Can we control the thermodynamic equilibrium of a sample of matter using light with well-chosen characteristics? Since the seminal papers by Einstein and Kastler, this question has accompanied the development of atomic physics and quantum optics. From the 1970s onwards, the development of tunable laser sources shed new light on this question, with proposals for cooling gases of neutral atoms or ions. The answers that emerged went far beyond the most optimistic initial predictions. Laser cooling of atomic particles makes it possible to lower the temperature of a gas from room temperature to a range between millikelvin and microkelvin, or even below in certain special cases.
Cold atoms are ubiquitous in time and frequency metrology experiments, as well as in most high-precision measurements in atomic physics. Radiative cooling has also paved the way for the production of quantum gases such as Bose-Einstein condensates, in which large numbers of particles accumulate in a single microscopic state. In fact, it enables us to approach the limit where the thermal wavelength of gas atoms becomes comparable to the distance between particles. It should be noted, however, that radiative cooling does not generally enable the condensation threshold to be reached directly. It is followed by evaporative cooling, which lowers the temperature by one or two orders of magnitude.
Radiative cooling has been applied to numerous atomic species - over thirty to date. The only decisive factor is the availability of continuous laser sources of sufficient intensity to resonantly excite an atomic transition. The aim of this lecture, consisting of six lessons of 1h30 each, has been to present the evolution of the main ideas behind radiative cooling, and to discuss their performance and limitations. We have not attempted to describe all the proposed methods, but have concentrated on a few important principles:
- the Doppler effect, which provides an atomic response to the light wave that depends on atomic velocity;
- the Sisyphus mechanism, which forces the atom to climb more potential hills than it descends;
- the use of black states, which involves hiding atoms in darkness, i.e. accumulating them in states where they are effectively decoupled from light.