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Only one year ago, cesium took first place in the International Chillout (IC) competition when researchers at the National Institute of Technology (NIST) in Gaithersburg MD chilled cesium atoms to 700 nanokelvins (less than one millionth of a degree above absolute zero). Now a group of researchers at the Joint Institute for Laboratory Astrophysics (JILA) at NIST and at the University of Boulder CO have gone even colder using the upstart rubidium.
A team headed by Eric A. Cornell used a novel technique to cool neutral rubidium atoms confined in a new magnetic trap to 200 nanokelvins - one five millionth of a degree above absolute zero. At this temperature, the average velocity of individual rubidium atoms is about 4 millimeters a second. That is about as fast as a speedy snail.
The scientists used a specially designed, non-uniform magnetic field to trap a large number of rubidium atoms in a small space. By gradually lowering the intensity of the field, the researchers let the faster, or hotter atoms escape, leaving behind the cooler, slower moving ones. Eventually they ended up with about 50,000 ultra-cold atoms packed into a volume 23 microns (a few hundredths of a millimeter) in diameter (don't try to chill your beer with this). The process is essentially one of evaporative cooling, the same physical principle that ordinary refrigerators use, only about a billion times better.
The temperature was measured by using a laser to make the atoms fluoresce, and observing the motion of the cloud through a microscope. Since there is no theoretical lower limit to the temperatures achievable using evaporative processes, scientists hope to get even colder in the near future. Get back in the ring cesium, and FIGHT FIGHT FIGHT!
Reference, Science News 6/3/95, V147.