The optical 88Sr lattice clocks and stabilized fibre links: A frequency reference for the VLBI system over a 15.5-km link and an absolute measurement of the clock transition over a 330-km link

We report a system of two independent strontium optical lattice standards with
⁸⁸
Sr probed with a single shared ultra-narrow laser. We achieved frequency stability (frequency between two standards) of 7 × 10
^-17
. The absolute frequency of the clock transition can be measured by the use of an optical frequency comb referenced to the UTC(AOS) and UTC(PL) via the 330-km stabilized fibre optic link of the OPTIME network. The 15.5-km stabilized fibre optic link between National Laboratory for Atomic, Molecular, and Optical Physics (KL FAMO) and Toruń Centre for Astronomy made it possible to use the optical clocks as a frequency reference for the 32-metre precise parabolic antenna of the radio telescope in the Toruń Centre for Astronomy participating in the VLBI networks. We report the world's first astronomical VLBI measurements referenced to an optical atomic clock.