Nuclear Clocks and the Search for More Precise Time

Oct 16

7 min read

Scientific laboratories have now demonstrated the first operating prototypes based on atomic nuclei rather than electrons. The devices are promising, but they are still research instruments—not replacements for the best atomic clocks yet.

The world’s most precise clocks do not measure time with gears or quartz. They use carefully controlled quantum transitions in atoms. A laser is tuned to a frequency that makes an electron move between two energy levels, and electronics count the resulting oscillations. That method already supports satellite navigation, telecommunications, power-grid synchronization, financial networks and international time standards.

Nuclear Clocks and the Search for More Precise Time

A new class of devices is now testing a different reference: the atomic nucleus itself. Two independent teams in Vienna and Beijing have reported operating thorium-229 nuclear clocks in studies published in Nature in October 2026. Their clocks can stabilize a laser by probing a transition inside the nucleus of thorium-229 embedded in calcium-fluoride crystals. 1 2 3

The result is a working demonstration of a long-standing idea. It is also a reminder that a prototype can be scientifically successful while remaining far from a finished commercial instrument.

Why use a nucleus instead of an electron?

An ordinary optical atomic clock observes an electron cloud around an atom. That cloud responds to electric and magnetic fields, temperature shifts, blackbody radiation and mechanical motion. Researchers suppress those effects with vacuum chambers, shielding, cooling and elaborate control systems.

A nucleus is much smaller and more tightly bound. Its protons and neutrons are therefore less exposed to many external disturbances that affect electrons. A nuclear transition can also occur at a very high frequency, which provides more oscillations to count in a given interval. Both features could support a more stable and accurate frequency reference. 4

The candidate that makes the concept technically possible is thorium-229. Most nuclear transitions require X-rays or gamma rays, which are difficult to produce and control with the narrow linewidth needed for a clock. Thorium-229 has an unusually low-energy excited nuclear state, called the isomer ²²⁹ᵐTh. The transition can be driven with vacuum-ultraviolet light near 148 nanometres. 4 5

That unusual energy gap is the foundation of the new clocks. It gives researchers an optical handle on the nucleus without requiring an impossible laser system.

Image: NIST/JILA, “How a Nuclear Clock Works.”

From a measured transition to a clock

A measurement of a nuclear resonance is not automatically a clock. To make a clock, researchers must complete a feedback loop:

A narrow-linewidth laser is directed at the thorium-doped crystal.
The laser frequency is scanned around the nuclear resonance.
Detectors measure a change in absorption or fluorescence when the laser reaches the correct frequency.
Electronics calculate the frequency error.
A feedback system corrects the laser and keeps it locked to the nuclear transition.

The stabilized laser then acts as the oscillator. Its frequency can be compared with another clock or used as a reference for timing and measurement.

The 2024 JILA/NIST experiment supplied several key components of this architecture. The researchers measured the thorium-229 transition with a specially developed ultraviolet laser, used an optical frequency comb to count the light cycles, and directly compared the nuclear transition with a strontium atomic clock. NIST described that work as a complete set of technologies needed for a clock, while also noting that it was not yet a fully operating nuclear clock. 4

The 2026 experiments closed the loop.

Two approaches reach operation

The Vienna group, led by researchers at the Vienna Center for Quantum Science and Technology and TU Wien, used a calcium-fluoride crystal containing thorium-229 and a continuous-wave vacuum-ultraviolet laser. Its system interrogates the crystal through absorption and feeds the measured error back to the laser. The published setup uses a 148-nanometre source, a high-finesse optical cavity, a photomultiplier detector and a frequency comb linked to a ytterbium-ion clock. 1

During continuous operation, the Vienna clock reached fractional frequency instability in the low 10⁻¹⁴ range. Its reproducibility from one day to another was less favorable, around 5 × 10⁻¹³ in the reported configuration. The researchers traced part of that limitation to local differences inside the crystal: the laser samples a small region, and microscopic strain or inhomogeneity can shift the observed line centre. 1

The Beijing team, led by Tsinghua University, used a related solid-state platform but optimized the laser and optical readout. Its continuous-wave source operates near 148.4 nanometres, while a phototube-based absorption measurement provides the error signal needed to steer the laser. The team reported a short-term fractional instability of 5 × 10⁻¹³/√(τ/s), where τ is the averaging time. Its clock reached approximately 2 × 10⁻¹⁴ over the measured averaging interval, and transitions measured in independently fabricated crystals agreed at about the 10⁻¹³ level. 2 3

The two systems do not produce identical numbers because they use different laser powers, detector arrangements, crystal properties and control strategies. Public commentary on the papers describes the Beijing device as roughly six times more stable in the short term than the Vienna device, while the Vienna experiment has the important strength of a direct comparison with a ytterbium-ion clock. 3

Image: image result associated with the nuclear-clock experimental setup.

