Absolute measurement of the ${}^1{S}_0$ - ${}^3{P}_0$ clock transition in neutral ${}^{88}$Sr over the 330 km-long stabilized fibre optic link

Abstract

We report a stability below $7\times 10{}^{-17}$ of two independent optical lattice clocks operating with bosonic ${}^{88}$Sr isotope. The value (429228066418008.3(1.9)${}_{syst}$(0.9)${}_{stat}$~Hz) of the absolute frequency of the ${}^1{S}_0$ - ${}^3{P}_0$ transition was measured with an optical frequency comb referenced to the local representation of the UTC by the 330 km-long stabilized fibre optical link. The result was verified by series of measurements on two independent optical lattice clocks and agrees with recommendation of Bureau International des Poids et Mesures.