text: Laser cooling includes several techniques where atoms, molecules, and small mechanical systems are cooled with laser light.  The directed energy of lasers is often associated with heating materials, e.g. laser cutting, so it can be counterintuitive that laser cooling often results in sample temperatures approaching  absolute zero. Laser cooling relies on the change in momentum when an object, such as an atom, absorbs and re-emits a photon (a particle of light).  For example, if laser light illuminates a warm cloud of atoms from all directions and the lasers frequency is tuned below an atomic resonance, the atoms will be cooled.  This common type of laser cooling relies on the Doppler effect where individual atoms will preferentially absorb laser light from the direction opposite to the atoms motion.  The absorbed light is re-emitted by the atom in a random direction. After repeated emission and absorption of light the net effect on the cloud of atoms is that they will expand more slowly.  The slower expansion reflects a decrease in the velocity distribution of the atoms, which corresponds to a lower temperature and therefore the atoms have been cooled.  For an ensemble of particles, their thermodynamic temperature is proportional to the variance in their velocity. More homogeneous velocities between particles corresponds to a lower temperature. 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.
  The 1997 Nobel Prize in Physics was awarded to Claude Cohen-Tannoudji, Steven Chu, and William Daniel Phillips "for development of methods to cool and trap atoms with laser light".