Why thorium-229 is difficult to work with

The most promising isotope is also a demanding material. Thorium-229 is rare, radioactive and difficult to incorporate into a transparent, uniform crystal. The transition is weak, so the detected signal can be a tiny change in the transmitted vacuum-ultraviolet light. The laser must be narrow and powerful enough, and the detector must distinguish the resonance from technical noise.

The Beijing team reported producing a crystal from only 1.4 micrograms of thorium-229, equivalent to about 10 kilobecquerels, in collaboration with Chinese materials and metrology laboratories. The result shows that a practical solid-state platform can use a very small quantity of the isotope, although crystal quality and reproducibility remain central engineering problems. 3

Temperature is another source of error. In the Beijing apparatus, the crystal was held near 301 kelvin with stability better than 10 millikelvins. The Vienna group reported that its crystal temperature remained within roughly ±0.5 kelvin during operation, while the relevant transition showed a temperature dependence of about 1.8 kilohertz per kelvin near room temperature. Better thermal control and more uniform crystal growth should reduce these shifts. 1 2

These constraints explain why it is premature to say that nuclear clocks have already replaced conventional atomic clocks. The best optical atomic clocks currently have lower systematic uncertainty and a longer record of repeatability. The nuclear clocks have demonstrated a robust principle and a functioning feedback system, but their performance must improve substantially before they can define time or replace existing standards.

Image: experimental illustration found through image search.

What could nuclear clocks be used for?

If the technology becomes more stable, compact and reproducible, nuclear clocks could complement rather than immediately displace atomic clocks.

Navigation and synchronization. Satellite navigation systems depend on precisely synchronized clocks. A smaller and more resistant clock could improve timing in spacecraft, autonomous systems and deep-space missions. High-precision references could also support communication networks and distributed sensors.

Geodesy and Earth science. Clocks measure gravitational redshift: a clock at a different gravitational potential runs at a slightly different rate. More accurate clocks could map changes in height, underground water, ice sheets or volcanic structures through time and frequency comparisons.

Tests of fundamental physics. The thorium-229 transition is unusually sensitive to possible changes in fundamental constants because its low energy results from a near-cancellation between large nuclear contributions. Comparing a nuclear clock with an atomic clock could reveal tiny shifts that would point to new physics. 1 4

Dark-matter searches. Some models predict that ultralight dark matter would create periodic changes in quantities such as the fine-structure constant or nuclear parameters. The Vienna team used its clock in a search for such fluctuations. It did not detect dark matter, but the experiment showed that the nuclear clock can already operate as a precision physics instrument. 1

The likely path is incremental. Researchers must improve laser power and linewidth, crystal purity, thermal control, magnetic shielding, line-centre reproducibility and independent clock comparisons. Only after those tests will the community know whether nuclear clocks can outperform the most advanced optical clocks in real operating conditions.

Closing Thoughts

The strongest part of this story is not the promise of a clock that never loses a second. It is the shift in what scientists can control. For decades, a transition inside a nucleus was a compelling theoretical reference but an impractical laboratory target. The 2026 results show that researchers can now drive that transition with a laser, read the response and use it to stabilize the laser itself.

That achievement deserves careful optimism. Nuclear clocks are not yet the universal replacement for atomic clocks, and their present systems are still sensitive to crystal conditions and laboratory noise. Their value is already clear, though: they provide a new precision instrument with a different pattern of sensitivities. In metrology and fundamental physics, a second kind of ruler can reveal effects that the first ruler cannot see.

What is the practical takeaway here?

Nuclear clocks based on thorium-229 have moved from a proposed concept to operating laboratory prototypes. They currently trail the best atomic clocks in overall performance, but their nuclear reference may eventually offer greater resistance to environmental disturbances, smaller form factors and new capabilities for precision measurement.

What is the relevance of this?

The technology could affect future navigation, telecommunications, geodesy and time synchronization. Its deeper relevance is scientific: comparisons between nuclear and atomic clocks may test whether fundamental constants remain constant and may search for weak signals from dark matter or other physics beyond current models.

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