The first stand-alone nuclear clock is ticking in Vienna
A decisive breakthrough: TU Wien is now home to the world’s first prototype of a new timekeeping technology that is expected to set major precision records
image:
Luca Toscani de Col (top right) working on the nuclear clock in the Vienna lab.
view moreCredit: Matthias Heisler, TU Wien
For decades, researchers around the world had been working towards this goal – and now major advances are following in rapid succession. Vienna is now home to the world’s first nuclear clock that stabilizes itself, as is also customary in atomic clocks. It has been shown that this system remains stable for more than 24 hours without intervention. This technology has the potential to significantly surpass the precision of previous atomic clocks. It is an important step towards a new kind of high-performance metrology, allowing a range of physical quantities to be measured with previously unattainable precision.
The Background: From Atomic Nucleus to Clock
For decades, it had been suspected that thorium atomic nuclei possess a very special property that makes them an ideal tool for extremely precise measurements: they have two different energy states whose energies are extremely close together. Because the energy gap between these two states is so exceptionally small, it is possible to use a laser to deliberately “switch” the atomic nucleus from one state to the other. In other atomic nuclei, the energy gaps are much larger, which is why they do not respond to laser light.
In April 2024, the team led by Prof. Thorsten Schumm at the Institute of Atomic and Subatomic Physics at TU Wien, together with the team led by Prof. Ekkehard Peik at PTB Braunschweig, succeeded for the first time in finding this long-suspected nuclear transition: they were able to show that thorium nuclei can be excited with laser beams. In the autumn of the same year, the team demonstrated that this can indeed be used to build a high-precision clock: the thorium nucleus excitation apparatus was coupled to a conventional optical atomic clock, and the thorium nuclei were used as a timekeeper.
Crucial: The Ability to Self-Stabilize
Strictly speaking, however, this was not yet the kind of nuclear clock that can be used to set precision records. “What you really want is a self-stabilizing nuclear clock,” explains Prof. Thorsten Schumm. “The basic idea is simple: you have a laser and you have thorium. The laser changes the energy state of the thorium nuclei, and the thorium nuclei are used to stabilize the frequency of the laser.”
At the heart of the nuclear clock is a crystal containing thorium atoms, produced at TU Wien. This crystal is irradiated with a laser. “The oscillation of this laser light can be used for timekeeping, but the laser frequency can shift slightly from time to time, for example due to temperature fluctuations,” explains Thorsten Schumm. “For high-precision measurements, you therefore need a mechanism to keep the laser frequency exactly stable, so that the clock continues to tick with precisely the same rhythm.”
In ordinary atomic clocks, atoms and the energy states of their electrons are used for this purpose – Thorsten Schumm’s team is now using thorium nuclei instead. These thorium nuclei absorb laser light, but only when the laser frequency is exactly right. If the frequency moves even slightly away from the optimum value, the absorption decreases measurably. In that case, the laser frequency is automatically readjusted, and the clock continues to tick precisely. This has now resulted in the first self-regulating nuclear clock – without necessarily having to rely on a conventional atomic clock.
Highly Precise, but Not Yet in the World-Record Range
“The great advantage of the new nuclear clock in Vienna is that, if you use atomic nuclei rather than atoms, much higher precision is possible in principle,” says Thorsten Schumm. Atomic nuclei are more than ten thousand times smaller than atoms; they react much more weakly to external disturbances and are therefore much more reliable timekeepers.
The precision of the new nuclear clock was investigated over the course of a day and is approximately 10 to the power of minus 15 – corresponding to an error of roughly one second in 30 million years. “This is not yet at the level of the world’s best optical atomic clocks, but for a first prototype it is a fantastic result,” says Thorsten Schumm. The precision is now to be drastically improved through several measures, such as stronger lasers and better thorium crystals.
The thorium crystal and the laser beam
Credit
TU Wien
The TU Wien team: Thomas Riebner, Luca Toscani de Col, Thorsten Schumm, Fabian Schaden, Felix Schneider, Ira Morawetz (left to right)
Credit
Matthias Heisler, TU Wien
Journal
Nature
Article Title
A thorium-229 optical nuclear clock with feedback loop
Article Publication Date
7-Oct-2026
The nuclear clock has begun ticking
Two papers by PTB/TU Wien collaboration published in Nature
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A part of the research team. From left: Luca Toscani de Col, Felix Schneider, Fabian Schaden, Ira Morawetz, Maksim Okhapkin, Sebastian Lahs, Johannes Tiedau, Vishal Lal (Foto. TU Wien)
view moreCredit: TU Wien
It was the critical breakthrough on the path to a new class of atomic clock. Some two and a half years ago, Ekkehard Peik and Thorsten Schumm achieved the world’s first ever laser excitation of an atom’s nucleus. Exciting parts of an atom with laser light is the basic principle behind every atomic clock, but until now these parts have always been electrons. Unlike electrons, however, the particles that make up atomic nuclei are crammed together tightly, and a laser’s energy is normally not enough to excite them. An exception is seen in the isotope thorium-229, and this presented a unique opportunity: “Because it has two very closely adjacent energy states, a laser is sufficient to alter the state of the atom’s nucleus,” explains Ekkehard Peik.
