Editing Laser cooling (section)

=== Doppler cooling ===

{{main|Doppler cooling}}
Doppler cooling is by far the most common method of laser cooling. It is used to cool low density gases down to the [[Doppler cooling limit]], which for [[rubidium]] (a popular choice in the field of atomic physics) is around 150&nbsp;[[wiktionary:microkelvin|microkelvin]].  It is often often combined with a magnetic field gradient to realize a [[magneto-optical trap]].

In Doppler cooling, initially, the frequency of light is tuned slightly below an [[electronic transition]] in the [[atom]]. Because the light is [[Laser detuning|detuned]] to the "red" (i.e.,&nbsp;at lower frequency) of the transition, the atoms will absorb more [[photon]]s if they move towards the light source, due to the [[Doppler effect]]. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion. In each scattering event the atom loses a [[momentum]] equal to the momentum of the photon. If the atom, which is now in the excited state, then emits a photon spontaneously, it will be kicked by the same amount of momentum, but in a random direction. Since the initial momentum change is a pure loss (opposing the direction of motion), while the subsequent change is random, the probable result of the absorption and emission process is to reduce the momentum of the atom, and therefore its [[speed]]&mdash;provided its initial speed was larger than the recoil speed from scattering a single photon. If the absorption and emission are repeated many times, the average speed, and therefore the [[kinetic energy]] of the atom, will be reduced. Since the [[temperature]] of a group of atoms is a measure of the average random internal kinetic energy, this is equivalent to cooling the atoms.

When atoms are Doppler cooled in three dimensions, traditionally by 6 counter-propagating red-detuned laser beams, this is called [[optical molasses]] because the atoms move slowly, as if they are moving through molasses.