For decades, physicists have dreamed of creating a clock so accurate that it could redefine how humanity measures time. Now, that dream has become a reality. Two independent research teams—one in China and the other in Europe—have successfully built working nuclear clocks, achieving a milestone that scientists have pursued for years.
The breakthrough was made by researchers led by Beichen Huang at Tsinghua University and Luca Toscani De Col at the Vienna Center for Quantum Science and Technology in Austria. Their studies, recently published as preprints on arXiv, demonstrate that nuclear clocks are no longer just a theoretical idea. They are now real devices capable of measuring time with extraordinary precision.
Many scientists believe these clocks could eventually outperform even the best atomic clocks in existence today, opening the door to revolutionary advances in science, navigation, and our understanding of the universe.
Why Accurate Timekeeping Matters
Timekeeping may seem simple, but it is one of the foundations of modern technology. Systems such as GPS navigation, internet communications, financial networks, and scientific experiments all depend on highly accurate clocks.
Today, the most precise clocks are atomic clocks. These devices keep time by measuring the frequency of light emitted when electrons move between different energy levels inside atoms.
Because these frequencies are extremely stable, atomic clocks can measure time with incredible accuracy. Some of the best atomic clocks would lose or gain less than a second over billions of years.
Yet physicists have long believed that an even better type of clock could be built—a nuclear clock.
What Is a Nuclear Clock?
A nuclear clock works on a principle similar to an atomic clock, but with one important difference.
Instead of measuring transitions in electrons surrounding an atom, a nuclear clock measures transitions that occur inside the atom’s nucleus itself. The nucleus contains protons and neutrons, which can also occupy different energy states.
When a nucleus changes from one energy state to another, it emits or absorbs energy at a very specific frequency. By measuring this frequency, scientists can use it as an incredibly stable timekeeping signal.
The major advantage is that the nucleus is much better protected from outside disturbances than electrons are.
Electric fields, magnetic fields, temperature changes, and other environmental effects can slightly influence electron behavior. The nucleus, however, is deeply shielded within the atom, making it far less sensitive to these disturbances.
This means a nuclear clock has the potential to be even more stable, accurate, and reliable than atomic clocks.
The Special Role of Thorium-229
Although the idea sounds straightforward, creating a nuclear clock has proven extremely difficult.
The biggest challenge is finding a suitable nucleus.
Most atomic nuclei require enormous amounts of energy to trigger transitions between nuclear states. Such transitions usually involve gamma rays, which are difficult to control and measure precisely.
Fortunately, one remarkable exception exists: thorium-229.
Thorium-229 possesses a unique nuclear transition with an unusually low energy level. This energy is small enough that scientists can stimulate it using laser light rather than high-energy gamma radiation.
Among all known elements, thorium-229 is currently the only nucleus suitable for building a practical nuclear clock.
Because of this unique property, researchers around the world have spent years trying to harness thorium-229 for precision timekeeping.
A Difficult Technical Challenge
Even with thorium-229, the task was far from easy.
The required laser light lies in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum, around 148 nanometers.
Generating and controlling light at these wavelengths is extremely challenging. Standard optical equipment does not work well in this region, and the light can be absorbed by air and many materials.
Scientists needed highly specialized equipment capable of producing stable vacuum ultraviolet laser beams while maintaining exceptional precision.
For years, these technical obstacles prevented researchers from creating a working nuclear clock.
Now, both teams have finally overcome them.
Two Teams, One Historic Achievement
Although working independently, both research groups adopted a similar overall strategy.
They embedded thorium-229 nuclei inside crystals made of calcium fluoride. These crystals provide a stable environment for the nuclei while allowing researchers to probe them using laser light.
The teams then directed finely tuned continuous-wave lasers at the crystals, targeting the unique nuclear transition of thorium-229.
While the basic approach was similar, each group optimized its system differently.
The Chinese team used a more powerful laser system to interact with the thorium nuclei.
Meanwhile, the European team employed crystals containing a higher concentration of thorium atoms, increasing the number of nuclei available for measurement.
Both methods successfully demonstrated functioning nuclear clock systems.
Different Ways to Prove Success
The two groups also chose different methods to verify that their clocks worked.
The Chinese researchers focused on stabilizing their laser frequency.
They successfully locked their vacuum ultraviolet laser to the thorium nuclear transition. This allowed the laser frequency to remain extremely stable over long periods.
Their measurements achieved a fractional frequency instability approaching one part in ten trillion after a day of operation—a remarkable level of precision.
The European researchers took a different path.
Instead of simply testing clock performance, they used their nuclear clock as a scientific instrument to search for evidence of ultralight dark matter.
Dark matter remains one of the biggest mysteries in modern physics. Scientists believe it makes up much of the universe’s mass, yet it has never been directly detected.
If ultralight dark matter exists, it could cause tiny periodic changes in the energy levels of thorium-229.
The team searched for these subtle shifts but found no evidence of dark matter.
However, the experiment demonstrated that the nuclear clock’s sensitivity matched—or even exceeded—that of today’s best atomic clocks.
Why This Breakthrough Matters
The creation of working nuclear clocks is more than just an improvement in timekeeping.
It represents a completely new tool for exploring the laws of nature.
Because nuclear clocks are expected to be extraordinarily precise, they could help scientists test some of the deepest questions in physics.
For example, physicists often assume that fundamental constants—such as the strength of electromagnetic forces—remain unchanged throughout the universe and across time.
But what if these constants are slowly changing?
Nuclear clocks may be sensitive enough to detect tiny variations that current instruments cannot observe.
Such discoveries could fundamentally change our understanding of the universe.
Future Applications
Although these first nuclear clocks are still experimental devices, researchers are optimistic about their future.
As the technology improves and becomes more compact, nuclear clocks could be used in many practical applications.
Future navigation systems could achieve unprecedented accuracy.
Gravitational sensing technologies could become more sensitive, helping scientists study Earth's interior, monitor geological activity, and detect subtle changes in gravity.
Space missions could benefit from ultra-precise timing systems that improve navigation across vast distances.
Nuclear clocks could also support new tests of Einstein’s theories and provide powerful tools for searching for dark matter and other unknown forms of physics.
A New Era of Precision
The successful construction of working nuclear clocks marks a historic moment in science.
For decades, nuclear clocks existed only as a theoretical possibility. Now, thanks to the efforts of teams in China and Europe, they have become reality.
While further refinement is still needed, these pioneering devices demonstrate that nuclear timekeeping is achievable and incredibly powerful.
As researchers continue improving the technology, nuclear clocks may soon become one of the most important scientific instruments ever created—helping humanity measure time more accurately than ever before while unlocking new secrets of the cosmos.
References: (1) L. Toscani De Col et al, A thorium-229 optical nuclear clock with feedback loop, arXiv (2026). DOI: 10.48550/arxiv.2606.04997 (2) Beichen Huang et al, A nuclear clock based on 229Th, arXiv (2026). DOI: 10.48550/arxiv.2606.08870
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