All the time in the world

Do you know what time it is? Are you sure?

Scientists from the UK and America have proven that an atomic clock based in the British National Physical Laboratory (NPL) is the most accurate long-term timekeeper in the world.

In a study to be published online today and in the October 2011 issue of the scientific journal Metrologia, the accuracy of the caesium fountain clock, as it is known is confirmed. This is useful, because the clock in question is used to define International Atomic Time (IAT).

Similar clocks are based in labs in the US and Japan and the average is used as the IAT - a concept which is crucial for global communications, satellite navigation, surveying and financial transactions. It is hoped that since the UK atomic clock is now so accurate, similar methods can be used to fine-tune the Japanese and US timepieces.

"The improvements that we report in our paper have reduced significantly the caesium fountain clock's two largest sources of measurement uncertainties - Doppler shifts and the microwave-lensing frequency shift," said NPL Project Leader Krzysztof Szymaniec.

Kurt Gibble of Penn State University who's team contributed to the work explains:
"One of the improvements that our model contributed is an improved understanding of the extremely small Doppler shifts that occur in caesium fountain clocks."

While the acoustic Doppler shift of a train is well known in everyday life and to Leaving Cert physics students, he explained that Doppler shifts for light are too small for people to notice. "If you are walking down the sidewalk while looking at a red traffic light, your eyes cannot perceive the small Doppler shifts resulting from your movement that shift the light toward the blue end of the spectrum," Gibble said. "This change in color is just 1/100 millionth of the difference between red and blue. In the NPL-CSF2 clock, our model now shows that these Doppler shifts are even 100 million times smaller than that."

The other major source of measurement uncertainties - microwave lensing - results from the forces that microwaves in the clock exert on the atoms used to measure the length of a second. "An international agreement on the definition of the second is of fundamental importance in timekeeping," Szymaniec said.

He explained that the length of a second, by international agreement, 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.


Image: 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. (Credit: National Physical Laboratory, United Kingdom)