Thorium-229 Nuclear Clocks Could Transform Timekeeping

A long-awaited advance in chronometry could reshape the boundaries of timekeeping.

Physicists have created working clocks using thorium-229 atoms. Rather than relying on electron oscillations, these devices use the repeated energy changes within atomic nuclei.

The result is especially notable because two separate groups, one in Europe and the other in China, have independently reached this milestone.

Both groups have reported their landmark findings in arXiv preprints.

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

From atomic clocks to thorium-229 nuclear clocks

First developed in the 1950s, atomic clocks are so accurate that they would not lose a single second over billions of years.

They measure time through the highly regular ‘ticking’ of electrons as laser stimulation moves them between energy states.

The concept of a nuclear clock was first suggested in 2003. Instead of following electrons, it would time energy transitions in an atom’s nucleus. This has been much harder to accomplish, since nuclear transitions generally demand far more energy than electronic ones and therefore lie beyond the capabilities of most lasers.

There is, however, a compelling reason to pursue nuclear clock technology.

Because electrons occupy an atom’s outer regions, they – and the atomic clocks that use them – are more exposed to environmental effects.

A nucleus, on the other hand, sits deep within the atom’s centre and is considerably less vulnerable to external disruption.

In principle, this could make nuclear clocks more stable than current atomic clocks. They could also provide potent ways to investigate phenomena including dark matter and potential variation in nature’s fundamental constants.

As described in the 2003 paper, thorium-229 is a particularly promising candidate because its transition state has exceptionally low energy, placing it within the scope of precision laser spectroscopy.

In 2024, teams in Austria and Germany achieved several advances: they induced the thorium-229 energy transition and then made it ‘tick’.

The remaining task was to turn that ticking into a functioning timekeeping clock.

That is now what the two research groups have accomplished.

How the new thorium-229 clocks work

Each team based its clock on thorium-229 nuclei set into calcium fluoride crystals and examined with vacuum-ultraviolet laser light. Their methods then took different paths.

The European group’s apparatus functioned as a fully self-contained clock, with the thorium nucleus continuously stabilising a laser frequency.

Its researchers benchmarked the device against a well-established ytterbium-ion atomic clock, showing both stable performance and long-term operation.

They further used it to look for evidence of hypothetical ultralight dark matter, establishing fresh limits for 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.

The Chinese group, led by Tsinghua University physicist Beichen Huang, pursued a somewhat different objective.

They evaluated their clock using two crystals produced independently, to determine whether their ticking remained consistent.

The two clocks produced almost identical frequencies, tackling an important obstacle for solid-state nuclear clocks.

Were the crystal surroundings to change the nuclear frequency in an unpredictable way, every device would need separate calibration.

Instead, the strong match indicates that nuclear clocks could ultimately serve as reproducible standards, rather than isolated laboratory demonstrations.

Nuclear clocks as reproducible standards

"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."

These new instruments do not yet surpass the leading atomic clocks – which, admittedly, have had a 70-year head start – but they establish that nuclear clocks are more than a theoretical ambition.

They are capable of operating in real-world conditions.

Moreover, if a 2024 prediction by Technical University of Vienna physicist Thorsten Schumm is borne out, they could exceed today’s finest atomic clocks in only a few years.

The clocks are detailed in preprints posted to arXiv, here and here.