Leveraging the unrivaled performance of optical clocks in applications in
fundamental physics beyond the standard model, in geo-sciences, and in
astronomy requires comparing the frequency of distant optical clocks
truthfully. Meeting this requirement, we report on the first comparison and
agreement of fully independent optical clocks separated by 700 km being only
limited by the uncertainties of the clocks themselves. This is achieved by a
phase-coherent optical frequency transfer via a 1415 km long telecom fiber link
that enables substantially better precision than classical means of frequency
transfer. The fractional precision in comparing the optical clocks of three
parts in 1017 was reached after only 1000 s averaging time, which is
already 10 times better and more than four orders of magnitude faster than with
any other existing frequency transfer method. The capability of performing high
resolution international clock comparisons paves the way for a redefinition of
the unit of time and an all-optical dissemination of the SI-second.Comment: 14 pages, 3 figures, 1 tabl