Laser cooling basics
When light is scattered on an object (which might be a mirror or a single atom), it exerts a force on this. This radiation pressure was already regarded by Kepler as an explanation for the fact that comet tails always point away from the sun. When narrow-band laser light is used, atoms which fly towards the laser light can be slowed down by selecting the frequency of the laser somewhat below a resonant frequency of the atom. Owing to the Doppler effect, this light is brought to resonance for the moving atom and can be absorbed. The photons which are subsequently radiated by the atom have a somewhat higher frequency (and thus a higher energy) than the absorbed laser photons. The lacking amount of energy is taken from the kinetic energy of the atoms - they are cooled down. Since the atoms return from the optically excited state to the ground state after some 10 ns, the absorption-emission process repeats very rapidly. With an arrangement of six laser beams the motion of the atoms can be slowed down in all directions so that a cloud of cold atoms in a so-called "optical molasses" as outlined in the figure is obtained.
Using this arrangement, the temperature of the atoms drops to the range of a few microkelvin. At 1 mK, the mean velocity of a caesium atom is only in the range of 10 mm/s, whereas at room temperature (300 K) it is still 150 m/s (about 500 km/h). When flying through the resonator of an atomic clock, a laser-cooled atom thus is available for a much longer time and allows its resonant frequencies to be more measured precisely.
If an atom or ion is stored in a trap, it can only perform a vibrational motion in a limited space. The amount of photons scattered during cooling is sufficient to observe individual atoms. By laser cooling, the amplitude and the energy of the vibrational motion can be reduced to the minimum given by the laws of quantum mechanics.