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Friday, August 26, 2011

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




Caesium clock at NPL (NPL)



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.



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