Optical lattice clocks: Hz-level spectral width with sub-Hz reproducibility

State-of-the-art optical clocks have surpassed microwave clocks in this century, causing discussions to redefine the SI second based on an optical transition. The superiority of optical standards was clearly revealed by two of optical-optical comparisons, one of which is a remote comparison of two strontium lattice clocks, and the other is a characterization of a single Ca+ ion clock using a Sr lattice clock as a frequency reference. The former for the first time demonstrated the frequency reproducibility of physically separated clocks at the 10−16 level. The latter has confirmed the capability of single Ca+ clocks to reach the ~10−16 instability.