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UK's atomic clock 'is world's most accurate'

2011-08-26 12:08:19

By Jason Palmer Science and technology reporter, BBC News, Teddington

An atomic clock at the UK's National Physical Laboratory (NPL) has the best

long-term accuracy of any in the world, research has found.

Studies of the clock's performance, to be published in the journal Metrologia,

show it is nearly twice as accurate as previously thought.

The clock would lose or gain less than a second in some 138 million years.

The UK is among the handful of nations providing a "standard second" that keeps

the world on time.

However, the international race for higher accuracy is always on, meaning the

record may not stand for long.

The NPL's CsF2 clock is a "caesium fountain" atomic clock, in which the

"ticking" is provided by the measurement of the energy required to change a

property of caesium atoms known as "spin".

By international definition, it is the electromagnetic waves required to

accomplish this "spin flip" that are measured; when 9,192,631,770 peaks and

troughs of these waves go by, one standard second passes.

Matching colours

Inside the clock, caesium atoms are gathered into bunches of 100 million or so,

and passed through a cavity where they are exposed to these electromagnetic

waves.

The colour, or frequency, is adjusted until the spins are seen to flip - then

the researchers know the waves are at the right frequency to define the second.

The NPL-CsF2 clock provides an "atomic pendulum" against which the UK's and the

world's clocks can be compared, ensuring they are all ticking at the same time.

That correction is done at the International Bureau of Weights and Measures

(BIPM) in the outskirts of Paris, which collates definitions of seconds from

six "primary frequency standards" - CsF2 in the UK, two in France, and one each

in the US, Germany and Japan.

For those six high-precision atomic pendulums, absolute accuracy is a tireless

pursuit.

At the last count in 2010, the UK's atomic clock was on a par with the best of

them in terms of long-term accuracy: to about one part in

2,500,000,000,000,000.

But the measurements carried out by the NPL's Krzysztof Szymaniec and

colleagues at Pennsylvania State University in the US have nearly doubled the

accuracy.

The second's strictest definition requires that the measurements are made in

conditions that Dr Szymaniec said were impossible actually to achieve in the

laboratory.

"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," he explained to BBC News.

"In many cases we can't remove these perturbations; but we can measure them

precisely, we can assess them, and introduce corrections for them."

The team's latest work addressed the errors in the measurement brought about by

the "microwave cavity" that the atoms pass through (the waves used to flip

spins are not so far in frequency from the ones that flip water molecules in

food, heating them in a microwave oven).

A fuller understanding of how the waves are distributed within it boosted the

measurement's accuracy, as did a more detailed treatment of what happens to the

measurement when the millions of caesium atoms collide.

Without touching a thing, the team boosted the known accuracy of the machine to

one part in 4,300,000,000,000,000.

But as Dr Szymaniec said, the achievement is not just about international

bragging rights; better standards lead to better technology.

"Nowadays definitions for electrical units are based on accurate frequency

measurements, so it's vital for the UK as an economy to maintain a set of

standards, a set of procedures, that underpin technical development," he said.

"The fact that we can develop the most accurate standard has quite measurable

economic implications."

What time is it, exactly?

The international time standard is maintained by a network of over 300 clocks

worldwide

These are sent by satellite and averaged at BIPM, a measurement institute in

France

But the "tick" of any one of them could drift out of accuracy, so BIPM corrects

the average using six "primary frequency standards" in Europe, the US and Japan

Their corrected result, "International Atomic Time", is occasionally compared

with the time-honoured measure of time by astronomical means

Occasionally a "leap second" is added or subtracted to correct any discrepancy