A technique for producing extremely low temperatures using lasers to slow down and trap atoms. The basic method is to direct a set of crossed laser beams at a sample of gas, with the wavelength set so that photons are absorbed by the atoms. One atom moving towards the photon beam will lose momentum on absorbing a photon and be cooled. An atom moving away from the incident photons will gain energy on absorption. Atoms moving towards the incident photons ‘see’ the incident photons as having a slightly different frequency than those moving away because of the Doppler effect, and it is possible to adjust the incident laser frequency by a small amount so that atoms are more likely to absorb when they are moving towards the oncoming photons. This results in a net cooling effect, a technique known as Doppler cooling. It produces a region of slow-moving atoms at the intersection of the laser beams – a state of matter sometimes called optical molasses. Further cooling, to temperatures below the theoretical limit for Doppler cooling, can be obtained by a mechanism known as Sisyphus cooling. Here the atom moves through a standing wave created by the laser. As it moves to the top of each ‘hill’ it loses energy and at the top it is optically pumped to a state at the bottom of the ‘valley’. Consequently, the effect is of an atom always moving up a potential gradient and losing energy. The name comes from the character Sisyphus in Greek mythology, who was condemned by the Gods continuously to push a boulder to the top of a hill, only for it to roll back down again when he reached the summit.
Work on laser cooling has also involved methods of trapping atoms. The magneto-optical trap ( MOT) uses six crossed laser beams together with an applied magnetic field to keep cooled atoms together. This allows a further method of cooling in which the height of the trap is lowered so as to let the more energetic atoms escape (a method known as evaporative cooling). Techniques of this type have led to temperatures less than 10−6 K and to the discovery of the Bose–Einstein condensate (see Bose–Einstein condensation).