Optical clocks represent the next generation of atomic clocks. With uncertainties and instabilities two orders of magnitude lower than the current realization of the definition of the second, they promise a wide field of applications, from fundamental physics to contributions to time scales serving metrological and industrial purposes. Remote comparisons are essential for both a full characterization of the clocks and these applications.
In recent years, common comparison techniques that use satellites and optical fibers have been improved. In particular, an enhanced satellite-based method was applied to carry out the first simultaneous comparison of five optical clocks and six microwave fountain clocks located in four different countries. The results of this comparison will be discussed and an overview over the latest developments of other techniques will be given.