SI second: from microwave towards optical realization

Present definition of SI second is based on the hyperfine transition frequency (in the microwave region) in Cesium atoms since 1967. In the last two decades, frequency standards based on optical transition have shown remarkable progress by outperforming the best microwave frequency standard by at least an order of magnitude in terms of relative frequency uncertainty. This paper presents an insight on the present realization of a microwave SI second and the roadmap towards future realization based on optical transition frequencies.