Laser cooling is a method of using laser light to cool gases to the µK temperature range (around one millionth of a degree above absolute zero). When combined with a magnetic "trap," atoms can be captured and held in a cooled state for study.
The basis for laser cooling originates in that photons, while without mass in the conventional sense, can impart their momentum onto atoms they hit. Imagine slowing down a bowling ball rolling towards you by throwing baseballs at it. In the same way, atoms heading towards the laser are slowed as they are bombarded by photons.
"But wait!", you say. "Won't the lasers accelerate atoms that were already moving away from them?"
The solution to this problem lies in Doppler shift. Like galaxies that are redshifted because they are moving away from us, atoms that are moving away from the lasers are redshifted and atoms that are moving towards the lasers are blueshifted. As only specific frequencies of laser light will be absorbed by the atoms, the laser can be set to a frequency that will only be absorbed by blueshifted atoms, those moving towards the laser, and thus atoms in the target gas will only be slowed, not accelerated.
Early laser cooling setups used six lasers at 90 degree angles to each other (IE, two opposing lasers on the x, y, and z axis), and cooled sodium atoms. The problem encountered in this type of setup was that gravity would quickly pull the target atoms out of the laser beams. Later, the magneto-optical trap (MOT) was added, which consists of two magnetic coils that create a magnetic field which trap the atoms in the intersection of the lasers. This allowed for cooled atoms to be captured for later study or experimentation.
The 1997 Nobel Prize in Physics was awarded to Steven Chu of Stanford University, Claude Cohen-Tannoudji of Collège de France and École Normale Supérieure, and William D. Phillips of NIST (National Institute of Standards and Technology) for their invention and improvement of laser cooling techniques.
Source information used in node: Press Release: The Nobel Prize in Phyiscs 1997, http://www.nobel.se/physics/laureates/1997/press.html