Vienna and Beijing teams demonstrate thorium clock prototypes

Teams at TU Wien in Vienna and Tsinghua University in Beijing have built and operated separate prototype clocks based on thorium-229 nuclei, demonstrating two ways to use nuclear transitions for precision timekeeping. The researchers described the devices in separate papers published in Nature. The prototypes mark a technical milestone, but neither has yet surpassed the best conventional atomic clocks, according to the researchers’ reports. Both devices use thorium-229 embedded in calcium fluoride crystals. Rather than measuring transitions in electrons around an atom, as conventional atomic clocks do, the prototypes use lasers to track changes between energy levels in the thorium nucleus. The nucleus is far smaller than the surrounding atom and may be less affected by some external disturbances, offering a possible route to greater stability and accuracy. The teams developed their systems independently and used different experimental approaches. In Vienna, researchers locked a laser to a thorium nuclear transition and compared the signal with a conventional ytterbium-ion clock.