Thorium Nuclear Clock Prototype Marks a Major Precision Breakthrough

A breakthrough that has taken decades to achieve was made only a few months ago, and scientists are already identifying what it could make possible. By measuring the separation between quantum energy states in a thorium nucleus, researchers have produced the first rudimentary nuclear clock.

Physicists coupled a strontium atomic clock to a crystal containing thorium nuclei, successfully demonstrating the essential technology required for the first fully developed nuclear clock.

Although that ultimate milestone has not yet been reached, it promises to usher in an entirely new era of exceptionally precise timekeeping.

Thorium nuclear clock technology

"With this first prototype, we have proven: Thorium can be used as a timekeeper for ultra-high-precision measurements," explains physicist Thorsten Strumm of the Vienna University of Technology.

"All that is left to do is technical development work, with no more major obstacles to be expected."

An atomic clock depends on the extraordinarily regular ‘ticks’ produced when laser stimulation causes atoms to move between energy states. These states are determined by the electrons moving around the nucleus at an atom’s centre.

Achieving the same result with the nucleus itself is considerably harder, however. Altering a nucleus’s energy state requires far more energy than changing the state of its electrons.

Why a nuclear clock matters

Nevertheless, a nuclear clock is highly sought after because it could be much more stable and accurate than an atomic clock. This would allow more exact measurements of the physical Universe, with consequences for fields ranging from navigation to the hunt for dark matter.

Earlier this year, scientists announced a measurement of the thorium nucleus’s energy jump, or the difference between its energy states. That result enabled Strumm and his colleagues to establish the precise energy needed to trigger the change between states-the process that would provide a nuclear clock’s tick.

Their next task was to show that this ticking could be turned into a clock, which Strumm and his colleagues have now achieved.

Linking a strontium atomic clock to thorium

The device demonstrated is not yet a complete nuclear clock, but it represents an initial move towards one. The strontium clock at JILA, part of the National Institute of Standards & Technology, runs on infrared light.

The researchers made a small calcium fluoride crystal containing thorium nuclei, whose energy states can be changed using vacuum-ultraviolet light.

To link that crystal with the atomic clock, they had to convert infrared light into ultraviolet light. They generated a frequency comb of infrared wavelengths and passed it through xenon gas, which interacts with the infrared light and produces ultraviolet wavelengths.

This created a combined frequency comb able both to excite the thorium nuclei’s transition and to synchronise it with the ticking of the strontium atoms.

The nuclear ticking produced so far is no more precise than the strontium atomic clock. Yet, now that the central principle has been proved, the technology itself is within reach-and close to being fully realised, according to the researchers.

"Imagine a wristwatch that wouldn't lose a second even if you left it running for billions of years. While we're not quite there yet, this research brings us closer to that level of precision," says physicist Jun Ye of JILA.

The team repeated the experiment numerous times, obtaining results consistent with those of an atomic clock on every occasion. The next stage is refinement.

"When we excited the transition for the first time, we were able to determine the frequency to within a few gigahertz. That was already more than a factor of a thousand better than anything known before. Now, however, we have precision in the kilohertz range – which is again a million times better," Schumm says.

"That way, we expect to overtake the best atomic clocks in 2-3 years."

The research has been published in Nature.