Tossing cold atoms like confetti, atomic-fountain clocks launch a new era in timekeeping

Science News, August 7, 1999 by Peter Weiss

Tossing cold atoms like confetti, atomic-fountain clocks launch a new era in timekeeping

In the 1970s, Judah Levine would periodically lug a shiny aluminum suitcase aboard a commercial airliner, strap it into the seat next to him, and head for France. Inside the heavy luggage was one of the world's most precise clocks. Levine's mission was to compare its time with that of clocks at the International Bureau of Weights and Measures near Paris, which keeps time for the world.

Hand carrying clocks to Paris "was the standard method then" for maintaining accurate world time, recalls the physicist, who works at the National Institute of Standards and Technology (NIST) in Boulder, Colo. Transmitting time by telephone or radio was too imprecise because of variable signal delays.

By the 1980s, however, the United States and much of the world were relying on orbiting satellites rather than scientist travelers to keep accurate time. Because each navigation satellite in the Global Positioning System (GPS) carries a good atomic clock of its own, national metrology laboratories use the spacecrafts' time and frequency transmissions to compare their ground-based master clocks. With the advent of GPS, Levine and other scientists gave up their flights.

Now, atomic-clock technology is entering a new phase. At the pinnacle of time and frequency measurement, a novel device known as the atomic-fountain clock is about to displace the reigning thermal; beam atomic clocks. The new instruments toss ultracold atoms upward and let them fall under the influence of gravity. Thermal-beam clocks, in contrast, use fast-moving streams of relatively hot atoms to mark time.

The use of cold atoms represents "a fundamental change in the way we build atomic clocks," says NIST's Steven Jefferts, also in Boulder. Chilling atoms improves atomic-clock performance because cold atoms move more slowly, making it possible to detect more precisely their response to microwaves of a critical frequency.

Already, the first clock of this new generation has joined the rank of the world's primary frequency standards--the most accurate clocks on the planet. The handful of clocks in that exclusive club, formerly only thermal-beam devices, enable the Paris bureau to determine whether the world's official clock is running fast or slow.

The unveiling of the first fountain clock in 1994 by a French team set off a scramble among standards labs around the world to make similar devices of their own. Some 15 countries have made it known that they intend to build fountain clocks. About a half-dozen of the clocks have already been built or are expected to be finished in the next year or two.

"It's a big, exciting time for the clock community," says Jefferts, one of the leaders in the construction of the first NIST fountain clock expected to become a primary standard.

Clock experts say that the advent of fountain clocks will spur demand for greater precision from many quarters--especially for military and civilian telecommunications and certain areas of astronomy and physics research. Projects to make fountain-type clocks adapted to zero gravity for the International Space Station are also under way in France and at NIST.

Ironically, the leap forward to fountain clocks is expected to turn back the clock, at least temporarily, to the days before the GPS. Because fountain clocks are too precise to be adequately compared via satellite transmissions, labs may again have to send atomic timing devices on airplanes to meet each other face-to-face.

Since the first thermal-beam atomic clock was built in 1949, designers have boosted the accuracy of such clocks from 1 second of error in 300 years to 1 second in 6 million years (SN: 5/1/93, p. 76). Moving at about 100 meters per second, atoms in a thermal beam get a kick to a new energy level as they pass through chambers filled with microwaves of adjustable frequency. The instrument tunes its microwave emissions to maximize production of the excited atoms. The tuned signal then serves as a frequency reference, or its oscillations can be counted off electronically to generate clock ticks.

In the past few years, the hope of much further improvement in these devices had dimmed. The thermal beam clock "was up against the wall," Jefferts says.

To burst through that wall, scientists have devised the atomic-fountain clock. It also tunes microwaves to the excitation of atoms in a cavity. However, the atoms are first cooled to microkelvin temperatures and then launched at a few meters per second up through the microwave cavity, which is kept in a vacuum. Before falling back down again, the atoms become momentarily motionless.

"They toss just as your car keys do," says Christopher R. Ekstrom of the U.S. Naval Observatory in Washington, D.C., who is building a fountain clock there for the military's timekeeping needs.

Slower atoms spend a longer time travelling through the device--about a half second for fountain atoms versus about 10 milliseconds for thermal-beam atoms. The precision of the clock can improve in rough proportion to that increase in time, or by a factor of 10 to 100. "The longer you can look at an atom in your [clock] the better you can do," Ekstrom notes.