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May-June 2008 Vol. 42, No. 3


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New Clocks: It's About Time

What time is it?” is more than a casual question to physicists, engineers, and other specialists whose work depends on ultra-precise measurements of time.

At present, the International Bureau of Weights and Measures located outside of Paris calculates global time by averaging data received from 300 atomic clocks at laboratories round the world. But this system of telling time may soon be out of date as researchers pursue ever more accurate time measurement.

Separate teams of researchers in Germany and the United States have succeeded in developing optical clocks that use lasers to capture strontium atoms and measure their frequencies. The new clocks can measure oscillation (pendulum swings) at higher ranges of frequency than the standard atomic clocks used today. As a result, future clocks may measure time far more precisely than today’s clocks.

Current atomic clocks measure the oscillation of the movement of cesium atoms. “Optical” clocks refer to the use of lasers to capture the atoms and measure their frequencies; the German and U.S. teams have both developed optical clocks using strontium rather than cesium atoms. The result could be clocks that measure time in intervals thousands of times smaller than now possible with cesium clocks, according to the Joint Institute for Laboratory Astrophysics (JILA), a partnership of the U.S. National Institute of Standards and Technology (NIST) and the University of Colorado–Boulder. JILA’s strontium clock has already surpassed the accuracy of NIST’s cesium clock, currently used for the U.S. time standard. The strontium clock would neither gain nor lose a second in more than 200 million years, compared with NIST’ s 80-million-year accuracy claim.

Researchers at Germany’s national metrology institute, Physikalisch- Technische Bundesanstalt (PTB), have also achieved success with their strontium-based optical atomic clock. Both the JILA and PTB methods use laser beams to capture strontium atoms that are supercooled and held for measurement. The laser excites some of the atoms, and then another laser forces the unexcited atoms to emit a light, which is then detected and measured in a “comb” of laser frequencies to accurately count the clock’s “ticks.”

In addition to strontium, other optical clocks are being designed based on calcium, mercury, aluminum, and ytterbium, each offering different advantages, according to JILA researchers. The pursuit of ever more precise time measurements is vital for synchronizing telecommunication networks and for deep-space communication.

It is not yet clear which specific optical clock that the world’s precision timing labs are designing will prove most effective—and thus win the race to become the future international time standard.—Cynthia G. Wagner

Sources: NIST, 100 Bureau Drive, Stop 1070, Gaithersburg, Maryland 20899-170. Web site www.nist.gov/public_affairs/clock/clock.html . JILA, University of Colorado, 440 UCB, Boulder, Colorado 80309-0440. Web site http://jilawww.colorado.edu/research/metrology.html . PTB, Bundesalle, 100, Braunschweig 38116, Germany. Web site www.ptb.de  .

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