Laser spotlight pinpoints atoms in motion.

Physicists who like to push atoms around may soon be able to tell exactly where those atoms went.

Adapting principles from magnetic resonance imaging, Kevin D. Stokes and his colleagues at Duke University in Durham, N.C., developed an optical method for determining the precise position of atoms moving in a beam. It provides greater resolution than any other approach, the researchers report.

Earlier this year, other researchers announced the development of atomicinterferometry techniques for deflecting beams of atoms ever so slightly (SN: 9/7/91, p.158). Although scientists can pinpoint the location of unmoving atoms, such as those in a solid surface, tracking the locations of moving atoms in a deflected beam has proven much more difficult and required the use of mechanical grids or slits. Such techniques "are relatively crude," says John E. Thomas, who heads the Duke group.

Over the past few years, he theorized a better way to locate moving atoms. First he'd overlay a series of parallel lines onto the area to be searched, with each line corresponding to a discrete energy level in a magnetic field. Then he would "mark" atoms that crossed a specific spot while traveling along one of the these lines, and tally them up.

In the Oct. 7 PHYSICAL REVIEW LETTERS, his team describes an experiment that puts those ideas into practice. It pinpointed atoms 1.7 micrometers apart -- and holds open the prospect of one day resolving the location of atoms to within 7 nanometers.

This "is a widely applicable techniques," asserts Harold J. Metcalf, a physicist at the State University of New York at Stony Brook. "It's capable of measuring atoms to a very high precision."

The Duke team establishes rows of energy lines by using two magnets to create a magnetic field whose strength varies. An energy gradient develops between the magnets, with the strongest at the top and the weakest at the bottom; all the lines run parallel to the magnets. Explains Thomas: the steeper the gradient, the more lines that get squeezed into a given space and the greater the technique's resolution. The researchers added two lasers to the setup, one atop j the other, such that their light would cross the magnetic gradient.

When the scientists direct a beam of atoms through this gradient, the atoms--depending on where they are dispersed along the width of the beam -- wind up traveling along different energy lines. An atom's position along the magentic gradient -- that is, which energy line that atom follows -- determines the frequency at which it vibrates, Thomas notes.

The two paired lasers act as a spotlight to illuminate an atom passing through one particular point. The scientists direct this "spotlight" by tuning the lasers to slightly different frequencies so that the difference between the two frequencies matches the frequency of atoms traveling along just one line of energy, says Thomas. When an atom passes through the spot where that energy line and the lasers intersect, the atom resonates and changes its energy level slightly. This "marked" atom then travels downstream and passes through a third laser. This laser excites any atoms with altered energy levels. A detector registers the pressence of these excited atoms.

Because the scientists knew precisely where in space they were looking, they can now know the exact location of any atom they saw there, says Thomas.

Metcalf predicts this method wil not only improve the quality of experiments involving atomic beams, atomic fountains (SN: 8/19/89, p.117) and laser colling (SN: 8/12/89, p.103) but also will help make possible the development of extremely precise atomic clocks and atomic gyroscopes. Indeed, he says about his current research with laser cooling, "What we do now is quite crude relative to John Thomas' technique."