Laser Cooling and Trapping

Absolute zero, as cold as it gets, resides at the very bottom of the temperature scale- zero in Kelvin, -273^o in Celsius. The table shows some important temperatures in these two scales. As objects get colder, their atoms move more slowly. For a gas, that means the atoms' average speed decreases. Putting the brakes on gas molecules reduces the temperature of the gas.

Laser trapping and cooling does just that. Photons, the quanta of light, carry momentum. When an atom absorbs a photon, the photon's momentum gives the atom a kick in the direction the photon was traveling. The atom can only absorb a photon if the photon's energy matches the energy difference between two of the atom's energy levels. Careful tuning of the laser frequency insures that only atoms moving toward the light can absorb photons and be slowed.

The crossed laser beams shown in the photo create a space picturesquely called "optical molasses." Atoms moving in this region, the bright area in the center of the picture, are trapped and cooled by absorption of photons from the crossed beams. With this technique, researchers have already reached temperatures lower than a millionth of a degree Kelvin. That's an average atomic speed on the order of centimeters per second.