ITA-JPN Broadband VLBI Experiment for Optical Clock Comparison

Abstract

NICT has developed a broadband VLBI (very-long-baseline interferometry) system for intercontinental frequency transfer. After domestic tests in Japan, a 2.4 m diameter VLBI station was exported and installed at Medicina astronomy observatory of INAF in Italy in 2018. We have started VLBI experiments for optical clock comparison on an Italy-Japan intercontinental baseline. Reference signals generated by an ytterbium (Yb) optical lattice clock at INRiM in Torino and by a strontium (Sr) lattice clock at NICT headquarters in Koganei are compared by a series of links, which include a coherent optical fiber link between INRiM and Medicina and intercontinental VLBI observation between Medicina and Koganei. A series of VLBI experiments was performed from Oct. 2018 to Feb. 2019. Yb and Sr lattice clocks were repeatedly operated throughout the same period for frequency comparison.