Nuclear Clocks: A Clock to Rule Them All?

The nuclear clock could represent the most revolutionary advance in atomic timekeeping since the cesium beam clock was invented in the 1950s. Instead of tapping energy jumps made by electrons, nuclear clocks would use jumps made by particles called protons and neutrons that are packed tightly inside the atomic nucleus.

The Nuclear Clock: The Next Leap in Timekeeping

Protons and neutrons, like electrons, can jump between energy levels. Rather than just one particle gaining energy, nuclear jumps involve protons and neutrons collectively shifting their arrangement to a state with a different energy. By tuning a laser to the “resonant frequency” of a nuclear jump and counting the number of times the tuned laser light passes a point in one second, scientists hope to create a new kind of timekeeper: the nuclear clock.

Unfortunately, shifts in the atomic nucleus almost always have resonant frequencies far greater than can be delivered by today’s lasers — or any laser that will exist in the foreseeable future. But in its mysterious way, the universe has created an exception: an isotope — or variety — of a naturally occurring radioactive metal called thorium. The nucleus of thorium-229 has an unusually low resonant frequency in the “vacuum” ultraviolet region of the electromagnetic spectrum, which has frequencies just a bit higher than the optical frequencies we can see. 

A clock that uses the thorium-229 resonant frequency could divide time far more finely than any that exists today. Better yet, an atom’s nucleus is much better shielded from environmental disturbances than its electrons are. That shielding could allow for extremely precise measurements of the nuclear resonant frequency, potentially making a nuclear clock the most stable timekeeper ever invented.

A Probe for New Physics and Practical Benefits

Physicists are especially excited about using nuclear clocks for ambitious experiments seeking to shed light on some of the deepest mysteries of the universe. Nuclear clocks could, for example, help probe the nature of gravity and dark matter, and search for a “fifth force” beyond gravity, electromagnetism and the strong and weak nuclear forces. 

How could a clock help explore such exotic possibilities? The atomic resonant frequencies used in atomic clocks are determined by certain special quantities: specifically, the mass of an electron and the fine-structure constant, which determines how strong electricity and magnetism are at the quantum scale. These qualities are known as fundamental constants because they play a powerful role in shaping the world we observe and cannot be derived from anything else.

All experiments to date suggest these constants are unchanging in time; in other words, they are truly constant. But many theories of physics that extend beyond the “standard model” (which includes all known particles and forces) allow these so-called fundamental constants to change. For example, some scientists speculate that dark matter — a mysterious substance that, according to astronomical observations, seems to pervade galaxies — could consist of vast fields whose strength fluctuates in space and time in ways that tweak particle masses or other “constants.” 

To search for such fluctuations, physicists have compared the ticking rates of different kinds of atomic clocks (whose energy levels are sensitive to fundamental constants in different ways) over periods of up to several years. So far, no deviations from the standard model have been seen. But even such “negative results” provide useful information: They have helped scientists rule out certain hypothesized dark matter particles and put constraints on how much certain constants could be changing.

A nuclear clock could change the game. That’s because it probes not just the electromagnetic force, as other atomic clocks do, but also the strong nuclear force, which holds protons and neutrons together inside the atomic nucleus. The way these two forces combine to set the nucleus’s resonant frequency makes the nuclear clock exquisitely sensitive to even the slightest changes in the fine structure constant. So a nuclear clock could pick up ultratiny signals related to dark matter or new forces that no existing instrument can detect.

In fact, this new era of exploration has already begun. NIST researchers and collaborators recently used measurements of the nuclear resonant frequency to rule out certain hypothesized forms of ultralight dark matter.

Scientists are also excited to use nuclear clocks to probe the strong nuclear force itself. To study this force today, physicists typically smash particles together at high energies in particle colliders and examine what comes out. 

By providing a way to measure the nuclear resonant frequency, which is determined by the strong nuclear force, nuclear clocks could offer a new, potentially much cheaper route to reveal the secrets of the nucleus and open a new era of precision measurement in nuclear physics.

Beyond advancing fundamental science, nuclear clocks could also deliver more practical benefits for industry and society. Today, scientists involved in precision timekeeping continue to use hydrogen masers for their long-term stability, even though other kinds of atomic clocks are much more accurate. With their built-in stability advantage, nuclear clocks could someday replace hydrogen masers in national time scales and navigation satellites. For similar reasons, nuclear clocks could also prove ideal for powering navigation during deep-space exploration, as spacecraft travel far beyond where they can be guided by Earthbound clocks.

Recent Progress

Physicists have recently cleared several major hurdles on the path toward building a nuclear clock. In 2024, three research groups, including one at NIST, reported that they had used lasers to trigger clock transitions within the thorium nucleus. Previously, the energy of these jumps had only been measured indirectly. 

The 2024 NIST experiment measured the thorium nuclear frequency with the highest precision and accuracy to date. These results have enabled physicists to focus their efforts on developing high-performance lasers that can measure the thorium nuclear resonant frequency.

And in 2026, research groups in Europe and China locked continuous-wave lasers to the thorium nuclear resonant frequency, an important step toward building a full-fledged nuclear clock that could push the frontiers of timekeeping.

To be sure, the accuracy and precision of these experiments still lag that of state-of-the-art optical clocks and even cesium fountain clocks, and a nuclear clock that outperforms the best optical clocks remains years away. For one thing, the ultraviolet lasers needed to accurately measure the thorium nucleus resonant frequency still need to improve greatly to catch up to visible-light lasers. To address this challenge, NIST researchers are working to improve the efficiency with which their continuous-wave ultraviolet laser generates power.

Another possible hurdle is that all nuclear clock experiments to date have used crystals to trap thorium atoms. This setup offers advantages: It allows physicists to probe many nuclei at once, enabling more stable and precise frequency measurements. And crystals could provide a platform for building compact, robust clocks for applications such as space exploration. 

But crystals also introduce complications. Electromagnetic fields from the particles that make up the crystal lattice slightly tweak the thorium nuclei, limiting how accurately scientists can measure the resonant frequency. To rival or exceed the accuracy of the best optical clocks, scientists will also need to find ways to fully isolate thorium atoms. For example, future nuclear clocks could use electromagnetic traps or optical lattices, similar to existing optical clocks.

Regardless, physicists, like the atoms they work with, are a restless bunch. Scientists are now striving to build a new generation of clocks to rule them all. Stay tuned!