a direction opposite to that of the other. This creates a magnetic field that has zero strength on the centerline midway between the two rings and that increases in all directions away from this point. An early trap of this type, built at the National Institute of Standards and Technology, held atoms within a volume the size of a golf ball. More recent traps, using stronger magnetic fields and better vacuums, have featured larger volumes. These have held atoms for a number of minutes, even hours.

THE ATOMIC FOUNTAIN

Nevertheless, whether trapped magnetically or optically, atoms are not in the pristine state that physicists would prefer to study. These trapping techniques all perturb the atoms in various ways. When caught in molasses, they are continually absorbing and reemitting photons, and their electrons are jumping up and down between energy levels. Use of the focused beam introduces rapidly varying oscillations in their electron densities. The magnetic trap, in turn, changes the spectral lines through the Zeeman effect. As a consequence, a current trend is to collect atoms in such a trap but not to study them there. Instead, a quick burst of tuned laser light can launch them out of the trap and into ballistic trajectories, within a vacuum. And just as a football has hang time when punted, these atoms have hang time within the chamber where they are available for study, away from the trap. The time for study approaches a full second if the chamber measures a meter from top to bottom. That is virtually an eternity; in principle, it suffices to permit measurement of spectral features down to line widths as narrow as 0.2 hertz. This is because longer observation times yield greater accuracy and precision.

In this fashion, Chu and his colleagues have built an atomic fountain. The apparatus collects atoms in a trap for some one-half second and then launches them upward at some 2 meters per second. Near the top of their trajectories, the instrument probes these atoms by applying two microwave pulses, separated in time by 0.25 seconds. These produce a transition between two closely spaced energy states, with the separation in time yielding superb measurement accuracy (see Figure 2.8). "In our first experiment," Chu notes, "we measured the energy difference between two states of an atom with a resolution of two parts in 100 billion." Line width was a mere 2 hertz. Furthermore, repeated observations made over 15 minutes resolved the center of the pertinent spectral line to 0.01 hertz. Current work with cesium atoms is improving the resolution still further, to as much as one part in 1015.

Chu has also gone on to develop an "atomic funnel" that produces a



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