In short: A nuclear clock takes its beat not from the electron shell of an atom but from a transition inside the atomic nucleus itself, in the isotope thorium-229. In October 2026, teams from TU Wien and Germany's national metrology institute PTB, and from Tsinghua University in Beijing, described the first working prototypes in Nature. They are not yet more accurate than the best atomic clocks. Their promise rests on the fact that a nucleus responds far more weakly to its surroundings than an electron shell does.
When a watchmaker regulates a wristwatch, they listen to it. The balance wheel beats five to ten times a second, and a well-adjusted watch gains or loses a few seconds a day. That is a relative deviation of a few parts in a hundred thousand. The best atomic clocks in the world are roughly ten trillion times more accurate. Even so, physicists have spent decades looking for an even steadier timekeeper. They have found it inside the atom, one level deeper than before.
What ticks inside a nuclear clock?
Every clock counts oscillations. Since 1967 the second has been defined by the caesium atom: as the duration of 9,192,631,770 oscillations of the radiation belonging to a particular transition in its electron shell. Optical atomic clocks work with light and therefore with much faster oscillations, about 429 trillion per second for strontium. The more finely a second is divided, the more precisely it can be counted, provided the beat itself stays steady.
A nuclear clock instead uses two states of the atomic nucleus. Such nuclear states are normally separated by thousands to millions of electronvolts; reaching them would take gamma rays, for which no precise lasers exist. Thorium-229 is the known exception. Its first excited state lies only about 8.4 electronvolts above the ground state. That corresponds to ultraviolet light with a wavelength of 148 nanometres and about 2,020 trillion oscillations per second. In 2024 a group at JILA in Boulder determined this frequency to within a few kilohertz by comparing it with a strontium clock.
Why did the search take almost 50 years?
Nuclear physicists found the first hints of this unusually low nuclear state as early as 1976. In 2003 Ekkehard Peik and Christian Tamm of PTB proposed using it as a clock. The problem was the search itself: the energy was known only roughly, and a laser had to hunt for an extremely narrow line across a wide range, rather like looking for one station on a dial whose position you know only vaguely, with a receiver that hears only a hair-thin slice at a time. In 2016 a group in Munich achieved the first direct detection of the excited state.
In spring 2024 researchers at PTB and TU Wien excited the thorium nucleus with laser light for the first time. The nuclei sat in calcium fluoride crystals with thorium built into them. This was still spectroscopy: the crystal was illuminated, then the experiment waited for an afterglow that lasts minutes. A clock, however, needs a feedback loop that measures and corrects continuously.
That is exactly what the two papers published in Nature in October 2026 show. In Vienna, a continuously emitting laser at 148 nanometres is sent through the thorium crystal. The quantity measured is how much light the nuclei absorb. If the laser drifts away from the resonance, absorption falls and the electronics pull it back. This clock ran for more than 24 hours without intervention and was compared with an optical ytterbium ion clock. The group in Beijing independently demonstrated a similar clock; there, the frequencies of two separately produced crystals agreed to about one part in ten trillion.
Is the nuclear clock already more accurate than the atomic clock?
No, not yet, and the researchers say so themselves. After one day, the Vienna clock reaches a relative stability of the order of 10−15. TU Wien translates this into an error of about one second in 30 million years. The best strontium clock at JILA, by contrast, already knows its systematic shifts to within 8 × 10−19, roughly a thousand times better. The two numbers do not describe exactly the same quantity, but they show the gap. Ekkehard Peik stresses that the stability now achieved does not yet set any world records.
The reason for the high expectations lies in the physics of the nucleus. It is more than ten thousand times smaller than the atom and surrounded by the atom's electrons. Electric and magnetic stray fields, which atomic clocks must laboriously shield and correct for, act on it far more weakly. Theoretical work therefore considers an inaccuracy close to 10−19 possible for single thorium ions held in a trap. The crystal version has a different appeal: it contains a great many nuclei at once and works at room temperature, without individual particles having to be held in a trap. That could make robust, compact clocks possible. Whether it can reach the accuracy of the best laboratory clocks is an open question; the crystal also shifts the resonance, and these effects are being measured now.
What does a clock measure when it looks deeper?
The history of timekeeping can be read as a retreat inward. First the sky served as a clock, until it became clear that Earth rotates irregularly. Then came the pendulum, the balance wheel and quartz, and finally the atom's electron shell. Each step sought a timekeeper less disturbed by the world. The nuclear clock continues this movement into a space about ten thousand times smaller than the atom.
It is precisely this shielding that makes it an instrument for the world outside. When two very good clocks run differently, either one of the clocks is at fault or nature is. Optical atomic clocks already measure that time passes minutely faster just one millimetre higher in Earth's gravitational field, an effect that also has to be accounted for in GPS. According to an analysis from 2025, the thorium transition responds about a thousand times more sensitively than the best atomic clocks to a possible change in the fine-structure constant, the number that sets the strength of the electromagnetic force. The Vienna group has already used its clock to search for certain forms of dark matter. It found nothing, but some of its limits are already tighter than earlier ones.
This shifts the question a clock asks. No clock measures time as such. It compares oscillations, and the 2024 result was literally a ratio: nucleus against shell, 4.707072615078. In the workshop, too, we regulate a watch against a better one. The nuclear clock opens the possibility of comparing timekeepers that respond differently to different forces of nature: the shell is governed by the electromagnetic force, the nuclear transition also by the strong nuclear force. If their ratio stays the same over years, we will know more precisely than ever that our measure of a second stands on solid ground. If it changes, we will have learned something new about nature.
A nuclear clock gives no one an extra minute. What it refines is the question of what we rely on when we say that two things lasted equally long.
Sources
L. A. Kroger and C. W. Reich, Features of the low-energy level scheme of 229Th as observed in the α-decay of 233U, Nuclear Physics A 259 (1976)
E. Peik and Chr. Tamm, Nuclear laser spectroscopy of the 3.5 eV transition in Th-229, Europhysics Letters 61 (2003)
C. J. Campbell et al., Single-ion nuclear clock for metrology at the 19th decimal place, Physical Review Letters 108 (2012)
L. von der Wense et al., Direct detection of the 229Th nuclear clock transition, Nature 533 (2016)
J. Tiedau et al., Laser excitation of the Th-229 nucleus, Physical Review Letters 132 (2024)
C. Zhang et al., Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock, Nature 633 (2024)
A. Aeppli et al., Clock with 8 × 10⁻¹⁹ systematic uncertainty, Physical Review Letters 133 (2024)
T. Bothwell et al., Resolving the gravitational redshift across a millimetre-scale atomic sample, Nature 602 (2022)
K. Beeks et al., Fine-structure constant sensitivity of the Th-229 nuclear clock transition, Nature Communications 16 (2025)
I. Morawetz et al., Continuous-wave laser absorption spectroscopy of the thorium-229 nucleus, Nature 657 (2026)
L. Toscani De Col et al., A thorium-229 optical nuclear clock with feedback loop, Nature 658 (2026)
B. Huang et al., A nuclear clock synchronized to 229Th, Nature 658 (2026)
TU Wien, Die erste selbststabilisierende Atomkernuhr tickt in Wien (press release in German), 7 October 2026