For more than 70 years, the world’s best clocks have counted the same thing: electrons jumping between energy levels inside an atom. Satellite navigation and the timestamps on the internet all trace back to that tick.
In October, Nature published two papers describing clocks that count something else: the atom’s nucleus.
Teams led by Shiqian Ding at Tsinghua University in Beijing and Thorsten Schumm at TU Wien in Vienna each built a working clock locked to a transition in the nucleus of thorium-229. Physicists proposed the idea in 2003. Until now, nobody had closed the loop and actually run one as a clock.
Both teams locked a laser to the thorium nucleus
A clock needs two things: something that oscillates at a perfectly regular rate, and a way to count the oscillations.
In both experiments, thorium-229 sits inside a calcium fluoride crystal a few millimetres long. A continuous laser shines through it at 148 nanometres, deep in the ultraviolet. That is shorter than the light of a germicidal UV-C lamp, and so short that air absorbs it.
When the laser hits exactly the right frequency, about 2 020 trillion oscillations per second, the nuclei absorb a little of it. A feedback loop keeps steering the laser back to that point. The laser becomes the clock’s pendulum, and the nucleus keeps it honest.
The setups are remarkably small:
- The Tsinghua crystal held about 1.4 micrograms of thorium-229, roughly a fortieth of the mass of a grain of table salt.
- The Vienna clock ran for about a day without anyone touching it.
The nucleus is shielded in a way electrons are not
Electrons sit on the outside of the atom, where stray electric and magnetic fields push them around. Atomic clocks go to great lengths to protect them, from vacuum chambers to lasers that hold single atoms still.
The nucleus is about 100 000 times smaller than the atom and buried at its centre. Outside fields barely reach it, so it can keep a steady tick even inside a solid crystal.
An atomic clock has to protect its electrons from the world. A nuclear clock lets the atom do that job.
Thorium-229 is the only known nucleus with a transition low enough in energy for a laser to reach. Almost every other nuclear transition needs X-rays or gamma rays.
The Beijing clock was about six times steadier over one second
Stability measures how much the tick wobbles from moment to moment. Over one second of averaging:
- Tsinghua: about 5 parts in 10 trillion
- Vienna: about 3 parts in 1 trillion, roughly six times more wobble
Averaging longer smooths out the noise. The Tsinghua clock reached a few parts in a quadrillion after about four hours of data. The Vienna clock reached the low range of parts in 100 trillion over its day-long run.
In everyday terms (our calculation, not the papers’): a clock as steady as the Tsinghua four-hour figure would drift by about one second in roughly 8 million years. The Vienna figure works out to about a second in 3 million years.
That isn’t a record. The best optical atomic clocks are steady to around one part in a billion billion (a typical value), which would be about a second over a span longer than the age of the universe. Stability also isn’t accuracy: the Tsinghua team could only pin down the absolute frequency to within about 185 kHz, mostly because of how precisely they could calibrate the crystal’s thermometer.
The Vienna clock has already been used to look for dark matter
The thorium transition depends on a delicate balance between the strong nuclear force and the electromagnetic force. If either drifted, the nucleus would feel it far more than electrons do. The Vienna team estimates its clock is 10 000 to 100 000 times more sensitive to such drifts than the ytterbium-ion atomic clock it was compared with.
Some theories suggest dark matter could be an extremely light particle that behaves like a slow wave washing through everything. Such a wave would make the constants of nature wobble very slightly, and a thorium clock would wobble with them.
The team compared its clock with the ytterbium clock at Austria’s national metrology office for about 23 hours and found no wobble. The null result still counts. For dark matter that would interact with quarks and the strong force, they report limits 100 to 1 000 times tighter than earlier atomic clock experiments. For dark matter that interacts with light, the limits match the best atomic clock comparisons.
The real promise is a precise clock inside a solid crystal
Today’s best clocks fill laboratories. A nuclear clock’s heart is a crystal at room temperature, which could one day make ultraprecise clocks small and robust enough to move around.
That matters beyond timekeeping. Clocks are also sensors. Comparing a nuclear clock with an atomic clock tests whether the laws of physics stay constant, because the two respond differently to the forces involved. The Vienna team projects that more laser power and better crystals could make the clock about 10 000 times more sensitive. That is a projection, not a result.
What the studies don’t show
- Neither clock beats the best optical atomic clocks, for steadiness or for accuracy. These are first demonstrations.
- In Vienna, the exact tick shifted by up to 1.7 kHz depending on where the laser passed through the crystal, probably because of strain. That limits how well the clock repeats from day to day.
- Both clocks are held back by weak ultraviolet light and small signals.
- The dark matter search covered about one day of data, so it rules out only certain signals over certain timescales.
- Both papers are peer reviewed. They circulated as preprints in June 2026.
A new kind of clock has started ticking
These clocks matter less for what they measure today than for what they make possible: a precise reference sealed inside a crystal, and a new way to ask whether physics itself holds still.
Treat this as the first tick, not the finish line.