Researchers at TU Wien in Vienna and Tsinghua University in Beijing have independently built the first nuclear clocks that keep time by counting the rapid squishing and unsquishing of thorium‑229 atomic nuclei. The isotope thorium‑229 is exceptionally rare, obtainable only from the decay of weapons‑grade uranium, and requires a laser tuned to 148 nanometres in the ultraviolet spectrum to drive the nuclear transition. Their results were reported in the journal Nature on Wednesday, earning praise from experts who called the work a major breakthrough for the emerging field of nuclear clocks.
Each clock operates by shining that 148‑nanometre UV laser on the thorium nucleus and measuring how quickly it flips between two shapes—an isomer that is squeezed like a football into a pear shape and then springs back. To keep the laser frequency stable, both teams added a feedback loop that measures and corrects any drift, similar to winding a pendulum clock, ensuring the tick‑tock remains steady. The Beijing team generated the needed UV laser using a heated cadmium gas, while the Vienna team secured access to thorium‑rich crystals to overcome the laser shortage.
This image shows the thorium crystal and UV laser apparatus used by the Vienna team to build their nuclear clock, as described in the article.
The Vienna device would lose about one second every few million years, while the Beijing version would lose roughly two seconds if it had been running since the asteroid impact that wiped out the non‑avian dinosaurs 65 million years ago. Although current atomic clocks remain about ten thousand times more stable, the nuclear clocks demonstrate a proof‑of‑principle for a new class of timekeeper that is less susceptible to external electromagnetic noise and could one day be more portable or serve as a sensor for dark matter, as a passing dark‑matter particle could impart a tiny wobble on the thorium nucleus detectable as a deviation in the clock’s output. Researchers say improved crystals and more powerful lasers will likely boost performance rapidly, characterizing the current results as just the beginning of the nuclear‑clocking field.