Coordinated international comparisons between optical clocks connected via fiber and satellite links

Optical clocks provide ultraprecise frequency references that are vital for international metrology as well as for tests of fundamental physics. To investigate the level of agreement between different clocks, we simultaneously measured the frequency ratios between ten optical clocks in six different countries, using fiber and satellite links. This is the largest coordinated comparison to date, from which we present a subset of 38 optical frequency ratios and an evaluation of the correlations between them. Four ratios were measured directly for the first time to our knowledge, while others had significantly lower uncertainties than previously achieved, supporting the advance toward a redefinition of the second and the use of optical standards for international time scales.