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."
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