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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 $10^{17}$ 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.
The advent of novel measurement instrumentation can lead to paradigm shifts in scientific research. Optical atomic clocks, due to their unprecedented stability and uncertainty, are already being used to test physical theories and herald a revision of
We investigate the leading systematic effects in ro-vibrational spectroscopy of the molecular hydrogen ions H2+ and HD+, in order to assess their potential for the realization of optical clocks that would be sensitive to possible variations of the pr
The Matter-Wave laser Interferometer Gravitation Antenna, MIGA, will be a hybrid instrument composed of a network of atom interferometers horizontally aligned and interrogated by the resonant field of an optical cavity. This detector will provide mea
Existing optical lattice clocks demonstrate a high level of performance, but they remain complex experimental devices. In order to address a wider range of applications including those requiring transportable devices, it will be necessary to simplify
We measure the dynamic differential scalar polarizabilities at 10.6 $mu$m for two candidate clock transitions in $^{176}mathrm{Lu}^+$. The fractional black body radiation (BBR) shifts at 300 K for the $^1S_0 leftrightarrow {^3D_1}$ and $^1S_0 leftrig