Optical frequency standards and clocks

Optical frequency standards based on single trapped ions or ensembles of neutral atoms have recently demonstrated stability and accuracy superior to that of the current generation of microwave primary frequency standards, with significant potential for further improvements. When combined with femtosecond optical frequency combs, these standards can be operated as optical clocks generating a direct microwave output signal, raising the possibility of a future redefinition of the second. In this introductory review, the principles of operation, current state-of-the-art performance and applications of optical frequency standards and clocks are described and the further developments that are likely to occur over the next few years are considered.