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.

从大约1985年开始，Steven Chu等人工作的成果推动了利用激光实现极低温的应用，使得温度低至10 -9 K的水平得以实现。实现纳米开尔文温度的首次重大进展是激光冷却。随着技术与技术的进步，这些进展被生动地称为“西西弗斯冷却”和“光学蜜糖”，从而实现了钠气的35 mK和铯气的3 mK的温度。激光冷却面临一个被称为多普勒极限的障碍，这一障碍部分通过使用磁场克服，这种技术被称为磁光学陷阱（MOT）。在这一时期，稀薄气体中玻色-爱因斯坦凝聚的探索正在形成，而这需要更低的温度。

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).

一旦实现了磁陷阱，人们意识到建立一个“深”陷阱，然后降低陷阱的壁面，将允许最活跃的原子逃逸，留下能量较低的原子集合，从而降低温度。这一过程被描述为蒸发冷却，它同时起到了降低温度和提高剩余被困原子密度的双重作用。这两种效应都有助于实现玻色-爱因斯坦凝聚（BEC）的目标。