Using atomic nuclei could allow scientists to read time more precisely than ever – what this research could mean for future clocks

Most clocks, from wristwatches to systems that manage GPS and the Internet, operate by following regular, repetitive movements.

To build a clock, you need something that works perfectly reproducibly. In a pendulum clock, this ticking is the regular swing of the pendulum: back and forth, back and forth, at roughly the same rate each time.

Our team of physicists is studying whether an even better clock could one day be built from the atomic nucleus. Today’s best clocks already use atoms to keep extraordinarily precise time. But in principle, a clock based on a nucleus – the tiny, dense nucleus at the center of an atom – rather than an atom’s electrons, could keep a more stable 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.

Ultra-precise clocks are more than just scientific curiosities. They play a key role in navigation, communications and international timekeeping. Improving timing accuracy can also open the door to new science.

How Atomic Clocks Work

In an atomic clock, researchers shine a laser at a material and carefully adjust the light until it triggers a specific atomic response, usually by pushing or exciting an electron from one energy level to another. They can see that this happened because atoms absorb laser light more strongly when its energy is perfectly matched.

This absorption occurs at an extremely precise frequency. Frequency is how often something repeats over time. For a pendulum, this is the number of swings back and forth per second. For light, it is the number of wave cycles that pass each second. The frequency of a light wave also determines its energy and, in the realm of visible light, its color.

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

To ensure that the ticking rate remains constant and the clock remains accurate, scientists closely match the energy of the laser to the energy needed to excite an electron in an atom.

Because the excitation energy of electrons is defined by the laws of physics, atomic clocks based on the same atom operate at the same rate everywhere in the universe – even ET would agree with your clock.

Using this energy to calibrate a clock, as atomic clocks do, is not without consequences, however. If something changes the energy of the atom, such as an unexplained magnetic field or the temperature of the room, the clock will run at a different rate.

Deep inside each atom is something even smaller: the nucleus. Today’s atomic clocks keep time by tracking changes in an atom’s electrons. A nuclear clock, on the other hand, would use excitation in the nucleus itself, which is much 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 an atom’s electrons. This makes it an attractive 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 during our research might help.

What makes thorium-229 special?

In an exceptionally rare case, the nucleus of the element thorium-229 has a property based on its two states: a ground state and an excited state of slightly higher energy. 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 was needed to push the thorium-229 nucleus into its excited state. It took almost 50 years – a feat we and other groups accomplished in 2024. This transition is happening at an extraordinarily high frequency, around 2 quadrillion – 2 * 10¹⁵ – cycles per second.

Next, to make sure your laser is at the right frequency to create a clock, you need to check that the core was properly excited. Until now, physicists thought the best way to do this was to look for the very faint flashes of light that excited nuclei usually emit.

However, this approach poses two problems.

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

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

Another way to listen to the core

In our work, we reversed the problem. Instead of trying to capture 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 – of thorium dioxide on a small metal disk. A laser tuned to the correct energy excited the thorium nuclei present in the sample. When some of these nuclei relaxed, they transferred their energy to nearby electrons, which could then leave the surface. We use carefully arranged electric and magnetic fields to guide these electrons to a detector.

By scanning the laser at different frequencies and recording the number of electrons detected, we were able to measure how closely the laser energy matched the energy needed to excite the nucleus. When the two matched exactly, the signal showed up clearly in the data, revealing the precise laser frequency at which the thorium-229 nuclei absorbed most strongly.

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

Measurement becomes much more powerful when combined with theory. Calculations can estimate how the type of material used shifts the energy needed to excite the thorium and how efficiently it converts the nucleus’ energy into emitted electrons. These calculations help researchers distinguish the intrinsic behavior of the nucleus from external effects caused by the solid surrounding it. This understanding is crucial for designing practical nuclear clocks.

Why this approach is important

Detecting electrons rather than 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 excluded. Previous approaches worked best only in materials where electrons were difficult to remove, limiting options. Our method relaxes this constraint, allowing scientists to explore materials that were previously impractical. This broader category of materials could facilitate the design and construction of future nuclear clocks.

Second, this method could make it possible to create a new type of nuclear clock, simpler and potentially easier to miniaturize. Instead of requiring sensitive light detectors, a clock based on this approach could tell the time by measuring a tiny electric current produced by emitted electrons.

What could nuclear clocks be used for?

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

Our work constitutes a first step in this direction. It does not provide a finite clock, but it removes a practical barrier and provides a new experimental tool for studying the behavior of the thorium nucleus inside solids.

This article is republished from The Conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you make sense of our complex world. It was written by: Eric R. Hudson, University of California, Los Angeles and Andrei Derevianko, University of Nevada, Reno

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Eric R. Hudson receives funding from ARO, DARPA, NIST, NSF, and RCSA.

Andrei Derevianko receives funding from NASA and the National Science Foundation.