For decades, nuclear clocks have existed as one of physics’ most tempting promises. A device that could keep time even more accurately than today’s best atomic clocks. Now, two independent research teams have turned this idea into reality.
Researchers in China and Europe have demonstrated working nuclear clocks based on the nucleus of thorium-229, a rare isotope whose unique properties have long made it the leading candidate for next-generation timekeeping.
The achievement represents much more than a new kind of clock. Since a nucleus is far better shielded from environmental disturbances than the electrons used in atomic clocks, nuclear clocks could eventually become the most precise timekeepers ever built.
Plus, this breakthrough could improve navigation, gravitational sensing, and tests of the laws governing the universe.
As the researchers note, “by making a laser-addressed atomic nucleus an operational clock reference, this work extends quantum metrology from electronic to nuclear transitions, and opens a new platform for compact clocks, solid-state nuclear quantum sensors and precision tests of fundamental physics.”
Turning a thorium nucleus into a clock
Modern atomic clocks keep time by measuring the frequency of light absorbed or emitted when electrons jump between specific energy levels inside an atom.
As these frequencies are extremely stable, counting their oscillations provides an exceptionally accurate way to measure time. Physicists have long envisioned building a clock using a nucleus instead of electrons. The idea is attractive because atomic nuclei are much smaller and more isolated from the outside world.
Stray electric and magnetic fields that can slightly disturb electrons have far less influence on the nucleus. In principle, that should allow a nuclear clock to achieve even greater stability and precision.
“Such a nuclear clock might rival or outperform current optical clocks based on electron-shell transitions in atoms or ions, is expected to be more robust against external perturbations, and provides enhanced sensitivity in clock-based tests of fundamental principles of physics,” the researchers explained.
However, the problem is that almost all nuclear energy transitions are far too energetic to be controlled with lasers. They typically require high-energy gamma rays rather than ordinary laser light.
Thorium-229 is a remarkable exception. Its nucleus possesses an unusually low-energy excited state that can be reached using ultraviolet laser light. For years, physicists recognized that this made thorium-229 the only known realistic candidate for a nuclear clock.
Even so, the task remained extraordinarily difficult. The required transition occurs at a wavelength of roughly 148 nanometers, deep in the vacuum ultraviolet region of the electromagnetic spectrum.
Producing stable laser light at this wavelength and precisely matching it to the nuclear transition pushed existing technology to its limits.
Two teams, one breakthrough
The two groups solved the problem using broadly similar strategies. One team, led by researchers at Tsinghua University, and another from the Vienna Center for Quantum Science and Technology, embedded thorium-229 atoms inside calcium fluoride crystals.
“Here, we implement a thorium-229 nuclear clock by stabilizing a continuous-wave laser to the 148 nm nuclear transition with rapid feedback based on continuous absorption spectroscopy,” the researchers from the Vienna Center note.
The crystal acts as a solid host that holds large numbers of thorium nuclei in place while allowing laser light to interact with them. Both groups then illuminated the crystals with highly specialized continuous-wave vacuum ultraviolet lasers tuned near the thorium nuclear transition.
Although the overall concept was similar, the teams optimized different parts of the experiment. The Chinese researchers relied on a more powerful laser system to drive the nuclear transition. The European team instead used crystals containing a higher concentration of thorium nuclei, increasing the number of nuclei available for measurement.
Once they had successfully excited the nuclei, the researchers needed to demonstrate that the system could function as a true clock rather than simply detecting the transition.
So did the system work?
The Chinese team locked the frequency of their vacuum ultraviolet laser directly to the nuclear resonance. In effect, the thorium nucleus became a frequency reference that continuously corrected the laser.
The researchers reported a fractional frequency instability approaching one part in 10 trillion after a day of operation, showing that the nuclear transition could serve as a stable timekeeping signal.
The European team chose a different test. Instead of focusing solely on clock performance, they used their nuclear clock as a scientific probe.
The researchers searched for ultralight dark matter, a hypothetical form of matter that could subtly alter the energy levels inside atomic nuclei. If such particles existed, they might produce tiny periodic shifts in the thorium transition frequency.
“We use the nuclear clock to constrain models of ultralight dark matter by searching for periodic fluctuations and slow drifts in the nuclear transition energy, on time scales between 20 sec and one day,” the Vienna team said.
No evidence of dark matter was detected. However, the experiment demonstrated that the nuclear clock was sensitive enough to rival or exceed the capabilities of the world’s best atomic clocks when searching for such effects.
Together, the two studies provide the strongest evidence yet that practical nuclear clocks are no longer a theoretical concept but an operational technology.
What happens next?
While these first-generation devices are still laboratory systems, they mark the beginning of a new era in precision measurement.
Future nuclear clocks could outperform today’s most advanced atomic clocks because the thorium nucleus is naturally protected from many sources of noise that limit current technologies.
According to the researchers, such clocks could eventually improve satellite navigation, provide more precise gravitational measurements, and enable new tests of fundamental physics.
The next major goal is to refine the technology, reduce its size and complexity, and compare multiple nuclear clocks against one another.