Eric R. Hudson, University of California, Los Angeles and Andrei Derevianko, University of Nevada, Reno

Most clocks, from wristwatches to the systems that run GPS and the internet, work by tracking regular, repeating motions.

To build a clock, you need something that ticks in a perfectly repeatable way. In a pendulum clock, that tick is the regular swinging of the pendulum: back and forth, back and forth, at nearly the same rate each time.

Our team of physicists studies whether an even better kind of clock could one day be built from the atomic nucleus. Today’s best clocks already use atoms to keep extraordinarily accurate time. But in principle, a clock based on a nucleus – the tiny, dense core at the center of an atom – rather than an atom’s electrons, could keep a steadier rhythm because it would be less sensitive to environmental disturbances such as temperature changes. In our research, published in the journal Nature, we measured and interpreted a unique nuclear property of thorium-229 in a crystal that could help make such nuclear clocks possible.

Ultraprecise clocks are more than scientific curiosities. They play key roles in navigation, communications and international timekeeping. Improvements in timing accuracy can also open doors to new science.

How atomic clocks work

In an atomic clock, researchers shine a laser on a material and carefully tune the light until it triggers a specific atomic response, typically by pushing or exciting an electron from one energy level to another. They can tell this has happened because the atoms absorb the laser light most strongly when its energy is exactly right.

That absorption happens at an exquisitely precise frequency. Frequency is how often something repeats over time. For a pendulum, it is the number of back-and-forth swings each second. For light, it is the number of wave cycles that pass each second. A light wave’s frequency also determines its energy and, in the visible light range, its color.

By detecting when atoms absorb the laser light most strongly, scientists can use the laser as a metronome. Rather than counting swings, these clocks count light waves.

To ensure the tick rate stays constant and the clock remains accurate, scientists closely match the laser’s energy to the energy needed to excite an electron in an atom.

Because the electron excitation energy is set by the laws of physics, atomic clocks based on the same atom tick at the same rate everywhere in the universe – even E.T. would agree with your clock.

Using this energy to calibrate a clock, like atomic clocks do, does not come without consequence, though. If anything changes the energy of the atom, like an unaccounted for magnetic field or the temperature of the room, the clock will tick at a different rate.

Deep inside every atom is something even smaller: the nucleus. Today’s atomic clocks keep time by tracking changes in an atom’s electrons. A nuclear clock, by contrast, would use an excitation in the nucleus itself, which is far more compact.

Because a nucleus is 10,000 times smaller than an atom, it is much less sensitive to temperature, electric fields and other environmental disturbances than the electrons in an atom. That makes it an appealing candidate for an even more stable clock.

The challenge is that nature does not make such a clock easy to build. The unique property we found in our research could help.

What makes thorium-229 special?

In one exceptionally rare case, the nucleus of the element thorium-229 has a property based on its two states: a ground state and a slightly higher-energy excited state. These states represent two different configurations of the nucleus, and scientists are able to use lasers to excite the nucleus from one state to the other.

The first step was to determine exactly how much energy is needed to push the thorium-229 nucleus into its excited state. That took nearly 50 years – a feat that we and other groups accomplished in 2024. That transition occurs at an extraordinarily high frequency, about 2 quadrillion – 2 * 10¹⁵ – cycles per second.

Next, in order to ensure your laser is at the right frequency to create a clock, you have to verify that the nucleus was indeed excited. Until now, physicists thought the best way to do that was to look for the very faint flashes of light that excited nuclei usually emit.

However, there are two problems with that approach.

First, in most materials, the thorium nuclei release their energy not as light, but through a process called internal conversion, where the energy is transferred to an electron in the material instead.

Second, even when light is emitted, it is extremely hard to detect. It lies in the vacuum ultraviolet, a part of the electromagnetic spectrum that air absorbs and is difficult to observe.

A different way to ‘listen’ to the nucleus

In our work, we flipped the problem around. Instead of trying to collect the light from the nucleus, we looked directly for the internal conversion electrons it produces.

We created a very thin layer – just a few dozen atoms across – of thorium dioxide on a small metal disc. A laser tuned to the right energy excited the thorium nuclei in the sample. When some of these nuclei relaxed, they transferred their energy to nearby electrons, which then could leave the surface. We use carefully arranged electric and magnetic fields to guide those electrons into a detector.

By scanning the laser across different frequencies and recording how many electrons we detected, we could measure how closely the laser energy matched the energy needed to excite the nucleus. When the two matched exactly, the signal appeared clearly in the data, revealing the precise laser frequency at which thorium-229 nuclei absorb most strongly.

We also measured how long the excited nuclear state survived in this material before relaxing, giving us a direct window into how the surrounding material influences the nucleus.

The measurement becomes much more powerful when paired with theory.
Calculations can estimate how the type of material used shifts the energy needed to excite thorium and how efficiently it converts energy from the nucleus into emitted electrons. These calculations help researchers tell apart the nucleus’s intrinsic behavior from outside effects caused by the solid around it. That understanding is crucial for designing practical nuclear clocks.

Why this approach matters

Detecting electrons instead of light has two major advantages.

First, it opens the door to studying thorium-229 in a much wider range of solid materials, including some that researchers had previously ruled out. Earlier approaches worked best only in materials where electrons were hard to knock off, which limited the options. Our method relaxes that constraint, allowing scientists to explore materials that were not practical before. That broader category of materials could make it easier to design and build future nuclear clocks.

Second, this method could enable a new type of nuclear clock that is simpler and potentially easier to miniaturize. Instead of needing sensitive light detectors, a clock based on this approach could read out time by measuring a tiny electrical current produced by the emitted electrons.

What could nuclear clocks be used for?

One day, researchers may use nuclear clocks to test whether the fundamental constants of nature truly remain constant over long periods of time, or to search for signs of new physics, such as dark matter, in the universe. More stable clocks could also improve technologies that depend on synchronized timing, such as advanced navigation systems.

Our work is an early step in that direction. It does not provide a finished clock, but it removes a practical barrier and provides a new experimental tool for studying how the thorium nucleus behaves inside solids.

Eric R. Hudson, Professor of Physics and Astronomy, University of California, Los Angeles and Andrei Derevianko, Professor of Physics, University of Nevada, Reno

This article is republished from The Conversation under a Creative Commons license. Read the original article.