If you can make the photons of a laser light that hits atoms have more energy than those that leave the atoms, then the atoms get colder. The trick is to tune the energy of a laser photon to a value that is slightly below that of the energy of an electronic transition in the atom. Due to the Doppler red-shift of the photons, those atoms moving toward the beam absorb more photons than those moving in the opposite direction. Emitted photons leave the atom in random directions, and the result is a general loss of momentum and kinetic energy. The atoms get colder because temperature is proportional to the kinetic energy.
The atoms need to be at very low concentrations. The idea, was first suggested in 1975 by Theodore Hansch and Arthur Schawlow at Stanford University in California. Ten years later, Steven Chu of AT&T Bell Labs put it into practice. Sodium was cooled down to about 250 Kelvin using six lasers that provided three pairs of beams along each coordinate axis. By using optical pumping, the sodium was cooled even further to a temperature of 35 K and caesium to 3 K.
Physicist Dr. David Wineland pioneered the application of laser cooling to the next generation of even more accurate atomic clocks. Extremely cold atoms form themselves into a Bose-Einstein condensate, this being a pure quantum form of matter. In this state, atoms can be studied with greater accuracy. Interestingly, the main perturbing effect on laser cooled material is Earth's large gravitational field. The next generation of experiments will have to take place in the microgravity environment of nearby space.
