Physicists have built a nuclear clock for the first time

This has long been a dream of physicists: people have been talking about nuclear clocks for decades, but no one had managed to build one. Now it has been achieved – and in two different laboratories, using different methods.

An important caveat: both studies have so far been published as preprints — that is, scientific articles that have not yet undergone independent peer review by other scientists. This is a significant step, but it is too early to draw definitive conclusions.

Details

To understand what the breakthrough is, we need to recall how accurate clocks work in the first place. Any clock counts something: a pendulum, quartz oscillations, the second hand. The more stable what we are counting is, the more accurate the clock.

The most accurate clocks today are atomic clocks. They count the oscillations of electrons as they ‘jump’ between energy levels within an atom. These oscillations are incredibly stable, which is why atomic clocks are only one second off in billions of years.

Nuclear clocks work in a similar way, but instead of counting electrons, they measure changes within the atom’s nucleus itself — where the protons and neutrons are located. And therein lies the main advantage: the nucleus is hidden deep inside the atom and is much better protected from external interference — random electric and magnetic fields. This means that clocks based on it can be even more accurate and reliable than atomic clocks.

The problem was that almost no nucleus is suitable for this. Of the entire periodic table, only one element is suitable — thorium-229. The ‘energy gap’ in its nucleus turned out to be exactly the right size to be triggered and measured using a laser. But there was a complication here too: a special laser is needed, operating in what is known as vacuum ultraviolet — a type of radiation that is very difficult to produce and hard to control.

Both teams solved the problem in the same way: they ‘implanted’ thorium nuclei into calcium fluoride crystals and directed a precisely tuned laser at them. From there, their approaches diverged. The Chinese team from Tsinghua University used a more powerful laser. The European team from the Vienna Centre for Quantum Science and Technology used a crystal with a higher concentration of thorium.

They also tested the results in different ways. The Chinese demonstrated that their device consistently ‘locks’ the laser frequency, coupling it to the nucleus with astonishing precision — the deviation was approximately one part in 10 trillion over a day of operation. The Austrians, meanwhile, immediately put their clock to work in the search for dark matter — the mysterious substance believed to make up the majority of the Universe’s mass. They found no signs of dark matter, but the device’s sensitivity proved to be no worse, and in some respects even better, than that of the best atomic clocks.

Why this matters

Nuclear clocks are not just ‘more accurate clocks’. They are a new tool for exploring the deepest mysteries of physics.

For example, they can be used to check whether fundamental constants — the basic values on which the laws of nature are based — are truly constant. If these constants change even slightly over time, ultra-precise clocks are capable of detecting it.

And if the technology can be scaled down to a compact size, nuclear clocks could find applications in navigation systems, in instruments that measure gravity, and in experiments that are simply beyond the reach of existing equipment today.

That said, it is worth repeating: this is only the beginning. The clock is working in the laboratory and has passed its initial tests — but there is still peer review, refinement and miniaturisation to come.

Background

Time measurement is one of the most precise fields of science, and it has a direct impact on everyday life. The accuracy of clocks determines, for example, the operation of satellite navigation systems such as GPS: an error of just a few nanoseconds translates into metres of error on the ground.

The idea of a nuclear clock has been around for a long time, but turning it into a working device was hindered precisely by the technical complexity of using the right type of laser. The breakthrough became possible after physicists in recent years learned how to precisely ‘hit’ the thorium-229 nucleus with a laser. The current work represents the next logical step: moving from simply ‘observing’ the nuclear transition to building a real clock based on it.

The fact that two teams independently achieved the result using different methods is particularly important. In science, it is precisely reproducibility – when other researchers arrive at the same conclusion – that serves as the main indicator of reliability.

Source

The work is based on two independent studies published as preprints on arXiv in 2026.

The first is by Luca Toscani De Cola’s team at the Vienna Centre for Quantum Science and Technology (Austria): ‘A thorium-229 optical nuclear clock with feedback loop’, DOI: 10.48550/arxiv.2606.04997. This group used a crystal with a high concentration of thorium and applied the clock to search for ultralight dark matter.

Second — the team led by Beichen Huang from Tsinghua University (China): ‘A nuclear clock based on 229Th’, DOI: 10.48550/arxiv.2606.08870. This group used a more powerful laser and demonstrated stable operation of the clock with an accuracy of about one part in 10 trillion per day.

In both cases, thorium-229 nuclei were embedded in calcium fluoride crystals and probed by a laser with a wavelength of around 148 nanometres.