Scientists in Vienna have built the worldโs first self-stabilizing nuclear clock, demonstrating that an atomic nucleus can independently stabilize the frequency used to measure time.
The system is based on the isotope thorium-229, whose nucleus possesses an unusually low-energy transition that can be excited using laser light.
Unlike conventional atomic clocks, which use transitions involving electrons surrounding an atom, the new clock uses an energy transition inside the atomic nucleus itself.
Researchers at TU Wien, working with collaborators including scientists at PTB Braunschweig, demonstrated that the thorium nuclei can continuously correct changes in the frequency of a laser, allowing the clock to operate independently without relying on another atomic clock for stabilization.
The prototype remained stable for more than 24 hours and achieved a relative precision of approximately 10โปยนโต, corresponding to roughly one second of error over 30 million years.
Although the device does not yet outperform the world's best optical atomic clocks, its successful independent operation marks an important step toward nuclear clocks that could eventually reach even greater precision.
The findings were published in Nature.
Why Do Scientists Need Better Clocks?
A clock may seem like a simple device for counting seconds.
At the highest levels of physics, however, timekeeping becomes one of the most precise forms of measurement ever achieved.
Modern atomic clocks can measure tiny differences in frequency with extraordinary accuracy.
They underpin international time standards and provide tools for testing fundamental physics.
Improving clock precision therefore means more than simply knowing the time more accurately.
It allows scientists to detect physical effects that would otherwise be far too small to measure.
Modern Precision Clocks Use Atoms
The most accurate clocks currently rely on atoms.
Atoms can exist only in specific quantum energy states.
When an atom moves between two of these states, it absorbs or emits electromagnetic radiation at an extremely precise frequency.
That frequency can act like the ticking of a clock.
Instead of counting the swings of a pendulum, an atomic clock counts oscillations associated with a quantum transition.
The Electron Provides the Reference in Atomic Clocks
In conventional atomic clocks, the relevant energy transition usually involves an electron.
Electrons occupy specific energy levels around the atomic nucleus.
A carefully tuned microwave or laser field can drive an electron between these levels.
Because the energy difference between the states is fixed by quantum mechanics, the associated radiation has an extremely reproducible frequency.
The clock uses that frequency as its reference.
But Electrons Are Exposed to Their Environment
Electrons occupy regions extending far beyond the nucleus.
This makes them sensitive to external conditions.
Electric fields can shift their energy levels.
Magnetic fields can produce additional changes.
Temperature and surrounding electromagnetic radiation can also influence clock performance.
Modern atomic-clock experiments work extremely hard to measure and correct these disturbances.
But they ultimately place limits on achievable accuracy.
Why Use the Nucleus Instead?
An atomic nucleus is dramatically smaller than the entire atom.
The nucleus contains protons and neutrons and occupies only a tiny region at the atom's centre.
Because nuclear states are so compact, they can be much less sensitive to many environmental disturbances.
This has led physicists to propose a different type of timekeeper:
a nuclear clock.
Instead of using an electron transition, the clock would use a transition between two energy states of the atomic nucleus.
Most Nuclear Transitions Require Too Much Energy
There is a major obstacle.
Typical nuclear energy levels are separated by much larger energies than electronic levels.
Driving such transitions generally requires extremely energetic radiation, such as gamma rays.
That makes them difficult to control using the precise laser technologies available for optical clocks.
For decades, one isotope has stood out as an extraordinary exception.
Thorium-229 Has an Unusual Nuclear Transition
The isotope thorium-229 possesses two nuclear states separated by an exceptionally small energy gap.
The excited state is known as a nuclear isomer.
Its energy is unusually low for a nuclear excitation.
That means the transition can be addressed using light at frequencies accessible to advanced laser systems.
This rare feature makes thorium-229 one of the strongest candidates for constructing a practical nuclear clock.
A Decades-Long Scientific Search
Scientists have known for decades that thorium-229 contains this unusually low-energy nuclear state.
But knowing that such a transition exists is not enough.
Researchers needed to determine its energy with extraordinary precision.
Then they needed to demonstrate that laser light could reliably excite the nucleus.
