Frequency comparison of lattice clocks toward the redefinition of the second

Strontium is the most popular species for optical lattice clocks. Recent reports of the accuracies from Boulder, U.S. and Tokyo reach 10⁻¹⁸ level, which is better than state-of-the-art caesium clocks more than one order of magnitude. While this achievement accelerates the discussion to redefine the second, the agreement of frequencies in separate laboratories is of critical importance. For this context, intercontinental comparison of Sr lattice clocks were demonstrated between Japan and Germany using a satellite-based technique. The frequency difference was consistent with zero with an uncertainty of 1.6 × 10⁻¹⁵.