Ultimate Clocks; September 2002; Scientific American Magazine; by W. Wayt Gibbs; 8 Page(s)

Dozens of the top clock makers in the worldconvened in New Orleans one muggy week this past May 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.

And 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 Agilent sells for $63,000, 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 one part in 10¹⁵ (usually written simply as 10^-15). That is 500 times the accuracy of NIST's best clock in 1975. But space-based clocks set to fly on the International Space Station by 2005 are expected to tick with uncertainties better than 10^-16. And successful prototypes of new clock designs--devices that extract time from calcium atoms or mercury ions instead of cesium--lead physicists to expect that within three years, accuracy will reach the 10^-18 range, a 1,000-fold improvement in less than a decade.