The world’s best clock, NIST-F1, has been improved over thepast few years and now measures time and frequency more than twice asaccurately as it did in 1999 when first used as a national standard,physicists at the National Institute of Standards and Technology (NIST)report.
The improved version of NIST-F1 would neither gain norlose one second in 60 million years, according to a paper publishedonline Sept. 13 by the journal Metrologia.* NIST-F1 uses afountain-like movement of cesium atoms to determine the length of thesecond. The clock measures the natural oscillations of the atoms toproduce more than 9 billion "ticks" per second. These results thencontribute to the international group of atomic clocks that define theofficial world time. NIST-F1 has been formally evaluated 15 times since1999; in its record performance, it measured the second with anuncertainty of 0.53 × 10-15
The improved accuracy isdue largely to three factors, according to Tom Parker, leader of theNIST atomic standards research group. First, better lasers, softwareand other components have made the entire NIST-F1 system much morereliable and able to operate for longer periods of time. Second, theatoms in the cesium vapor are now spread out over a much larger volumeof space, reducing the frequency shifts caused by interactions amongthe atoms. (The formerly round cloud of atoms is now shaped like ashort cigar.) Third, scientists are now better able to control magneticfields within the clock and quantify the corrections needed tocompensate for their effects on the atoms.
Improved time andfrequency standards have many applications. For instance, ultrapreciseclocks can be used to improve synchronization in precision navigationand positioning systems, telecommunications networks, and wireless anddeep-space communications. Better frequency standards can be used toimprove probes of magnetic and gravitational fields for security andmedical applications, and to measure whether “fundamental constants”used in scientific research might be varying over time—a question thathas enormous implications for understanding the origins and ultimatefate of the universe.
* T.P. Heavner, S.R. Jefferts, E.A. Donley, J.H. Shirley, T.E.Parker. 2005. NIST-F1: Recent improvements and accuracy evaluations.Metrologia (October 2005). Posted online Sept. 13.
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