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Ultraprecise Clocks Approach Chronometry’s Final Frontier

Atomic clocks are shrinking to microchip size, heading for space—and approaching the limits of useful precision

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Dozens of the top clockmakers in the world convened in new orleans one muggy week in May 2002 to present their latest inventions. There was not a mechanic among them; these were scientists, and their conversations buzzed with talk of spectrums and quantum levels, not gears and escapements. Today those who would build a more accurate clock must advance into the frontiers of physics and engineering in several directions at once. They are cobbling lasers that spit out pulses a quadrillionth of a second long together with chambers that chill atoms to a few millionths of a degree above absolute zero. They are snaring individual ions in tar pits of light and magnetism and manipulating the spin of electrons in their orbits.

Thanks to major technical advances, the art of ultraprecise timekeeping is progressing with a speed not seen for 30 years or more. These days a good cesium beam clock, of the kind Symmetricom sells for about $50,000, will tick off seconds true to about a microsecond a month, its frequency accurate to five parts in 1013. The primary time standard for the U.S., a cesium fountain clock installed in 1999 by the National Institute of Standards and Technology (NIST) at its Boulder, Colo., laboratory, is good to five parts in 1016 (usually written simply as 10−16). That is 1,000 times the accuracy of NIST's best clock in 1975. Successful prototypes of new clock designs—devices that extract time from aluminum or mercury ions instead of cesium—have recently attained accuracy in the 10−18 range, a 100-fold improvement in a decade.

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