Progress of Atomic Clocks toward the Redefinition of the Second : The Secondary Representations of the Second

Abstract

The rapid development of research on optical frequency measurement based on femtosecond combs has stimulated the field of frequency metrology, especially research on high-performance optical frequency standards. The International Committee for Weights and Measures (CIPM) has recommended four optical and one microwave frequency standards as "the secondary representations of the second" for future redefinition of the second. We introduce the history of the definition of the secondary, the development of the femtosecond frequency combs, and the progress of recent optical clocks.