Atom
Direction of motion
Light
Light
1
2
3
4
Direction
of force
6-23
Laser Cooling of Atoms
   Laser cooling of atoms can create atoms that move very slowly (equivalent to temperatures of microkelvins).  This allows long observation times.  The slow speed virtually eliminates Doppler shifts, and the long observation times allow high accuracy determinations of atomic transition frequencies, per the Heisenberg uncertainty principle, i.e., Et ~ h and E = h, so   ~ 1/t. Laser cooling promises frequency accuracies of parts in 1016.  The explanation of laser cooling is as follows.  The numbers correspond to the numbers in the illustration above:
1.  Consider two rays of light that bombard an atom.  One ray travels in the same direction as the atom,; the other moves in the opposite direction.  The frequency of the light is slightly lower than the frequency that the atom readily absorbs.
2.  From the atom’s perspective, the ray moving in the same direction as the atom is shifted down in frequency; the other ray is shifted up in frequency.
3.  The atom is likely to absorb the high-frequency light but not the low.  It is therefore pushed in a direction opposite its motion and slows down.
4.  The emission of the absorbed light pushes the atom in some random direction, but if the process is repeated many times, the emission exerts no net force.


Chu, Steven, ”Laser Trapping of Neutral Particles," Scientific American, February 1992,
pp. 71-76.