For years, these goals remained experimentally difficult.
A Major Breakthrough Came in 2024
A crucial advance occurred in April 2024.
A team led by Professor Thorsten Schumm at TU Wien, working with researchers led by Professor Ekkehard Peik at PTB Braunschweig, experimentally identified the long-sought nuclear transition.
The scientists demonstrated that laser light could directly excite thorium-229 nuclei.
This transformed the idea of a nuclear clock from a long-standing theoretical possibility into an experimentally accessible technology.
The Nuclear Transition Was Then Connected to an Atomic Clock
Further progress came later in 2024.
Researchers connected the thorium excitation experiment to a conventional optical atomic clock.
This allowed them to demonstrate that the nuclear transition could function as an extremely precise frequency reference.
The result showed that thorium-229 could indeed form the foundation of nuclear timekeeping.
But an important piece was still missing.
The Early System Could Not Yet Stabilize Itself
A practical clock needs a stable oscillator.
In optical clocks, that oscillator is typically a laser.
The light wave oscillates at an enormous frequency.
Those oscillations provide the regular rhythm used for timekeeping.
However, laser frequencies are not perfectly stable on their own.
Temperature changes, vibrations and other environmental effects can cause small frequency shifts.
The oscillator therefore needs a reference that continuously corrects these deviations.
The New Clock Uses Thorium as Its Own Reference
The Vienna researchers have now demonstrated exactly that capability.
Their system contains a specially prepared crystal containing thorium atoms.
A laser illuminates the crystal.
The frequency of that laser is tuned to the nuclear transition of thorium-229.
The thorium nuclei effectively tell the system whether the laser is operating at the correct frequency.
The Nuclei Only Respond to the Correct Frequency
Quantum transitions are highly selective.
A thorium nucleus absorbs the laser light most effectively when the laser frequency precisely matches the energy difference between the two nuclear states.
If the laser drifts away from that frequency, the interaction changes.
The amount of excitation detected from the nuclei decreases.
The clock's control electronics can identify that change.
A Feedback Loop Corrects the Laser
The system then uses this information to adjust the laser.
If its frequency shifts in one direction, the feedback mechanism pushes it back.
If it shifts in the other direction, another correction is applied.
The process repeats continuously.
This locks the laser frequency to the nuclear transition.
The thorium nuclei therefore become the frequency reference that stabilizes the clock.
Why the Feedback Loop Is So Important
Earlier experiments had already demonstrated nuclear excitation.
But an actual clock must do more than occasionally measure a transition.
It needs to maintain a stable frequency over extended periods.
The feedback loop turns the thorium system into an independent timekeeping instrument.
It no longer needs another atomic clock to continuously determine whether its laser is drifting.
This is why the experiment represents a major step toward a genuine stand-alone nuclear clock.
The Clock Ran for More Than 24 Hours
The researchers demonstrated that the system could remain stabilized for more than one day.
This long-duration operation is important.
An oscillator might appear highly precise over short intervals but drift substantially over longer periods.
Continuous nuclear stabilization prevented the laser frequency from wandering freely.
The result showed that the system could function as an operational clock rather than merely as a brief laboratory demonstration.
One Second in Roughly 30 Million Years
The prototype achieved a relative precision of about 10โปยนโต.
One way of expressing this level of performance is an error of approximately one second over 30 million years.
This does not mean researchers operated the clock for millions of years.
The figure is derived from its measured fractional frequency stability.
It provides an intuitive way to illustrate how small the clock's relative uncertainty is.
But Atomic Clocks Are Still Better โ For Now
Despite this impressive performance, the new nuclear clock has not yet surpassed the world's best optical atomic clocks.
Modern optical clocks can achieve substantially higher precision.
The Vienna device is an early prototype.
Its significance lies in demonstrating that the nuclear-clock concept can operate with its own stabilization mechanism.
Researchers now have a functioning platform that can be systematically improved.
Why Nuclear Clocks Could Eventually Win
The strongest advantage comes from the size of the nucleus.
An atomic nucleus is more than 10,000 times smaller than an atom.
