The nuclear clock has begun ticking

A collaboration led by PTB and TU Wien has demonstrated a functioning optical clock based on a laser-driven nuclear transition in thorium-229, reporting results in two Nature papers. Thorium-229 has an unusually low-energy excited nuclear state that can be driven with laser light, but the transition energy had to be matched to within about one millionth of an electronvolt. TU Wien produced thorium-doped calcium fluoride crystals containing many nuclei to boost signal, and PTB with the Max Born Institute built a compact, continuous-wave solid-state laser with very high frequency resolution. That laser enabled absorption-based detection instead of slow fluorescence (which had a ~10-minute time constant), permitting real-time feedback to lock the laser to the nuclear resonance.

Using this system the team ran the clock autonomously for over 24 hours and compared it to a leading optical atomic clock at the Austrian metrology institute. The work establishes the first nuclear-resonance clock and the first solid-state optical-clock architecture, and identifies dominant technical limits to improve stability toward parity with trapped-atom/ion clocks. Ancillary studies mapped site-dependent resonance shifts in the crystal lattice and performed an initial search for couplings between the Th-229 nucleus and forms of dark matter.