For about 70 years, the world’s best clocks have kept time by watching atoms. Now physicists have shown that the heart of an atom, its nucleus, can do the job too, and possibly much better one day.
In two papers published Oct. 7 in the journal Nature, two research groups described the first working “nuclear clocks.” Both are built around a rare form of the element thorium, called thorium-229. One of them, from the Vienna University of Technology (TU Wien) in Austria, goes a step further: it keeps itself in tune, with no help from a conventional atomic clock.
These are early prototypes, and they are not yet as stable as today’s record-holding atomic clocks. But the researchers say the approach has room to improve dramatically, and it could open up new kinds of measurements in physics.
What Was Announced
- Two clocks, two teams: A TU Wien group led by physicist Thorsten Schumm and a second group led by Shiqian Ding each reported a working thorium-229 clock in Nature.
- A self-steering clock: The Vienna device uses a feedback loop to keep its laser locked to the thorium nuclei on its own. It ran steadily for more than 24 hours without intervention.
- Precision so far: Over a day, the Vienna team measured a relative precision of about 1 part in 1015. Its university says that works out to roughly one second of error in 30 million years, though that is an extrapolation, not a 30-million-year test.
- Not the champion yet: Both teams’ clocks are currently less stable than the best optical atomic clocks, according to Nature’s summary of the work.
How Clocks Keep Time, in 60 Seconds
Every clock counts something that repeats. A grandfather clock counts the swings of a pendulum. A quartz watch counts the vibrations of a tiny crystal. Atomic clocks count something far steadier: the frequency of light (or microwaves) that atoms absorb when an electron jumps between two energy levels.
That frequency is set by nature, so it is the same for every atom of a given element. That is why the official definition of a second is based on atoms of cesium. The catch is that electrons sit on the outside of an atom, where stray electric and magnetic fields can nudge them slightly. Scientists go to great lengths to shield atomic clocks from those disturbances.
A nucleus is a different story. According to TU Wien, atomic nuclei are more than 10,000 times smaller than atoms, which makes them much less sensitive to outside interference. In principle, that means a clock that “ticks” with the nucleus could be more precise than one that ticks with electrons.
Why Thorium-229 Is Special
There has long been a practical problem: most nuclei only switch between energy states when hit with very high-energy radiation, such as gamma rays, which lasers cannot easily produce or control. Thorium-229 is the exception. Two of its nuclear energy states are unusually close together, close enough that an ultraviolet laser can nudge the nucleus from one to the other.
Researchers spent decades trying to pin down that transition. In April 2024, the TU Wien team and colleagues at Germany’s national metrology institute, PTB Braunschweig, reported that they had excited the thorium nucleus with a laser for the first time. Later in 2024, the team linked that setup to a conventional optical atomic clock, using the atomic clock as the reference that kept everything in step.
Both of the new clocks embed thorium-229 in a calcium fluoride crystal, according to a press release from Nature’s publisher. The thorium crystal used in Vienna was made at TU Wien.
The light needed sits in the vacuum ultraviolet, at a wavelength of roughly 148 nanometers, far shorter than anything the human eye can see. Building lasers that work reliably at that wavelength is one of the main engineering hurdles, and it is a big reason the field has moved step by step: first find the transition, then measure it against an atomic clock, and only now build a clock that runs on the nucleus alone.
What Makes the Vienna Clock “Self-Stabilizing”
The trick is a feedback loop. The thorium nuclei absorb laser light only when the laser is tuned to exactly the right frequency. If the laser starts to drift, the nuclei absorb less light, and the drop is measurable. The system detects that and steers the laser back on target, again and again.
In other words, the nuclei themselves become the reference. “What you really want is a self-stabilizing nuclear clock,” Schumm said in the university’s announcement. Calling it a first prototype, he added that “for a first prototype it is a fantastic result.”
The Nature paper from the Vienna group is titled “A thorium-229 optical nuclear clock with feedback loop.” The second paper, “A nuclear clock synchronized to 229Th,” comes from the team led by Ding, which has been reported as based in Beijing.
How Good Is It?
To put “1 part in 1015” in perspective: it is like measuring the distance from New York to Los Angeles, about 2,450 miles, and being off by only about the width of two strands of DNA. Precision at that level is routine for top laboratory clocks, but it is remarkable for the very first device of a brand-new kind.
The Vienna clock reached a relative precision of about 10−15 when measured over a day. That is impressive for a first attempt, but the best optical atomic clocks still do better. TU Wien is upfront that its device does not yet match them.
The longer-term promise is the bigger story. According to the Nature press summary, Schumm’s team estimates that further work could improve stability by around four orders of magnitude, meaning roughly 10,000 times. The planned upgrades are fairly concrete: more powerful lasers and higher-quality thorium crystals.
Why It Matters Beyond Telling Time
Most people will never own a nuclear clock. But ultra-precise clocks quietly support a lot of modern life. Satellite navigation systems such as GPS, for example, work out your position by comparing timing signals, so better clocks generally mean better measurements. The researchers themselves frame the payoff more broadly: a new tool for measuring physical quantities with unprecedented precision.
Some of the uses already being discussed include:
- Testing the laws of physics: Very stable clocks can check whether supposedly fixed constants of nature change over time.
- Hunting for dark matter: According to Nature’s summary, one experiment by Schumm’s group used its clock to search for dark matter at a sensitivity competitive with the best atomic clocks. It did not report a detection.
- Navigation and geophysics: Coverage of the work has pointed to possible future portable clocks for deep-space navigation and for mapping tiny gravity differences underground.
The Catch
- These are first-generation prototypes. Neither is yet more stable than the best atomic clocks.
- The “one second in 30 million years” figure is an extrapolation from a one-day measurement, not a long-term test.
- The big gains depend on engineering work that has not been done yet, especially better lasers and crystals.
- Headlines saying nuclear clocks “could make atomic clocks obsolete” are getting ahead of the evidence. The researchers say nuclear clocks could eventually surpass atomic clocks, not that they already do.
What Comes Next
The Vienna team plans to boost laser power and improve its thorium crystals, which it hopes will substantially raise precision. Having two independent groups report working clocks at the same time also matters: in physics, results become much more convincing when different teams, using different setups, can reproduce them.
For now, the milestone is simple to state. After decades of searching for a usable nuclear “tick,” scientists have built clocks that actually run on one.
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Sources
- ScienceDaily (Vienna University of Technology): Scientists build a nuclear clock that could make atomic clocks obsolete
- TU Wien via idw: Press release on the self-stabilizing thorium nuclear clock
- Nature press release: The first working nuclear clocks
- Nature: A thorium-229 optical nuclear clock with feedback loop
- Nature: A nuclear clock synchronized to 229Th
- Interesting Engineering: World’s first self-stabilizing nuclear clock