A caesium fountain clock
A caesium fountain clock that keeps the United Kingdom's atomic time is now the most accurate long-term timekeeper in the world, according to a new evaluation of the clock that will be published in the October 2011 issue of the international scientific journal Metrologia by a team of physicists at the National Physical Laboratory (NPL) in the United Kingdom and Penn State University in the United States. This image shows the clock, NPL-CsF2, which is located at the National Physical Laboratory in Teddington, U.K. The whole device is approximately 8.2 feet (2.5 m) high. Atoms are tossed up 3.2 feet (1 m), approximately 12 inches (30 cm) above the cavity that is contained inside a vacuum vessel. The large external cylinder screens the atoms inside the clock from the relatively large and unstable external magnetic field. National Physical Laboratory,

A caesium fountain clock, responsible for keeping United Kingdom's atomic time is declared as the most accurate long-term time keeper in the world.

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

The national primary standards for the measurement of time are averaged to produce International Atomic Time and Universal Coordinated Time. These time scales are used worldwide for vital processes including global communications, satellite navigation and surveying, and time stamping for the computerized transactions of financial and stock markets.

The methods that were used to develop the British clock can also be used to evaluate the caesium fountain clocks of other countries, which will be helpful in improving the world's most accurate methods of keeping time.

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.

Other authors of the paper are Ruoxin Li and Professor of Physics Kurt Gibble at Penn State. The physicists evaluated caesium fountain clock with physical measurements at NPL and mathematical models developed at Penn State.

We now know that the NPL clock is so precise that it has to be considered as an atom interferometer, said Gibble.

Scientists claimed the clock would lose or gain less than a second in 138 million years, which makes 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.