In a feat of precision engineering, scientists at TU Wien in Austria and Tsinghua University in China have independently constructed the first nuclear clocks—timekeepers that harness the energy transitions of an atomic nucleus rather than the electron shell. The two devices, described in separate papers published in Nature on Wednesday, mark a significant departure from the atomic clocks that have underpinned global timekeeping since 1949.
Conventional atomic clocks, such as those used in satellite navigation and telecommunications, measure time by tracking the oscillations of electrons as they jump between energy levels. The new nuclear clocks, by contrast, use a laser to flip the energy state of the nucleus itself—specifically, of thorium-229, a rare isotope whose nucleus can be excited by ultraviolet light at a wavelength of 148.4 nanometres. This is the only known nucleus with such a transition, making it uniquely suited for clockmaking.
The teams built lasers that emit precisely this ultraviolet light and aimed them at small crystals of calcium fluoride doped with thorium-229. The key achievement was that the laser remained locked to the nuclear transition, keeping time with a stability of about one part in a quadrillion (10⁻¹⁵) over the test period. That level of precision rivals the best atomic clocks, but the nuclear approach offers potential advantages in robustness and sensitivity.
Why nuclear clocks matter
Timekeeping is the silent backbone of modern infrastructure. GPS satellites rely on atomic clocks to calculate positions, and telecommunications networks use them to synchronise data streams. More precise clocks could improve these systems, making navigation more accurate and enabling faster, more reliable communication. But the real promise of nuclear clocks lies in fundamental physics.
Because the nucleus is far smaller and more isolated than the electron cloud, it is less susceptible to external disturbances such as electric and magnetic fields. This makes nuclear clocks ideal for testing the constancy of fundamental constants—like the fine-structure constant, which governs the strength of electromagnetic interactions. If these constants vary over time, nuclear clocks could detect the drift, a possibility that would have profound implications for theories of everything.
The research teams did not specify exact applications, but physicists have long speculated that nuclear clocks could help probe dark matter, gravitational waves, and the boundary between quantum mechanics and general relativity. The new devices are a proof of concept, not yet ready for deployment, but they open a new experimental window.
The development also highlights the collaborative—and competitive—nature of European and Chinese research. While the Vienna and Beijing groups worked independently, their simultaneous publication underscores the global race to build ever more precise instruments. This comes at a time when EU-China trade talks are reaching a decisive stage, and the European Parliament has recently urged a tougher EU stance on China. Yet in the realm of science, cooperation and competition often go hand in hand.
For Europe, the achievement reinforces the continent's strength in precision metrology. TU Wien has a long tradition in quantum optics and atomic physics, and this breakthrough could bolster the EU's position in the development of next-generation timekeeping. It also raises questions about the future of the international time standard, which is currently defined by caesium atomic clocks. Nuclear clocks might one day replace them, but that would require international agreement and further refinement.
The immediate impact, however, is scientific. As the authors note, nuclear clocks could become a powerful tool for exploring the fundamental laws of nature. They might also find practical uses in geodesy—measuring the Earth's shape and gravitational field—since the nuclear transition is sensitive to gravity. That could improve our understanding of sea-level rise and tectonic movements, with implications for climate monitoring and disaster preparedness.
While the two teams celebrate their success, the path to practical nuclear clocks is still long. The lasers are complex, the thorium-229 is rare and difficult to handle, and the stability needs to be improved by several orders of magnitude. But the principle is now proven, and that is a giant step.
As Europe and China navigate their complex relationship, this joint milestone—though achieved separately—serves as a reminder that scientific progress often transcends political boundaries. The next generation of clocks may well be nuclear, and they will likely be built by teams on both continents.
