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Time up for atomic clocks

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Scientists are plotting a new era of hyper-exact timekeeping, spelling the end of the atomic clock in its current form. Very accurate clocks are vital in telecommunications, GPS, and other modern technological applications.

Traditional Caesium-based atomic clocks have been around since the mid-50s. They work by detecting microwave emissions from the Caesium atom, which occur at a very steady rate. Since 1967 that rate has been the fundamental frequency on which the international definition of a second is based. Prior to that, seconds had been defined in terms of the Earth's rotation, which is relatively variable.

The new clocks will work using optical rather than microwave frequencies, and ions rather than atoms. In timekeeping, the higher the frequency, the more stable the time signal. A team at the National Physics Laboratory (NPL) is using a light emitting Strontium ion trapped and cooled by lasers to push the accuracy of clocks.

The positively-charged ion sits in a spinning "saddle" of positive charge either side of it in the device, which is made of a 10cm tube of a glass composite, which is very stable to temperature fluctuations. Normal diode-type lasers hold it in place, cooling it to near-absolute zero and cause it to emit photons at an extraordinarily steady rate.

Helen Margolis from NPL says her team's Strontium version of an optical clock has the advantage over other ions like Mercury and Aluminium that the lasers it requires are common commercial types already cheaply produced.

Strontium clocks have now reached the point where their only point of reference for accuracy, the old-style Caesium clocks, can't compete. Margolis says the next step will be to have several Strontium clocks to compare against each other.

As well as having practical applications in navigation and telecoms, how well we can pin down the length of a second is a fundamental issue in physics. Indeed, basic units of length are defined by how far light travels in various time periods. ®

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