One of Britain's cesium fountain clocks, responsible for keeping the country's atomic time, is declared as the world's most accurate long-term time keeper.

According to physicists at the National Physical Laboratory (NPL) near London, the accuracy of the clock was estimated by evaluating the uncertainties of all the physical effects that are responsible to cause frequency shifts in the clock's operation.

These physical effects include atomic interactions with external fields, clashes between atoms, and the construction of the atomic clock's subsystems, for example, its microwave cavity. There are two largest sources that cause measurement uncertainties - the Doppler effect and microwave-lensing.

Dr. Krzysztof Szymaniec, the project leader at NPL, said that by tinkering with the cesium fountain clock, scientists were able to lessen its margin of error to exceptional levels. They successfully reduced the two largest sources of uncertainties.

Together with other improvements of the cesium fountain, these models and numerical calculations have improved the accuracy of the UK's cesium fountain clock, NPL-CsF2, by reducing the uncertainty to 2.3 × 10-16 - the lowest value for any primary national standard so far, said Szymaniec.

The clock, twice as accurate as it was believed earlier, is one of a group of cesium fountain clocks that were built by timing labs in Europe, the U.S. and Japan as national primary frequency standard for the measurement of time, TG Daily reported.

Scientists claimed the clock would lose or gain less than a second in 138 million years, making it the most accurate clock in the world.

The new evaluation of the clock will be published in the October 2011 issue of the international scientific journal Metrologia. An early posting of the paper on the journal's online site will occur on Friday.