Overview
- The Vienna/PTB team ran a thorium-229 clock autonomously for more than 24 hours on Wednesday, using the nuclear resonance in a calcium‑fluoride crystal to lock a continuous 148 nm vacuum‑UV laser.
- The prototype’s measured instability followed about 3 × 10⁻¹² divided by the square root of averaging time and approached the 10⁻¹⁵ range after a day of operation, but it still lags the best optical atomic clocks.
- Researchers traced the main reproducibility limit to the host crystal, which produced resonance shifts of roughly 5 × 10⁻¹³ between runs and will require more homogeneous thorium-doped crystals to remove local stress and inhomogeneity effects.
- The team compared the clock signal over a stabilized fiber link to an ytterbium-ion clock for roughly 23–24 hours and found no periodic variations from ultralight dark matter, setting new upper limits on certain couplings.
- An independent Chinese group reported a near-simultaneous functioning prototype, and near-term work aims to combine stronger, more homogeneous VUV lasers with improved crystals and compact frequency-comb technology to close the gap to state-of-the-art optical clocks and enable practical sensors and navigation devices.