Laser cooling refers to a number of techniques in which atomic and molecular samples are cooled down to near
absolute zero through the interaction with one or more
laser fields. All laser cooling techniques rely on the fact that when an object (usually an atom) absorbs and re-emits a
photon (a particle of light) its
momentum changes. The
temperature of an ensemble of particles is larger for larger
variance in the velocity distribution of the particles. Laser cooling techniques combine
atomic spectroscopy with the aforementioned mechanical effect of light to compress the velocity distribution of an ensemble of particles, thereby cooling the particles. [File:Doppler laser cooling.svg|thumb|upright|200px|Simplified principle of Doppler laser cooling: The first example of laser cooling, and also still the most common method (so much so that it is still often referred to simply as 'laser cooling') is
Doppler cooling. Other methods of laser cooling include: