Trapped ion optical clocks

Abstract
A single laser-cooled ion confined in a radiofrequency ion trap comes close to the spectroscopic ideal of an absorber at rest in a perturbation-free environment. Narrow optical transitions in such systems are therefore very promising for the realisation of optical frequency standards with accuracy significantly exceeding that of current microwave primary frequency standards. When combined with femtosecond optical frequency combs, these standards can be operated as optical clocks generating a direct microwave output, raising the possibility of a future redefinition of the SI second. In this article, the fundamental aspects and current state-of-the-art of trapped ion optical clocks are reviewed, and the improvements that are likely to occur over the next few years are considered.