It takes time to perfect NIST’s atomic clock


Steven Jefferts knows better than to make lofty predictions about F2. After all, it took his colleagues and him almost a decade of tweaking to get F1 into the position as the world’s best timekeeper.

F1 is nothing like a traditional clock, though. It’s a laser-cooled cesium fountain, a device that determines the passing of a second by measuring the oscillations of cesium atoms. “Nine billion and change” oscillations add up to a second, said Jefferts, a physicist at the National Institute of Standards and Technology in Boulder, Colo., and designer of F1. F1 can so accurately track the oscillations that it is projected to neither gain nor lose a second in 60 million years. Jefferts and his team published their record results in the journal Metrologia last fall.

Since 1999, F1 has set the standard in the United States and is a contributor to the International Atomic Clock. The data is used to synchronize Global Positioning Systems, for instance, and for defense applications ranging from navigation to code breaking.

“Time applications are not the place where huge accuracy is needed,” Jefferts said. “It’s when you’re trying to synchronize at high speeds. With GPS, if you just let the clock wander off on its own, eventually it will tell you a wrong time and give a positioning error.”

From left, physicists Steven Jefferts, Elizabeth Donley and Tom Heavner are mastering the art of accuracy by improving NIST’s atomic clock. Photo courtesy of NIST.
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Jefferts and co-workers Elizabeth Donley and Tom Heavner think in terms of reducing uncertainties rather than increasing accuracy. “There’s a laundry list of things that can mess up the clock,” Jefferts said. “They add up to the uncertainties.”

The NIST group earned their title as having the most accurate of the clocks through incremental changes that improved reliability: Swapping Jefferts’ homemade laser apparatus for a commercial product and better monitoring of subsystems allowed them to conduct uninterrupted run times. They even found ways to overcome the shortcomings of their lab space, an add-on to the NIST facility.

“We’re the poster child for how bad an environment can be,” Jefferts joked.

That will change this year. Jefferts and his crew in the Time and Frequency Division are moving to better digs within NIST’s building. The new space will hold F2, a second-generation fountain that may be in place by the summer. F2 is expected to have capabilities that will whittle away some remaining uncertainties. But based on his experience, Jefferts cautions, it will take some time before F2 has a hope of besting its predecessor.
- Candace Stuart