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.