Because the nuclear wavefunctions are concentrated in such a tiny region, many external electric and magnetic disturbances have less influence on the transition.
That could make the nuclear reference intrinsically less sensitive to environmental noise.
With sufficiently advanced lasers and materials, this may allow nuclear clocks to surpass electronic atomic clocks.
Smaller Does Not Automatically Mean Perfect
A nuclear clock is not completely immune to environmental effects.
The thorium nuclei are still embedded inside a material.
Their surroundings can influence the measured transition.
Temperature, lattice properties and other interactions must still be understood.
Researchers therefore need to characterize these effects carefully.
But the nuclear transition offers a fundamentally different and potentially more protected reference.
Why a Crystal Is Used
The Vienna system places thorium atoms inside a specially engineered crystal.
This allows researchers to work with many thorium nuclei simultaneously.
A larger number of nuclei can provide a stronger optical signal.
That makes it easier to detect whether the laser is precisely tuned to the transition.
The solid-state architecture may also eventually allow compact nuclear-clock devices.
The Crystal Must Be Exceptionally Good
Embedding nuclei inside a crystal creates its own challenges.
Imperfections in the crystal lattice can change the local electromagnetic environment.
Different thorium atoms might then experience slightly different conditions.
That can broaden the nuclear transition and reduce clock performance.
Improving crystal quality is therefore one of the main paths toward higher precision.
Better Lasers Could Also Improve the Clock
The researchers expect stronger and more advanced laser systems to improve performance.
A more powerful laser can interact with the thorium nuclei more effectively.
Better laser stability can also reduce noise before the nuclear feedback system even begins making corrections.
These improvements could allow the nuclear transition to be measured much more precisely.
Why Ultra-Precise Timekeeping Matters
Better clocks can transform measurements that appear unrelated to time.
Frequency can be measured with extraordinary precision.
Many other physical quantities can then be translated into frequency shifts.
A highly sensitive clock effectively becomes a sensor for anything capable of changing its quantum transition.
This turns precision timekeeping into a tool for fundamental science.
Gravity Changes the Rate of Clocks
Einstein's theory of general relativity predicts that clocks run at different rates in different gravitational potentials.
A clock located slightly closer to Earth ticks more slowly than one positioned higher up.
Modern optical clocks are already sensitive enough to detect surprisingly small height differences.
Even more precise nuclear clocks could improve such measurements further.
Clocks Could Become Extremely Sensitive Gravity Sensors
If clock precision improves substantially, researchers could compare clocks at different locations and measure tiny gravitational differences.
Those differences can reveal changes in gravitational potential.
This could create new approaches to relativistic geodesy, where scientists use precision clocks to measure height and gravitational structure.
A sufficiently sensitive clock could effectively measure gravity through time.
Testing Whether Fundamental Constants Really Stay Constant
Nuclear clocks could also be useful for investigating fundamental constants of nature.
Physics assumes that quantities such as the fine-structure constant maintain fixed values.
Some speculative theories predict that these constants could vary extremely slightly over time or space.
Different types of atomic and nuclear transitions respond differently to such changes.
Comparing nuclear clocks with atomic clocks could therefore provide unusually sensitive tests.
Searching for New Physics
Ultra-precise clocks can also be used to search for phenomena not included in current physical theories.
For example, certain hypothetical forms of dark matter could interact weakly with ordinary matter and temporarily shift atomic or nuclear energy levels.
Networks of precision clocks might detect correlated changes.
Nuclear clocks could provide a new type of detector because their transitions involve different physics from electronic atomic transitions.
Nuclear Clocks Could Complement Atomic Clocks
The emergence of nuclear clocks does not mean atomic clocks would suddenly become useless.
Different clocks respond differently to external influences.
Comparing several types of clock is scientifically valuable.
An optical atomic clock may respond strongly to one physical effect while a nuclear clock responds differently.
Comparisons can therefore help researchers distinguish genuine new physics from ordinary environmental disturbances.
A Clock Can Test Quantum Mechanics and Relativity Together
Precision clocks occupy an unusual position in physics.
Their operation depends on quantum mechanics because the ticking frequency comes from discrete quantum energy levels.
