Precision Timekeeping: Optical Atomic Clocks

Optical atomic clocks are the most precise measurement devices ever built, with accuracies and stabilities now reaching the 18th digit and continuing to improve rapidly. Achieving this level of precision requires the integration of multiple state-of-the-art laser technologies with control over the internal and external quantum states of individual atoms, and engineering the correlations between atoms is also becoming increasingly important. Traditional applications of precision timekeeping such as navigation, network synchronization, and unit definitions, which presently make use of radio-frequency atomic clocks, will soon benefit from the enhanced stabilities and accuracies afforded by optical atomic clocks. In addition, the rapid advancements in optical atomic clock precision and their unique sensitivities to physical phenomena are giving rise to new applications, such as geodesy, quantum many-body physics, and searches for new physics beyond the Standard Model.

In this chapter we give an overview of the basic concepts underlying optical atomic clocks, including clock stability, clock accuracy, the ultra-stable lasers used as local oscillators, and the optical frequency combs used for optical-to-optical and optical-to-microwave frequency stability transfer. We then describe the experimental techniques and limiting systematics for the two current flavours of optical clock: single ion clocks and neutral atom optical lattice clocks. We conclude with an outlook presenting current research into future-generation optical clock technologies and a brief discussion of some of the emerging novel applications of optical clocks.