Intercontinental optical clock comparison using the geodetic VLBI technique in K-band

Comparing distant atomic clocks is very important for international timekeeping, global positioning and tests of fundamental physics. Optical clocks are the most technologically advanced devices for frequency generation with a stability of 10−18. In the near future they could be used in the redefinition of the SI second replacing the current one defined using the microwave transition of a Cs atom. Optical fiber link networks allow the most performing optical clocks to be compared on distances up to two thousand kilometers, but for longer distances clock comparisons are limited by the performances of satellite frequency transfer techniques. In this presentation we show the use of high-frequency geodetic VLBI as an alternative technique for long distance frequency transfer. A K-band 24-hour experiment involving six antennas between Europe and Korea was carried out in order to estimate the clock rate between the H-masers of Medicina and KRISS sites. These masers were connected and calibrated against two Ytterbium lattice optical clocks in INRiM (Italy) and KRISS (Korea). The fractional frequency difference between the optical clocks was thus evaluated.