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Monthly milestones ... atomic timekeeping 60--years on.

Last month marked the 60th anniversary of atomic timekeeping, a revolutionary technology that underpins much in the modern world. From global communications and satellite navigation to the systems behind our transport and financial networks, the maintenance of a stable and accurate timescale is essential for our society to function.

In an attempt to move away from definitions in terms of the earth's rotation, physicist Louis Essen spent the early 1950s developing his pioneering atomic clock at the National Physical Laboratory. On 3 June 1955, he started to control the UK radio time signals using atomic units of time derived from the atomic clock, effectively marking what he called "the death of the astronomical second and the birth of atomic time".

Essen showed that atoms could provide a more stable reference time interval. By using microwaves to excite electrons from one energy level to another within atoms of caesium, he was able to stabilise the microwaves at a precise and reproducible frequency. Much as a grandfather clock depends on the swinging of a pendulum, his prototype atomic clock relied on this frequency to mark the passing of time.

Although different elements have since been used in other clocks, the caesium clock has remained the standard of time and frequency. Since 1967, the SI second has been defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

Since Essen's pioneering work, the accuracy of atomic clocks has improved by a factor of 10 or so every decade. The main improvement in caesium clock accuracy has been achieved through the use of laser-cooled caesium atoms in laboratory clocks known as 'caesium fountains'.

The current primary clock at the National Physical Laboratory, the caesium fountain NPL-CsF2, contributes to the international timescale UTC (coordinated universal time) and is the reference for the national timescale UTC. It is more than 300,000 times more precise than Essen's original clock, which is now housed in London's Science Museum.

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Title Annotation:Engineering extras: Innovation: Careers: Ideas
Publication:Professional Engineering Magazine
Date:Jul 1, 2015
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