Connecting a narrow Th-229 nuclear clock transition with Sr optical clock via a VUV frequency comb

We observed a narrow (~100 kHz) 229Th nuclear clock transition in Th-doped CaF2 crystals using a VUV frequency comb. The VUV comb directly links the 229Th nuclear transition frequency to the JILA Sr optical clock, enabling determination of the absolute 229Th clock frequency. These results represent a milestone in building solid-state nuclear clocks for precision measurements and fundamental physics.