Optical Molasses

With a pair of lasers, tuned slightly below the Cs resonance ("to the red"), one can cool atoms to very low temperatures by taking advantage of the Doppler effect. An atom moving toward one laser will see that laser's frequency shifted up, or closer to resonance, while the other laser's frequency will be shifted down. This causes a higher rate of scattering from the laser opposing the atom's motion and the light pressure slows the atom down to zero average velocity. The atoms can be cooled to about one microKelvin, or one one-millionth of a degree above absolute zero.

This can be done in one, two, or three dimension. Because of the large accelerations possible, the force on atoms is very viscous and this technique is referred to as optical molasses.

 

A University of Colorado Interactive Java Applet that demonstrates laser cooling.