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out of date as researchers pursue ever more accurate time measurement.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
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
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
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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