CAesium Fountain atomic clock with the world's best long-term accuracy

A caesium fountain clock that keeps the United Kingdom's atomic time is now the most accurate long-term timekeeper in the world. 

This has been ascertained by 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. Credit: National Physical Laboratory, United Kingdom.

The atomic clock housed in Britain's National Physical Laboratory (NPL) is the world's most accurate, according to new research.

The clock is a caesium fountain clock, meaning that the "tick" is provided by the measurement of the energy required to change the caesium atoms' spin.

Caesium atoms are placed into a cavity, and exposed to electromagnetic radiation of different wavelengths. Once the spin "flips", the waves are at the right frequency to define what a second is.

In the case of caesium, that quantity is defined as 9.2GHz (or, to be appropriately exact, 9,192,631,770Hz). When the spin flips, the clock operators can set the frequency at that point, and work backward to determine the exact length of a second.

The international Bureau of Weights and Measures takes readings from a selection of "primary frequency standards", in France, the US, Germany, Japan -- and, the most accurate of them all, in the UK.

A team led by NPL's Krzysztof Szymaniec and colleagues at Pennsylvania State University found that Britain's atomic clock was accurate to one part in 4,300,000,000,000,000, nearly doubling the accuracy found when the clocks were last measured in 2010. That level of precision means that NPL's clock wouldn't stray by more than a second in 138 million years.

While that might seem like overegging the pudding in terms of making sure your alarm clock goes off in time for you to get to work, the definition of most electrical units are based on these measurements, and given the vast amounts of energy and data pouring through the world's computer systems, even a tiny change can have measurable economic impact.

"The frequency we measure is not necessarily the one prescribed by the definition of a second, which requires that all the external fields and 'perturbations' would be removed," Szymaniec stated. "In many cases we can't remove these perturbations; but we can measure them precisely, we can assess them, and introduce corrections for them."

"It's vital for the UK as an economy to maintain a set of standards, a set of procedures, that underpin technical development," he added.