Doppler cooling

Doppler cooling is a mechanism that can be used to trap and cool atoms and is sometimes used synonomously with laser cooling. Doppler cooling is based on the doppler effect in that if something is moving towards a wave, in this case an electromagnetic wave, the frequency seen by the moving object will be higher than that of an object not moving relative to the source of the wave. This principle is used to trap atoms using a laser red detuned from a specific frequency corresponding to an electronic transition of the atom being trapped.

If an atom is traveling toward a laser, the frequency seen by the atom will be shifted up (to the blue), which corresponds to an electronic transition in the atom. The atom will then absorb a photon from the direction of the laser and become excited, it will slow due to a momentum tranfer from the photon when this absorption occurs. When the atom falls back to its orignal state a photon is emitted, this photon is emitted in a random direction. Because the photons are emitted in a random direction, the net momentum tranfer from the emission of photons over a long period of time will average to zero. Therefore, the net momentum change of the atom will be opposite of the direction of the laser beam. Using this mechanis atoms in a gas can be slowed to a speed corresponding to a temperature known as the doppler limit.