Atomic clock 'race' heating up

JILA advance highlights promise of laser-based timekeeping

Martin Boyd stood amid three tables, each big enough to host a family feast. Rather than turkey and side dishes, they were crammed instead with dozens of lenses and mirrors, vacuum chambers and mysterious boxes.

In the eternal dusk of the JILA basement lab, blue laser light bounced about the painstakingly arranged hardware of the first table, invisible infrared light on the second and green light on the third.

Martin Boyd in a JILA lab
Martin Boyd, a doctoral student at the University of Colorado, calibrates at the JILA basement lab on Wednesday. Boyd and his colleagues have been working on an experiment that could lead to advances in atomic timekeeping.

The University of Colorado physics doctoral student and his colleagues have built and upgraded this experiment for five years in JILA Fellow Jun Ye's University of Colorado-National Institute of Standards and Technology lab.

The three lasers conspire to chill a cadre of strontium atoms to less than a millionth of a degree above absolute zero. A fourth laser then zaps them to see precisely how fast their nuclei spin.

It could be the future of atomic timekeeping. Such optical atomic clocks aim to use lasers to probe atoms or ions for accuracy 100 times better than today's best microwave-based cesium fountain clocks , which maintain the current global time standard. The Ye group work, published in today's edition of the journal Science, counts as another feather in strontium's cap as a potential successor to cesium.

The cesium fountain clock , first developed 50 years ago, is approaching its limits, said Steven Jefferts, a NIST physicist who has helped enhance the U.S. standard NIST-F1 cesium fountain clock to the point where it is accurate to one second in 80 million years. The next-generation NIST-F2, which he and colleagues expect to have running sometime next summer, will triple the F1's accuracy. But then that's it for microwave-based clocks , he said.

"I don't think there's doubt in anybody's mind that, sometime in the foreseeable future, the transition to optical clocks is going to happen," he said.

The leap from microwaves to lasers could enhance the sensitivity of deep-space antenna arrays, among other applications.

The JILA strontium clock harnesses two key innovations. One is an optical trap using laser light to capture about 10,000 atoms in the troughs of the laser's waves.

The second innovation is a laser capable of maintaining an extremely exact frequency. CU Ph.D. student Andrew Ludlow developed that laser, whose fundamental designs descend from work done by JILA Fellow and 2005 Nobel Prize in physics winner John L. Hall.

The combination allowed the team to watch strontium atoms' electrons jump as they absorbed an extremely specific color of laser light. A precise knowledge of the laser's frequency - and thus the atom's spin - combined with the speed of atomic rotation resulted in one of the best time measurements ever reported.

But there is serious competition down the street. NIST physicist Jim Bergquist's mercury-ion optical clock is faster yet. Bergquist's NIST colleague Till Rosenband has an optical clock based on aluminum and beryllium ions whose accuracy, yet unpublished, is at least comparable. Ye being a NIST Fellow, the teams also collaborate.

Physicist Leo Hollberg's NIST optical frequency measurement group also has scientists working on ytterbium and calcium clocks . He said nobody knows which one succeed cesium as primary frequency standard.

"I'm of the opinion that any of these optical ones will be good, and perhaps find niche uses," Hollberg said.