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Past and Future of Optical Clocks Toward Redefinition of the Second

Following the major revision of four SI units in 2018, redefinition of the second based on optical clocks is becoming foreseeable. This presentation briefly shows how optical frequency standards have made progress to surpass microwave frequency standards, and the level they are presently at. The future redefinition also requires the advances in the capability of remote frequency comparison to confirm the frequency reproducibility of standards as well as to maintain International Atomic Time (TAI) compatible with an optical definition. Such issues to be resolved prior to the redefinition are also discussed.