Credit: Shutterstock
Scientists have turned an atomic nucleus into a working clock reference.
For decades, nuclear clocks were one of physics’ most ambitious ideas. Now, two independent research teams have demonstrated working clocks based on the thorium-229 nucleus.
Researchers at TU Wien in Austria developed one system, working with Germany’s national metrology institute, PTB. A separate team led by Tsinghua University in China independently demonstrated another operational thorium-229 nuclear clock.
This is more than another experiment showing that thorium-229 has unusual nuclear properties. The researchers have now used those properties as an active frequency reference that can control a laser. That is what turns the underlying nuclear transition into a working clock.
What makes a nuclear clock different?
Atomic clocks already measure time with extraordinary precision. They work by locking the frequency of electromagnetic radiation to a very specific transition between energy states of an atom’s electrons.
A nuclear clock changes the reference.
Instead of relying on electrons around the atom, it uses an energy transition inside the atomic nucleus itself.
That is difficult because nuclear transitions usually require much more energy than ordinary lasers can provide. Thorium-229 is an exceptional case. Its nucleus has an unusually low-energy excited state, or nuclear isomer, at about 8.4 electronvolts. Researchers can reach the corresponding transition with vacuum-ultraviolet laser light near 148 nanometres.
This makes thorium-229 uniquely useful. It provides a practical bridge between nuclear physics and precision laser technology.
What did the researchers actually build?
Both teams embedded thorium-229 nuclei inside calcium fluoride crystals and used highly specialized continuous-wave vacuum-ultraviolet lasers to interact with the nuclear transition.
The important step was feedback.
A laser’s frequency naturally drifts. For a clock to work, a reference must detect those changes and correct them.
In the Vienna system, the thorium nuclei absorb the laser light most strongly when its frequency matches the nuclear transition. If the laser moves away from that frequency, the change in absorption can be detected. A feedback system then adjusts the laser frequency and brings it back into alignment with the thorium nuclei.
In other words, the nucleus itself tells the laser when it is drifting.
TU Wien reports that its self-stabilizing system operated for more than 24 hours without intervention. After about one day of averaging, its measured fractional frequency instability approached the (10^{-15}) level.
The Chinese team independently built an operational clock using a 148.4-nanometre laser and thorium-229-doped calcium fluoride. Importantly, measurements using two separately fabricated crystals produced clock-transition frequencies that agreed at roughly the (10^{-13}) level, supporting the idea that this could become a reproducible solid-state clock platform rather than a result limited to one special crystal.
Why is this a major breakthrough?
The idea of a thorium-229 nuclear clock was formally proposed more than two decades ago, but several difficult pieces of physics and engineering had to fall into place.
Scientists had to determine the nuclear transition accurately, learn to excite it with laser light, build suitable thorium-containing materials, develop vacuum-ultraviolet laser systems, measure the transition with high precision, and finally create a feedback system that uses that nuclear transition to stabilize a clock laser.
Major progress accelerated recently. In 2024, researchers directly connected the thorium-229 nuclear transition to an optical atomic clock using a vacuum-ultraviolet frequency comb. That demonstrated the precision spectroscopy needed for a nuclear clock. The 2026 experiments have taken the next step by turning the nuclear resonance into an operational clock reference.
Why use a nucleus at all?
The nucleus is much smaller than the atom surrounding it and is relatively well shielded from many external electromagnetic disturbances.
In principle, that could make a nuclear transition less sensitive to environmental effects that can disturb electronic transitions in atomic clocks.
Thorium-229 also offers the possibility of putting large numbers of clock nuclei into a small solid crystal that works at room temperature. That could eventually lead to compact, robust nuclear clocks that still offer exceptional precision.
Potential uses include:
- Even more precise frequency and time standards
- Highly sensitive tests of whether fundamental constants change over time
- Searches for physics beyond the Standard Model
- New quantum sensors
- Tests for some forms of dark matter
The last point is not purely hypothetical. The Vienna researchers already used their nuclear clock measurements to search for changes in the thorium nuclear transition that could be associated with ultralight dark matter. Their experiment placed competitive constraints on some proposed dark-matter interactions.
Does it already beat today’s best atomic clocks?
No. The first operational thorium-229 nuclear clocks are major technological demonstrations, but they do not yet outperform the world’s best optical atomic clocks.
The current achievement is that scientists have established the nucleus as a practical, self-correcting clock reference. The researchers expect substantial improvements from stronger lasers, better thorium crystals, improved spectroscopy, and further control of environmental effects.
So claims that nuclear clocks have already replaced atomic clocks, or are already the most accurate clocks ever built, would be premature.
