Thorium-229 nuclear clocks take a major step beyond atomic clocks

A decades-long push in chronometry has reached a milestone that could shift the boundaries of timekeeping.

Physicists have now created working clocks using thorium-229 atoms, where the reference signal comes not from electrons, but from energy changes within the atomic nucleus itself.

Notably, this feat has been achieved independently twice: once by a team in Europe and once by a team in China.

Both groups have set out their results in arXiv preprints.

"The system presented in this work," writes the group led by physicist Luca Toscani De Col at the Technical University of Vienna, "constitutes the first implementation of a nuclear clock that operates as a stand-alone device."

From atomic clocks to nuclear clocks

Atomic clocks have existed since the 1950s, and their accuracy is so extraordinary that they would not drift by even a single second over billions of years.

They work by counting the exceptionally regular ‘ticks’ produced when electrons move between energy levels under laser stimulation.

A nuclear clock, proposed in 2003, would instead keep time by monitoring shifts between energy states inside the nucleus. Turning that concept into a practical device has been far harder, largely because nuclear transitions usually demand far higher energies than electronic transitions, beyond what most laser systems can deliver.

Why thorium-229 is the key

There is, however, a compelling motivation to pursue nuclear clocks.

Because electrons reside in the outer parts of the atom, they - and the clocks that rely on them - are more exposed to environmental disturbances.

The nucleus, in contrast, sits buried at the atom’s centre and is much less affected by external interference.

In principle, that added isolation could allow nuclear clocks to surpass the stability of today’s atomic clocks. It could also make them valuable instruments for investigating dark matter and for testing whether the fundamental constants of nature might change.

As the 2003 proposal highlighted, thorium-229 is a particularly strong candidate because its nuclear transition is unusually low in energy, placing it within reach of precision laser spectroscopy.

In 2024, researchers in Austria and Germany reported multiple advances: they induced the thorium-229 transition and then succeeded in making it ‘tick’.

The remaining challenge was to turn that ticking behaviour into a clock capable of timekeeping.

Two independent thorium-229 nuclear clocks: Europe and China

That is the step the two new teams have now taken.

In both cases, the clocks were built around thorium-229 nuclei embedded in calcium fluoride crystals, and the nuclei were probed using vacuum-ultraviolet laser light. From that shared starting point, the projects then pursued different goals.

Europe: a stand-alone nuclear clock and dark matter constraints

The European group built a fully self-contained, stand-alone clock, using the thorium nucleus as a continuous reference to stabilise a laser’s frequency.

To validate performance, they compared the device with a well-established ytterbium-ion atomic clock, showing stable, long-term operation.

They also employed the clock to look for signatures of hypothetical ultralight dark matter, and in doing so placed new constraints on several suggested models.

"Drawing benefit from the enhanced sensitivity of the thorium-229 transition, these constraints compete with the best atomic clocks concerning dark matter coupling to photons and go beyond previous measurements regarding coupling to the strong force and quarks," they write in their paper.

China: reproducibility in solid-state nuclear clocks

The Chinese team, led by physicist Beichen Huang at Tsinghua University, concentrated on a different obstacle: whether a solid-state nuclear clock’s ticking would be consistent from one crystal to another.

They evaluated their clock using two separately produced crystals to check for agreement.

The frequencies from the two clocks matched very closely, directly addressing a central difficulty for solid-state nuclear time standards.

If the crystal surroundings were to shift the nuclear frequency in an unpredictable way, each clock would need its own bespoke calibration.

Instead, the near-identical results indicate that nuclear clocks could, in time, become reproducible standards rather than unique laboratory builds.

"By making a laser-addressed atomic nucleus an operational clock reference," the Chinese team writes, "this work extends quantum metrology from electronic to nuclear transitions, and opens a new platform for compact clocks, solid-state nuclear quantum sensors, and precision tests of fundamental physics."

The new instruments do not yet exceed the performance of the very best atomic clocks - which, in fairness, have had a 70-year head start - but they demonstrate that nuclear clocks have moved beyond theory.

They can operate, and they do so outside purely conceptual designs.

And if Technical University of Vienna physicist Thorsten Schumm’s 2024 prediction turns out to be right, nuclear clocks could even overtake today’s top atomic clocks within only a few years.

The clocks have been described in preprints uploaded to arXiv, here and here.