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How to make a nuclear clock tick

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How to make a nuclear clock tick

Nuclear clocks may hold the key to unlocking the secrets of the universe
(photo: CC0 Public Domain)

By Caleb Davis

Thorsten Schüm is a watchmaker, but not the kind who sits in front of a workbench covered in springs and wheels, with a magnifying glass stuck in one eye. No, it makes a timepiece that is in a completely different class.

Atomic clocks may sound familiar enough—but if Schum’s research goes according to plan, it could lead to the creation of a nuclear clock. And instead of just telling the time, it can help unravel some of the universe’s best-kept secrets.

“It’s still a dream,” says Schüm, a professor at the Vienna University of Technology in Austria. “No one knows how to do it.”

His intention is to change that and, in the process, shed light on some of the fundamental interactions in nature.

A fraction of a second

At the base of a clock can lie anything that oscillates at regular intervals and can be counted. The first watches were mechanical. Many wristwatches today use the electromechanical oscillations of the quartz crystal.

However, clock technology went up a notch in the 1950s with the advent of atomic clocks.

Atoms consist of a nucleus surrounded by an orbiting cloud of electrons. The ticking time of the atomic clock depends on the “quantum transitions” that these electrons make.

This is done in the following way. Electrons can absorb a burst of energy that moves them from the “ground” to an “excited state” with more energy. They can then return to their ground state by releasing this portion of energy on the way down.

These energy transitions occur at a certain frequency that can be used to tell time. All this happens amazingly fast.

One second, for example, is officially defined as 9,192,631,770 oscillations of a portion of the energy that excites an atom of cesium 133.

Atomic clocks are precise because they produce an awful lot of oscillations, or “ticks”. Therefore, if the counting mechanism misses one or two of them, it is generally not a big problem when they are more than 9 billion per second.

Nuclear clocks are different. The “ticking” does not depend on the electrons, but rather on the vibrations of the nucleus itself. They are many times faster than the “ticks” of electronic transitions.

But, as Schum states, the work of creating a working nuclear clock continues.

A happy coincidence

His interest in solving the nuclear mystery arose partly by chance.

It turns out that a rare isotope of the element thorium 229 is the easiest material from which to build a nuclear clock. This is because it is considered to have the slowest “ticks” of all the cores. Also, the institute where Schum works is one of the few places that has access to this material.

Thorium 229 does not occur naturally. It is obtained only from the nuclear decay of certain types of uranium.

The Vienna University of Technology has an agreement with the Oakridge National Laboratory in the US that allows it to obtain known amounts of thorium 229 from the remnants of uranium used in nuclear tests decades ago.

It did not escape Schum’s attention that his own name and the name of the element were derived from that of the mythical Norse god Thor.

“It blew my mind,” he says.

Its about time

Since 2020, Schum has been conducting fundamental research into creating a nuclear clock as part of the EU-funded ThoriumNuclearClock project, which will continue until early 2026.

He and his colleague, Professor Eckerhard Pike of Germany’s National Metrological Institute in Braunschweig, are the principal investigators, along with Mariana Safronova of the University of Delaware in the US and Peter Tiroff of the Ludwig and Maximilian University of Munich (LMU) in Germany.

In order for the nuclear clock to start ticking, it must receive a boost from a laser system at just the right energy level. For most cores, however, the required energy frequency is far beyond what is available with current laser technologies.

The thorium 229 nucleus is one of the largest stable nuclei in existence. It was thought to be able to adopt a very low-energy state that current lasers can reach—though no one understands exactly how or why it does this.

“To begin with, it wasn’t even very clear that this state of thorium 229 existed,” Schum says.

It is now known to exist. In 2020, Schum and colleagues published a measurement of the isotope’s energy level. Since then, they have continued to build on this knowledge.

All this paves the way for a real test of the watch. Schum and his fellow researchers are working on building a laser that is specifically designed to excite thorium at just the right frequency.

They soon plan to point this laser at trapped thorium atoms for the first time, in an attempt to get them ticking.

“We are very excited about the outcome of this experiment, as this has never been done before,” says Pike. “We, as well as others, have tried similar experiments with thorium 229 in the past without success. This time we think we are much better prepared.”

Crystal clear

For these experiments, thorium atoms will be trapped in atom traps—a very delicate task. At the start of his work with the ThoriumNuclearClock, Schüm led a two-year EU-funded project called CRYSTALCLOCK, which aimed to develop a simpler design and readout mechanism for a single nuclear clock.

The idea was to create a crystal composed of calcium fluoride in which thorium 229 atoms were dispersed. This provided a solid material that was much easier to work with than atom traps.

Schum and his colleagues, including Dr. Thomas Sikorsky, have published a paper demonstrating that these thorium-doped crystals could be mined in 2022. The next step will be to understand how the ticking of these crystals.

According to Schum, a technique called nuclear tomography could be adapted for this purpose, and the whole process would be much easier than using thorium atoms in traps.

Natural forces

All these efforts make sense, not because more precise clocks are needed, but because humanity’s fundamental understanding of how reality works can be put to the test.

The best theories of physics explain that there are four fundamental forces in the universe: gravity, electromagnetism, the small nuclear force, and the large nuclear force. The impact of these forces is known, and the corresponding numbers are often called fundamental “constants”.

However, it is not known whether the interaction of these forces was and will always be the same. There are signs that the forces were much more intense in the distant past, around the Big Bang, and that they may still be changing, albeit by very little.

Atomic and nuclear clocks may make it possible to put this to the test. The “ticking” of the atomic clock is primarily affected by the electromagnetic interaction, so if the speed of the ticking starts to change, this will indicate a change in the fundamental force.

However, electromagnetism is very weak, which is why atomic clocks, despite their incredible accuracy, may never be able to detect this change.

In contrast, the “ticking” of nuclear clocks is affected by the great force. Therefore, if and when a nuclear clock is created, it could be used to track if there are changes in major power within certain periods of time.

“The transition from atoms to nuclei is not done to have a better clock,” says Schum. “In reality, the first nuclear clock is likely not to be as good as the best atomic clocks. The point is more to have a completely new technology with which the great power can be basically tested”.

The research in this article was funded by the European Research Council (ERC) and the Marie Skłodowska-Curie (MRC) Action of the EU. It was first published in Horizon, the EU’s research and innovation journal.

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