Cooling by Magnetic Trapping

Starting in about 1985 with the work of Steven Chu and others, the use of lasers to achieve extremely low temperatures has advanced to the point that temperatures of 10^-9 K have been reached. The first big step toward the nanokelvin temperatures was laser cooling. With advances in technology and techniques colorfully dubbed "Sisyphus cooling" and "optical molasses", temperatures as low as 35 mK for sodium and 3 mK for cesium were obtained. The laser cooling faced a hurdle known as the Doppler limit, which was partially overcome using magnetic fields in what was called a magneto-optical trap (MOT). During this period the quest for Bose-Einstein condensation in dilute gases was forming, and it needed yet lower temperatures.

Once a magnetic trap had been achieved, it was realized that the establishment of a "deep" trap and then the lowering of the walls of the trap would allow the most energetic of the atoms to escape, leaving a collection with lower energies and a lower temperature. This was described as cooling by evaporation, and it had the dual effect of lowering the temperature and raising the density of the remaining trapped atoms. Both of these effects helped toward the goal of establishing the Bose-Einstein Condensate (BEC).