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Most precise clock

Cesium atoms keep precise time in this atomic clock. A new, improved version of the clock uses ytterbium.

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It would take more than 50 billion years for a new atomic clock to gain or lose a second. The clocks that determine official U.S. time link the duration of a second to the frequency of light that coerces cesium’s electrons to jump to a higher energy level.
 
Physicist Andrew Ludlow of the National Institute of Standards and Technology in Boulder, Colo., and his team swapped in ytterbium atoms for cesium and then trapped groups of the atoms with lasers, precisely measuring that frequency. Ludlow and colleagues suggest May 24 at arXiv.org that such precise timekeeping could improve global positioning systems and test the limits of Einstein’s theory of general relativity, which posits that clocks at varying elevations tick at slightly different rates.

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