Optical Atomic Clock Could Provide More Accurate Definition of a Second

By Will SoutterJul 10 2013

An international research project based at the Paris Observatory has developed an optical atomic clock based on strontium. The clock could lead to a new, more accurate definition of the second, improving on the accuracy and stability of existing atomic clocks.

This new optical clock is the latest in a line of ever-improving super accurate clocks. Our current time standards are set at specialist facilities like the Paris Observatory, and NIST in the USA, using a type of atomic clock called a caesium fountain.

This has been the standard for defining the length of an SI second since the 1960s, but various attempts have been made recently to create a clock which will generate a more accurate and more reliable standard.

Caesium-based atomic clocks rely on a transition between energy levels within the caesium atom which is triggered by microwave radiation. This produces a very precise signal with a well-known measureable frequency, which has a relative uncertainty factor of just 3 parts in 10¹⁶ (ten thousand million million) - clocks based on this standard frequency only lose a second every 100 million years.

This new atomic clock, reported in Nature Communications, replaces the microwave-range transition in caesium with an optical transition in strontium. Optical frequencies are tens of thousands times higher than those of microwave radiation, giving the strontium clock a correspondingly higher accuracy.

In the experiment at the Paris Observatory, two separate strontium clocks were shown to agree with each other perfectly within the range that the equipment was able to measure - giving a total uncertainty factor of under 1.5 in 10¹⁶. The two clocks also agreed with the three standard caesium atomic clocks at the Observatory, at least to the extent that the caesium clocks could measure.

Another type of caesium clock has been proposed, which would use an optical transition in a single caesium ion at it's standard. This could theoretically provide a relative uncertainty a full order of magnitude smaller than existing clocks.

However, because there are practical issues associated with building a clock around a single ion, and because measuring the average emission frequency from a larger sample of atoms is much more statistically robust, experts predict that this new strontium-based optical atomic clock is more likely to become the new standard.

Sources

• "Experimental realization of an optical second with strontium lattice clocks" - R. Le Targat et al, Nature Comms 2013
• Strontium optical clock - SYRTE, Paris Observatory
• Optical frequency standards - NPL