NPL's leading-edge optical atomic clocks have stabilities and uncertainties that surpass the performance of present-day primary frequency standards. As national standards, they provide the reference against which other clocks are evaluated. Building on the expertise gained in developing these laboratory systems, NPL are now engineering portable and compact versions, which will bring improved frequency and timing precision to navigation, sensing, communications and space-based applications.

Optical clocks generate a frequency output in the form of ultra-stable laser light. These frequencies are at several hundred terahertz, but can be divided down into microwave or radio frequencies, without loss of precision, using optical frequency combs. Optical clocks are at the leading edge of performance for both uncertainty and stability in frequency standards.

NPL is working on laboratory-based systems that will be used for the highest accuracy realisation of SI units, contributing to international time scales and ensuring consistency of time and frequency measurements around the globe. They are also used to test fundamental physical theories at unprecedented levels of precision. These compact and portable optical clocks will find much wider use; for example they could provide improved autonomy of on-board clocks in future satellite navigation systems. They could also be used as sensors of gravity potential for oil and mineral surveying or for monitoring volcanic activity, ocean currents and rising sea levels.

Key facts and data

• NPL optical clock types: Sr, Sr+ and Yb+
• Frequency uncertainty: approaching 1 part in 1018
• Frequency stability: approaching 1 part in 1016 at 1 s averaging   time
• Future gravity potential sensing capability corresponding to 1 cm height change at the Earth’s surface