First working nuclear clock heralds a new era in timekeeping

A team led by Thorsten Schumm at TU Wien has built what they describe as the first working nuclear clock. Using thorium nuclei embedded in a calcium fluoride crystal and an ultraviolet laser that “ticks” by comparing two frequencies around the nucleus’s known

For more than two decades, researchers have chased a timekeeper that doesn’t rely on electrons. Now, in a prototype that runs on thorium and an ultraviolet laser, scientists say they’ve crossed a line that once looked unreachable: a working nuclear clock.

The concept is starkly different from the atomic clocks used today. Most of the most accurate atomic clocks keep time by watching electrons. Those electrons sit in distinct energy levels around a nucleus. and they only switch levels when illuminated by light at a very specific frequency. Since the frequency of a wave is defined by how many waves pass a point in a set amount of time. counting those oscillations can act like the steady “ticking” of a grandfather clock. In practice. the clock nudges its laser until the electron transitions are maximally responsive. maintaining accuracy—clocks built this way are described as losing only a few seconds every billion years.

Nuclear clocks aim higher by changing what does the vibrating. Instead of electrons, they use atomic nuclei, which can also move between energy levels. The promise is that nuclei could be far more stable for timekeeping because nuclear transitions involve much higher energies and demand extremely precise excitation. In theory. that could stretch stability to seconds over hundreds of billions of years—so long that it would be older than the age of the universe. a timescale that could matter for experiments hunting for “exotic new physics.”.

Building one has been hard. Many nuclei need more energy to be excited than even today’s most energetic lasers can supply. For years. the path has centered on radioactive thorium: the laser frequency required to excite its nucleus was first discovered in 2023. and thorium can be stimulated with relatively little energy compared with most other candidates.

At the Vienna University of Technology in Austria. Thorsten Schumm and colleagues have now built a device made from thorium that they say is a working clock. It’s already showing promise in the hunt for dark matter particles. Schumm called it “the culmination of 15 to 20 years of research,” adding: “It’s amazing. Very few researchers actually see their dream become true.”.

The prototype also addresses a gap in earlier approaches. Previous systems demonstrated that thorium’s nuclear frequency could be excited by the right laser. but they lacked the distinct frequency adjustment mechanism that makes a clock a clock. Harry Morgan at the University of Manchester. UK. put it this way: “If there’s ever going to be a ‘this is it’ moment. it’s probably this.”.

Schumm’s team made the clock by embedding thorium in a crystal made from calcium fluoride and then shining an ultraviolet laser through it. The laser—serving as the clock’s ticking—periodically switches between two frequencies just above and just below thorium’s known nuclear frequency. The logic is feedback in its most direct form. If the slightly higher and slightly lower frequencies are absorbed by thorium equally, then the laser is tuned correctly. If absorption is unequal, the clock uses that difference to tune the laser back toward the right frequency.

As for performance, the nuclear clock does not yet match the stability of the best atomic clocks. It is running at tens of seconds lost every billion years. Schumm and his team describe it as a proof of principle and say they have not yet fine-tuned the system using the best available lasers and electronics.

Still. the prototype is behaving in a way that matters to anyone trying to turn a lab effect into a measurement instrument. Ekkehard Peik of PTB. the German national metrology institute. said the most impressive part to him was reliability: “What impressed me the most was that the system ran overnight and for 24 hours without user intervention.” He added: “This is something that has not been achieved so rapidly with other optical clocks.”.

The contrast with conventional atomic clocks goes beyond stability numbers. Nuclear transitions are shielded from the chaotic electromagnetic environment created by electrons. Because of that. the transition is described as very precise and not affected by the moving electrons that can introduce noise in electron-based systems. In practice. that means properties of nucleons can be measured without the same electronic “noise. ” which could make both the nuclear ticking and measurements of fundamental physical properties more precise.

There is also a practical advantage: the system does not need the extremely low temperatures or vacuum conditions that atomic clocks often require. Schumm said it plainly: “It’s really the most simple thing you can imagine.” He argues that this room-temperature operation could make the technology easier to miniaturise and deploy across experiments. He points to satellite tests of relativity as an example.

The timeline for improvement may be quick if the prototype can be upgraded. Eric Hudson at the University of California, Los Angeles, said that even though the current performance is below the state of the art, researchers can expect “orders-of-magnitude improvement in the near future.”

That future has another urgency. In this setup. Schumm and colleagues used the very high energies of the thorium nucleus to rule out possible dark matter particles. The logic is straightforward. If dark matter behaves like an electromagnetic-like force permeating the universe. it should subtly shift nuclear energy transitions in matter. including thorium. That would make the specific nuclear frequency the clock relies on measurably different. and the change would be obvious because of thorium’s high nuclear frequency.

Schumm compared it to everyday intuition: “It’s a little bit like if you want to measure the change of length [in a metal] because of heat temperature change,” he said. “The longer your stick, the larger the effect.”

nuclear clock thorium calcium fluoride crystal ultraviolet laser atomic nuclei metrology dark matter relativity PTB TU Wien