First Working Nuclear Clocks Built Using Thorium-229

Two independent teams — in Vienna and at Tsinghua University — have built the world's first working nuclear clocks based on the thorium-229 transition in CaF2 crystals, publishing in Nature. The Vienna group immediately ran the first dark matter search using a nuclear frequency reference, setting new limits on its coupling to the strong force.

• The thorium-229 transition sits near 8.4 eV, in the vacuum-ultraviolet range reachable by lasers
• The Vienna clock drifts one second per 30 million years, still ~1,800 times less stable than top atomic clocks
• The CaF2 crystal holds up to 10^15 thorium nuclei at room temperature
• The Vienna clock is the first to run as a stand-alone device with a nuclear-resonance feedback loop