LASER COOLING BY COHERENT SCATTERING. In early laser-cooling experiments inside atom traps, atoms were cooled when they encountered a laser beam coming at them with an energy that was slightly less than what is needed to be absorbed by an unmoving atom (promoting an electron from a lower to a higher energy level). But absorption can occur anyway if the atom's energy of motion equals the energy by which the laser beam is de-tuned. Thus the detuning compensates for the Doppler effect of the moving atom.

Now a pair of Stanford physicists, Vladan Vuletic (650-725-2356, vladan2@leland.stanford.edu) and Steven Chu (who won a Nobel prize for his work on laser cooling) are announcing a new cooling scheme, one in which the laser light is not absorbed but scattered. And scattered coherently in such a way that the atom loses a bit of energy in the encounter. The coherence helps to cool large samples because the scattered light circulates in an optical cavity and the scattering from one atom promotes scattering of light from other atoms.

The detuning relationship in the new scheme is not between the incident laser beam and the atom but between the scattered light and the cavity. The cavity permits some light modes to propagate but not others; the incident laser light can be slightly detuned below what the cavity will accept, thus encouraging scattering events in which the scattered light has just the resonant energy. Furthermore, because the incident laser beam is not related to any particular transition inside the target atom, the target can actually be any number of atoms in different states or even a molecule with a diversity of ground states (owing to internal degrees of motion arising from rotation or vibration of the molecule). Vuletic believes that this cooling method will be realized in the lab in the next 6 to 12 months. (Vuletic and Chu, Physical Review Letters, 24 April /pnu/2000/; Select Article.)