Past and Future of Optical Clocks Toward Redefinition of the SI Second

Following the major revision of four SI units in 2018, the redefinition of the second on the basis of optical clocks is becoming foreseeable. In this paper, how optical frequency standards have made progress to surpass microwave frequency standards and the level they are presently at are shown. Future redefinition also requires advances: 1) in remote frequency comparison to confirm the frequency reproducibility of standards and 2) in the capability of maintaining International Atomic Time compatible with an optical definition. Such issues to be resolved prior to the redefinition are also discussed.