But their rates are also affected by gravity according to general relativity.
That makes clocks powerful instruments for testing how two of physics' foundational theories behave under extremely precise conditions.
A nuclear clock introduces an entirely new type of quantum reference for these experiments.
From Electrons to the Atomic Nucleus
The progression of clock technology reflects increasingly precise control of matter.
Mechanical clocks used moving macroscopic components.
Microwave atomic clocks used transitions between atomic states.
Optical clocks moved to much higher-frequency electronic transitions.
Nuclear clocks take the next step inward.
Their reference lies inside the nucleus itself.
Higher Frequency Can Improve Precision
The frequency of a clock's reference oscillation matters.
A higher-frequency oscillator produces more cycles in a given amount of time.
This can allow time intervals to be subdivided more finely.
Optical atomic clocks gained much of their precision by using frequencies vastly higher than those used in microwave clocks.
The thorium nuclear transition lies in an optical-frequency region that can be accessed using advanced laser technology.
The Challenge Is Reading the Nuclear Signal
The nuclear transition can be extremely narrow.
That narrowness is advantageous for precision.
But it can also make the transition difficult to excite and detect.
Researchers need lasers with exceptional frequency control.
They also need sensitive techniques for measuring how many nuclei respond.
The successful feedback loop shows that the Vienna team can obtain enough information from the nuclei to control the laser continuously.
A Self-Stabilizing Clock Is a Crucial Engineering Milestone
The conceptual principle behind nuclear clocks has been discussed for decades.
The experimental transition was eventually found.
Laser excitation was demonstrated.
The nuclear reference was compared against an existing atomic clock.
Now, the system can stabilize itself.
Each of these milestones removes another obstacle separating nuclear clocks from practical precision instruments.
What Comes Next
The Vienna researchers plan to improve both the thorium-containing crystals and the laser systems.
Better crystals should provide cleaner and narrower nuclear signals.
More powerful and stable lasers should allow more precise interrogation.
Together, these developments could improve the clock's performance substantially.
The Goal Is to Surpass Optical Atomic Clocks
The current device is not yet the world's most accurate clock.
But researchers believe the fundamental properties of the thorium-229 nucleus provide room for significant improvement.
If environmental disturbances can be sufficiently controlled and the transition measured more precisely, nuclear clocks could eventually exceed the stability and accuracy achievable with electronic transitions.
That would represent a new generation of precision timekeeping.
A New Reference for Measuring Nature
The importance of the Vienna experiment goes beyond its present numerical accuracy.
For the first time, researchers have demonstrated a nuclear clock that can continuously use its own nuclear transition to stabilize its oscillator.
The nucleus is no longer merely an experimental signal measured against another clock.
It has become the clock's controlling reference.
That is the critical step required for an independent nuclear timekeeper.
From a Nuclear Curiosity to a Working Clock
Thorium-229 has attracted attention for decades because of its unusual nuclear energy structure.
For much of that time, a nuclear clock remained a compelling idea without a complete experimental implementation.
Recent advances have changed that rapidly.
The nuclear transition has been found.
It has been excited with lasers.
Its frequency has been compared with atomic standards.
And now, it has been used to continuously stabilize a clock for more than 24 hours.
The current prototype may not yet make atomic clocks obsolete.
But it establishes that nuclear timekeeping works as an independent system.
With better crystals, stronger lasers and further improvements in control, the tiny nucleus of thorium-229 could eventually become the foundation for the most precise clocks ever built.
Journal reference
L. Toscani De Col, T. Riebner, I. Morawetz, F. Schneider, N. Sempelmann, J. Schlachet-Lรฉpinay, F. Schaden, M. Bartokos, G. A. Kazakov, K. Beeks, B. Gerstenecker, M. Pimon, S. Lahs, A. Hellerschmied, T. Lercher, H. Denker, J. Premper, A. Niessner, M. Matus, M. ฤรญลพek, O. ฤรญp, V. Lal, G. Zitzer, V. Petrov, J. Tiedau, M. V. Okhapkin, E. Peik and T. Schumm. โA thorium-229 optical nuclear clock with feedback loop.โ Nature, 2026.