Laser cooling

Laser cooling includes several techniques where <a href="/facts/Atom/69syB8dH">atoms</a>, <a href="/facts/Molecule/N9ljSfFq">molecules</a>, and small mechanical systems are cooled with <a href="/facts/Laser/jk6PTvvT">laser</a> light. The directed energy of lasers is often associated with heating materials, e.g. <a href="/facts/Laser_cutting/3cLypxPr">laser cutting</a>, so it can be counterintuitive that laser cooling often results in sample <a href="/facts/Temperature/QyGe4gmj">temperatures</a> approaching <a href="/facts/Absolute_zero/U3UyPl2K">absolute zero</a>. It is a routinely used in atomic physics experiments where the laser-cooled atoms are manipulated and measured, or in technologies, such as atom-based quantum computing architectures. 
Laser cooling reduces the random motion of particles or the random vibrations of mechanical systems. For atoms and molecules this reduces Doppler shifts in spectroscopy, allowing for high precision measurements and instruments such as <a href="/facts/Optical_clock/MF8nsXDR">optical clocks</a>. The reduction in thermal energy also allows for efficient loading of atoms and molecules into traps where they can be used in experiments or atom-based devices for longer periods of time.
Laser cooling relies on the momentum change when an object, such as an atom, absorbs and re-emits a <a href="/facts/Photon/mFNhAEzA">photon</a> (a particle of light). Atoms will be cooled in one dimension if they are illuminated by a pair of counter-propagating laser beams that are detuned below an atomic transition. The laser light will be preferentially absorbed from the laser beam that counter-propagates with respect to the atom's motion due to the <a href="/facts/Doppler_effect/bzrK1BKf">Doppler effect</a>. The absorbed light is re-emitted by the atom in a random direction. After this process is repeated the random motion of the atoms will be reduced along the laser cooling axis. With three pairs of counter-propagating laser beams along all three axes a warm cloud of atoms will be cooled in three dimensions. The atom cloud will expand more slowly because of the decrease in the cloud's velocity distribution, which corresponds to a lower temperature and therefore colder atoms. For an ensemble of particles, their <a href="/facts/Thermodynamic_temperature/JXeJJPa6">thermodynamic temperature</a> is proportional to the <a href="/facts/Variance/ULBJKXD1">variance</a> in their velocity, therefore the lower the distribution of velocities, the lower the temperature of the particles.

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Laser cooling