and Schawlow on the one hand, and Wineland and Dehmelt on the other, came simultaneously to propose exploiting light from tunable lasers to create new thermodynamic equilibria. In the language we developed in the previous lecture, a monochromatic laser can be used to create an arbitrarily narrow spectral distribution of light: the natural width of the excited level of the atom replaces the width of the blackbody distribution. In other words, there is no longer a temperature imposed "from outside" by the incident light, and it is the parameters of the atomic transition used that determine the equilibrium temperature. As in Einstein's paper, it is the Doppler effect that is at the root of the frictional force that cools the atoms. The approach we followed was therefore very similar to that developed for blackbody radiation. We used Brownian motion theory and determined both a friction coefficient and a diffusion coefficient, to arrive at the Doppler limit linking the equilibrium temperature and the natural width of the atomic transition. Once the principle of these optical molasses had been established, we transposed it from velocity space to position space, substituting the Zeeman effect for the Doppler effect. We have thus arrived at the principle of the magneto-optical trap, which we have described and illustrated using recent experiments with both atoms and molecules.
