Researchers from TU Wien, Vienna, developed the first nuclear clock, according to a study published in Nature.
Scientists have been trying to build a nuclear clock for decades, but a team from TU Wien in Vienna finally managed to do it. The system relies on an atomic nucleus, which can be switched from one state to another using a laser. The team combined a high-precision optical atomic clock with a high-energy laser system and a crystal containing thorium atomic nuclei. The thorium atomic nucleus is the time-keeping device, making it the world’s first nuclear clock.
“With this first prototype, we have proven: Thorium can be used as a timekeeper for ultra-high-precision measurements. All that is left to do is technical development work, with no more major obstacles to be expected,” says Prof. Thorsten Schumm.
Every clock needs a timekeeper. For example, in a wall clock, the pendulum’s swing works to keep the time. Atomic clocks use electromagnetic wave oscillation, with oscillations counted as time intervals. However, the frequency of a laser changes slightly over time, and it needs to be readjusted.
“That’s why, in addition to the laser, you need a quantum system that reacts extremely selectively to a very specific laser frequency,” explains Thorsten Schumm (TU Wien). The authors suggest caesium or strontium atoms. When they are hit with laser light at a specific frequency, the electrons of these atoms move between two quantum states, and this can be used for timekeeping.
However, if this could be done with an atomic nucleus, it would allow for even greater precision. Atomic nuclei are smaller than atoms and react less to disturbances, such as electromagnetic fields from outside. The main problem is that switching atomic nuclei between two states requires a thousand times more energy than the photons of a laser can deliver.
The only exception is thorium. “Thorium nuclei have two states of very similar energy, so you can switch them with lasers,” explained Prof. Schumm. “But for this to work, you have to know the energy difference between these two states very precisely. For many years, research teams around the world had been searching for the exact value of this energy difference in order to be able to switch thorium nuclei in a targeted manner – we were the first to succeeded, this is the result we published in April.”
In simple words, ultra-short infrared laser pulses consisting of a series of different infrared frequencies cause xenon atoms to produce UV light in a very precisely predictable way. This UV light is then sent onto a crystal containing a thorium nucleus. “This crystal is the central element of the experiment,” says Thorsten Schumm. “It was produced at TU Wien, in Vienna, and several years of development work were required to develop the necessary expertise.”
This first prototype doesn’t increase precision compared to atomic clocks, but that was not the intention. “Our aim was to develop a new technology. Once it’s there, the increase in quality comes naturally, that has always been the case,” said Thorsten Schumm. “The first cars weren’t any faster than carriages. It was all about introducing a new concept. And that’s exactly what we’ve now achieved with the nuclear clock.”
The authors believe this technology should make more accurate measurements in the future. From geology to astrophysics, thorium-based technology could deliver important advances. This extreme precision could be used to study the fundamental laws of nature and assess whether the constants of nature are perhaps not perfectly constant at all but maybe change in space and time.
Zhang, C., Ooi, T., Higgins, J.S. et al. Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock. Nature 633, 63–70 (2024). https://doi.org/10.1038/s41586-024-07839-6