Teams in Vienna and Beijing build first nuclear clocks using thorium-229

Two independent research teams, one in Vienna and one in Beijing, have built the first nuclear clocks. These clocks keep time by counting the squishing and unsquishing of thorium-229 atomic nuclei. Thorium is a heavy, radioactive element. The results were reported in the journal Nature on Wednesday, Oct. 7, 2026.

Every clock works the same basic way: it counts ticks. A grandfather clock counts the swing of a pendulum. An atomic clock counts the rapid oscillations of electrons. A nuclear clock counts changes in the nucleus itself.

The new clocks are very precise. They would lose about one second every few million years. One of them sits at TU Wien in Vienna, where a laser is aimed at a thorium crystal held in a vacuum chamber.

Experts who reviewed the study before publication called the results a major breakthrough and a significant milestone. Eric Hudson, a physicist at the University of California, Los Angeles, works on nuclear clocks but was not involved in this research. He said the field is "moving at breakneck speed."

These clocks will not replace atomic clocks any time soon. The worldwide network of atomic clocks supports GPS and the official definition of a second. The best atomic clocks are also still about 10,000 times more reliable, and would take more than 100 billion years to lose one second.

Still, nuclear clocks have potential advantages. One day they could be more reliable than the leading atomic clocks, and they could be made more portable.

They could also be used to search for certain kinds of dark matter. Dark matter is the invisible material whose clumps make up 83 percent of the matter in the universe. If a dark matter particle passed through the thorium nuclei, it could show up as a wobble in the clock's otherwise steady ticking.