Implementing the revised second

The synchronisation of clocks around the world is essential for commerce, communications and navigation. It relies on precise methods for comparing frequency standards anywhere on Earth, at any time.

National Metrology Institutes (NMIs) realise the unit of time, the second, using laboratory based primary frequency standards based on the caesium-133 transition frequency that defines the SI second. Recently, optical frequency standards based on atoms trapped at close to absolute zero temperature have achieved accuracies that exceed those of the precise NMI caesium atomic clocks. This opens up the possibility for a future redefinition of the second using optical frequency standards. In order to prepare for this it is necessary to relate the frequencies of optical atomic clocks to the caesium frequency that defines the second and investigate the use of fibre optics to improve links between geographically remote NMI atomic clocks. EMRP project High-accuracy optical clocks with trapped ions has taken a significant step towards a future redefinition of the second based on optical clock technologies.

Defining the second

The SI base unit for time, the second is already defined in terms of a fundamental property of nature – the time taken for 9 192 631 770 oscillations to occur between two energy levels in a caesium-133 atom. The link between the 70 laboratories with over 420 highly accurate atomic clocks that contribute to the international time scale (Coordinated Universal Time - UTC) is made by the International Bureau of Weights and Measures (BIPM).