induced charge and flies toward the comb, collecting at a place on the comb where the static charge is strongest.

In 1968 the Soviet physicist Vladilen Letokhov proposed that atoms immersed in laser light, with their rapidly varying redistribution of charges, would behave like the bits of paper. This would happen if the laser were tuned to a frequency well away from that of any spectral line; the atom's electrons then indeed would oscillate in response to the light's rapidly changing electric field. By contrast, if the laser had a spectral-line frequency, the electrons would tend to absorb and reemit photons, which would quickly push them out of the trap. Then, just as the paper bits are attracted to the strongest region of charge on the comb, the atoms feel an attraction toward the strongest or most intense region of laser light. One produces such an intense region simply by bringing the light to a sharp focus.

In practice, one locates this focus within the intersection of the sets of beams that form optical molasses. The lasers then feature two different frequencies: There are molasses beams that are close to a spectral line, along with the focused beam that is well away from such a line. The resulting arrangement can then produce a large increase in the density of trapped atoms. At AT&T Bell Laboratories, for example, molasses beams alone give a density of 106 atoms per cubic centimeter. The focused beam then raises this density a millionfold.

These techniques, however, leave something to be desired. The volume within the focus is quite small; it can be as low as a billionth of a cubic centimeter. In addition, these traps leak atoms badly. The reason is that, although they are immersed in a vacuum, a physicist's vacuum is far from perfect. It contains residual atoms that bump into the trapped ones and knock them away. Some improvement is possible; better vacuums give longer trapping times. In addition, Chu at Stanford reports creating a "super molasses" by misaligning his laser beams in a particular way. This holds particular atoms for as long as a minute, compared with storage times with conventional molasses of closer to 1 second. But in seeking long storage times together with large volumes, researchers have turned to the use of magnetic fields or a combination of magnetic and laser fields.

Atoms, even when left alone, have a small ability to act like bar magnets and to respond to a magnetic field. This effect is weak and comes into play only at vanishingly low temperatures, but such temperatures characterize laser-cooled atoms, which means that magnetic traps can operate successfully. Such a trap features two current-carrying rings, mounted like barrel hoops, with the current in one ring flowing in



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