Current status of frequency standards toward the redefinition of the SI second

Abstract

In 1967, the SI second was defined as the duration of 9,192,631,770 cycles of the microwave transition between hyperfine levels of the ground state of the cesium-133 atom. A primary frequency standard realizes this definition using cesium atoms while rigorously evaluating all sources of systematic bias to achieve a well-characterized, low-uncertainty realization of the second. Currently, the best cesium-based standards achieve systematic uncertainties at the level of 1-2 x 10-16; however, this performance has plateaued over the past 15 years. In contrast, optical frequency standards have made rapid progress since around 2005, with state-of-the-art systems now demonstrating systematic uncertainties in the 10-19 range. This substantial improvement has sparked active discussions about redefining the SI second based on optical transitions.