High-accuracy optical clocks with trapped ions

Atomic clocks form the basis of international time keeping and are widely used in navigation, communications and computer network management. A new generation of atomic clocks that is based on optical reference frequencies rather than on microwaves promises significantly improved accuracy and stability. Ultra-precise optical clocks using laser-cooled trapped ions could provide the new frequency standards needed for a future redefinition of the second but their operational performance needs to be optimised so that this can happen.

The EMRP project High-accuracy optical clocks with trapped ions (Ion Clock), developed a selection of trapped ion optical clocks as potential successors for the current primary NMI-based caesium atomic clocks. The project optimised their design and generated data to assess their performance.

The project:

• Developed more advanced ion trap designs for single and multiple ions which minimise heat load and temperature differences in the trap structure under real operating conditions. These are major contributors to measurement uncertainties for these types of frequency standards.
• Developed novel and efficient methods to generate the laser radiation required to excite and cool the trapped ions, an important milestone in the realisation of a new optical frequency standard.
• Performed measurements of absolute frequencies generated by single-ion optical clocks against primary caesium clocks to ensure optical frequency standard traceability.
• Performed the first comparison of remote single-ion optical clocks using the Global Positioning System and geodetic data processing. This demonstrated that this world-wide method for frequency transfer can be used in assessing the precision of optical clocks.

This EMRP project has taken a significant step towards a future redefinition of the second based on optical frequency standards. Efficient sharing of next generation ion traps developed in the project, has resulted in two new highly accurate European optical clocks. Optical clocks based on trapped ytterbium ions have been demonstrated to have the highest accuracy of all the various types of trapped ion optical clocks presented so far, making them suitable candidates for next generation frequency standards. These particular ion based optical clocks offer a very favourable combination of operational reliability and high accuracy for future realisations of the second.

As a result of the project’s experimental comparison of optical and atomic caesium based clocks, international agreement has been reached on ytterbium reference frequencies that can now be used as representations of the SI seconds in the optical frequency range.

EMPIR project 17FUN07 CC4C builds on this work.