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British atomic clock is the world's most accurate.

It’s beyond punctual. An atomic clock housed at Britain’s National Physical Laboratory (NPL) is the most accurate long-term timekeeper in the world, according to a new evaluation by a team of physicists at NPL and Penn State.

The clock, known as NPL-CsF2, is one of an elite group of caesium fountain clocks that have been built by timing labs in Europe, the United States and Japan as their national "primary frequency standard" for the measurement of time. These national standards are averaged to produce International Atomic Time and Universal Coordinated Time, which are used as time scales worldwide for such critical processes as global communications, satellite navigation and surveying, and time stamping for the computerized transactions of financial and stock markets.

"An international agreement on the definition of the second is of fundamental importance in timekeeping," says NPL project leader Krzysztof Szymaniec. The agreed-upon length, he says, is the "transition frequency between two ground-state sublevels of a caesium 133 atom."

To measure this frequency, caesium fountain clocks probe laser-cooled caesium atoms twice as they travel through the clock’s microwave cavity—once on their way up and again on their way down. To achieve an accurate assessment of the clock’s frequency, Szymaniec and Penn State professor of physics Kurt Gibble used both physical measurements and mathematical models.

Scientists estimate the accuracy of a caesium fountain clock by evaluating the uncertainties of all the physical effects known to cause frequency shifts in the clock’s operation, including atomic interactions with external fields, collisions between atoms, and the construction of the atomic clock's subsystems. The two largest sources of these measurement uncertainties are frequency shifts caused by the Doppler effect and microwave-lensing.

The models developed by Gibble’s group helped to reduce significantly the impact of the latter two effects, the researchers report in the journal Metrologia. "Together with other improvements of the caesium fountain," Szymaniec says, "these models and numerical calculations have improved the accuracy of the UK’s caesium fountain clock, NPL-CsF2, by reducing the uncertainty to 2.3 x 10-16—the lowest value for any primary national standard so far."

At that rate, the NPL clock is accurate to within one second over 138 million years.

Kurt Gibble, Ph.D., is professor of physics in the Eberly College of Science, keg15@psu.edu.

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Most recently updated September 7, 2011.
Most recently updated September 7, 2011.