Atomic fountain springs from a light touch.

Atomic fountain springs from a light touch

Using pulses of laser light, a team of physicists has succeeded in creating an atomic fountain. The laser pulses push the atoms up; gravity brings them down. During the descent, scientists can probe the freely falling atoms with microwaves, obtaining extremely precise measurements of transitions from one atomic energy level to another. Such a scheme may form the basis of an atomic clock for establishing a time standard.

"The atomic physics community was dreaming of making [atomic fountains] in the early 1950s, but they didn't have techniques for cooling down and manipulating atoms," says Steven Chu of Stanford University. "We had to string together a lot of tricks that we have been developing in laser cooling. Once we had those tricks, it was actually fairly easy."

To create an atomic fountain, Chu and his colleagues first use a laser beam to slow a stream of sodium atoms moving toward the laser (see illustration). They then store the slowed atoms in a special trap created by a combination of magnetic fields and three laser beams positioned at right angles to each other (SN: 7/23/89, p.52). After a final cooling stage, during which the atoms reach a temperature of roughly 50 microkelvins (a tiny fraction of a degree above absolute zero), the trap is turned off and laser pulses from beneath launch the atoms upward. Following a ballistic trajectory, the atoms soar into a microwave waveguide, where the transition from one energy level to another occurs.

Such a fountain makes it possible to measure atomic properties very precisely. According to the Heisenberg uncertainty principle, the longer an atom can be observed, the more precisely researchers can determine the frequency of an energy-level transition. Chu's fountain allows a 0.25-second measurement time, when the atoms are free of any perturbing electric and magnetic fields that would otherwise affect the measurements. The relatively long time the atoms spend freely falling allows the team to measure the frequency of a microwave transition in the sodium atom to within 2 hertz, better than the 26-hertz precision of the present U.S. time standard.

The atomic fountain is only one of several possible ways to build a precise atomic clock. Other researchers are investigating the use of single ions, which can be stored for long periods of time in magnetic traps (SN: 8/12/89, p. 103), or designing traps with specially shaped magnetic fields to hold neutral atoms without unduly perturbing them. Which approach works best for making a practical atomic clock remains to be seen.

Meanwhile, Chu and his colleagues, who report their achievement in the Aug. 7 PHYSICAL REVIEW LETTERS, are in the middle of deciding whether to perform the same experiment with cesium atoms--which ought to yield even more precise measurements -- or to try building an apparatus to measure optical rather than microwave frequencies.