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Absolute Zero

01.10.1998

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Dr. Nir Davidson. Going for zero

Think of a Zero. Chances are your mind didn't land on laser beams, drift to Doppler effects, or free associate atoms of gas flying through the air at the greatest of ease.

That's what Dr. Nir Davidson and his colleagues from the Institute's Physics of Complex Systems Department are doing ­ -- giving lots of thought to zero. For them, absolute zero is absolutely worthy of consideration.

This is a story about turning science fiction into science function. The application of Davidson's work, cooling and trapping atoms, could be employed, for example, in building highly-accurate atomic clocks. Waves of cooled atoms could be focused, using "lenses" of laser light to create a kind of pen or etching tool, to "draw" electronic circuits much smaller than those existing today. They could be used to measure atomic constants, to confirm or deny various generalizations in quantum theory, and to research some of the basic symmetry laws of nature.

There's just one problem: Making atoms cool their heels long enough to study them. While we may neither feel nor see them, atoms of gas are constantly flying about at an incredible speed, making it extremely difficult to research their properties, let alone isolate them for application. It's difficult to conceive of, but within a completely still room, gas atoms are flying about at an average speed of 620 mph (1,000 km/h). Even at a temperature of only 3°K (-460°F, or -270°C), the atoms are still moving at about 62 mph (100 km/h).

That's where absolute zero comes in. Only at the temperature of about one-thousandth of a degree above absolute zero do the atoms "calm down" slightly and slow to a speed less than that of the pace of a walking human.

One method of cooling gas atoms is based on six "laser cannons" that "shell" groups of atoms from all directions and "catch" them almost without motion in a vacuum. Light particles, known as photons, hit the atom like a ping pong ball hitting a billiard ball. If enough ping pong balls hit amoving billiard ball in the direction contrary to its line of motion, its progress will be arrested until making it come to a full stop.

More accurately, the laser beams directed toward the atoms have a wavelength (color) slightly longer than the wavelength of the light absorbed by the atoms being cooled. Thus, the atoms at rest hardly "feel" the hits by the photons, whereas the atoms moving toward one of the beams in an attempt to leave the area they are being confined in, find themselves involved with the Doppler effect. The Doppler effect "shortens" the wavelength of the laser beam so that its photons are absorbed by the atom, which slows down its progress. This method is known as "Doppler cooling."

Like everything else in life, the Doppler cooling method also has its limitations. As a result, more advanced laser cooling methods have been developed over the years, such as the Sisyphus and the Raman cooling methods. In this way scientists have succeeded in cooling atoms to a temperature of less than one-third of a millionth of a degree above absolute zero.

To exploit these cooled atoms, the researchers must store them for as long a period of time as possible ­ hopefully, for several seconds ­ in a special trap made out of light rays. The "industry" of making light traps capable of trapping atoms istoday one of the hottest frontiers of science.

Dr. Davidson, who is working on this front, says that one should differentiate between the "illuminated" traps in which atoms of material are attracted and trapped in the illuminated area of the trap, and the dark traps, in which the atoms are repelled by the light and are trapped in the darkened part of the trap. Dr. Davidson himself first demonstrated the "dark trap" as part of his postdoctoral work at Stanford University, California, in the laboratory of Prof. Steve Chu. Chu won this year's Nobel Prize in Physics.

The "dark trap" is similar to a kind of "room" whose walls are made of laser beam light, in the center of which is darkness. The gas atoms located in the dark area try to escape from the trap, but each time they hit one of the walls of light, they're repelled.

The main advantage of the "dark trap" as opposed to the regular light traps, derives from a significant reduction in the distortions the light has on the trapped atoms (both in changing their energy levels and in the random distribution of photons from the trapping laser).

Until now, Dr. Davidson has succeeded, using his dark trap, in reducing these distortions by a factor of one thousand or more, over several seconds. His current research is directed toward further reducing the distortions, in order to extend containment time and to improve the trap's properties.

The field of science is, in a manner of speaking, about performing research until scoring a hit. In the matter of atoms of gas, cooling and trapping them, it means Dr. Davidson and team are going for zero.
 

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