This knowledge led the PTB physicist in 2003 to propose the principle of a nuclear clock based on thorium. Yet it would take some 20 years before he, in collaboration with Thorsten Schumm, was able to precisely identify the required laser frequency. It was like searching for a needle in a haystack, according to Peik: “To excite the nucleus, the energy of the transition, which is known only approximately, has to be matched by a laser to within one millionth of an electron volt.”
The success of this endeavor was made possible by Thorsten Schumm and his team at the Technical University of Vienna, who had grown special thorium-doped calcium fluoride crystals incorporating large numbers of thorium nuclei that could be hit simultaneously. This approach dramatically increased their chances of finding the right laser frequency.
But there was still one crucial step missing on the path to a true clock. “The first laser system we used required an area of several square meters on optical tables and could be operated only in pulsed mode,” explains Ekkehard Peik. Joining forces with the Max Born Institute in Berlin, PTB physicists worked intensively to solve this problem. In December of 2025 they presented their solution: a compact, continuously-emitting solid-state laser with very high frequency resolution.
The first tick
This laser allowed the team to detect nuclear excitation with the required sensitivity. “In our initial experiments, we had irradiated the crystals with the laser for two minutes at a time, then blocked the laser beam and observed and measured the fluorescent light emitted by the excited nuclei,” relates Peik. “This process is slow, with a time constant of around 10 minutes. That is too slow to stabilize the laser frequency to the nuclear resonance as is required for optical clock operation.” With the new laser, far more stable in both frequency and power, it was now possible to measure the laser power absorbed by the nuclei instead of measuring the emitted fluorescence. The laser no longer needed to be blocked to detect excitation, and the signal experienced no time delay. The signal could therefore be used to tune the laser frequency to the nuclear resonance frequency over both short and long timescales.
At this point, the most important technical prerequisites had been met. Using the laser system built by PTB and the thorium-doped calcium fluoride crystals produced at TU Wien, the team in Vienna was then for the first time able to show that its clock could operate stably for over 24 hours without user intervention. During this operation, the clock was compared with an optical atomic clock at the Austrian metrology institute (BEV–PTP) in Vienna.
Remarkably rapid success
Considering the length of time required for the development of other optical clocks, we can say that the rate of progress made here has been exceptionally high, particularly for such a young field of research. In several respects, moreover, this nuclear clock has broken new ground: It is the first clock based on a nuclear resonance, and it uses a solid-state material as its timekeeping element—unlike all other optical atomic clocks currently under development, which rely on atoms or ions stored in vacuum.
The new measurement capabilities opened up by the nuclear clock have already been applied to various physical investigations, which are also presented in the two published papers. One examined the tendency of the thorium nucleus to occupy different positions within the calcium fluoride crystal lattice. This results in a nuclear resonance spectrum that provides information about the microscopic structure of the system—information that can be used to optimize the clock. Another involved an initial test conducted to search for possible couplings of the Th-229 nucleus to certain forms of dark matter. This represents a preliminary application of the nuclear clock to address a fundamental and as yet unanswered question in physics.
Global recognition
Even before appearing in Nature, the outcomes of this research had generated strong interest at scientific conferences—further fueled by reports, just weeks after the Vienna results were announced, of similar experiments by a collaboration of Chinese institutes led by Tsinghua University. Within just a few years, the thorium-229 nuclear clock research begun in Braunschweig and Vienna has attained worldwide recognition.
And the upcoming future appears promising, as Ekkehard Peik reports: “The stability we have now achieved with the nuclear clock does not yet set any world records when compared to that of the best optical atomic clocks based on trapped atoms or ions. We can, however, now clearly identify where the current technical limits lie and which properties of the laser systems and thorium crystals need further improvement in order to make our clock truly competitive.”
es/ptb
PTB researcher Ekkehard Peik with the nuclear clock experiment at PTB.
Credit
(Photo: PTB)
The original scientific publications
I. Morawetz et al.: Continuous-wave laser absorption spectroscopy of the thorium-229 nucleus. Nature 657, 626-631 (2026), DOI: 10.1038/s41586-026-11011-7
L. Toscani et al.: A thorium-229 optical nuclear clock with feedback loop. Nature 7 October, 2026, DOI: 10.1038/s41586-026-11084-4
Journal
Nature
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
A thorium-229 optical nuclear clock with feedback loop
Article Publication Date
7-Oct-2026
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