What is "laser cooling"

n. (context physics English) the process of using the force exerted by a laser to reduce the temperature of a gas, or the spread of velocities of a beam of atoms

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. [[ laser cooling.svg|thumb|upright|200px|Simplified principle of Doppler laser cooling:

1

A stationary atom sees the laser neither red- nor blue-shifted and does not absorb the photon.

2

An atom moving away from the laser sees it red-shifted and does not absorb the photon.

3.1

An atom moving towards the laser sees it blue-shifted and absorbs the photon, slowing the atom.

3.2

The photon excites the atom, moving an electron to a higher quantum state.

3.3

The atom re-emits a photon. As its direction is random, there is no net change in momentum over many absorption-emission cycles.

]] 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:

• Sisyphus cooling
• Resolved sideband cooling
• Raman Sideband Cooling
• Velocity selective coherent population trapping (VSCPT)
• Cavity mediated cooling
• Use of a Zeeman slower
• Electromagnetically induced transparency (EIT) cooling