Nuclear clocks are the next big thing in ultra-precise timekeeping. Recent publications in the journal Nature propose a new method and new technology to build the clocks.
Timekeeping has become more precise as humans have engineered more effective clocks, but even a quartz analog wristwatch loses a minute over the course of a week. Incredibly accurate timing is essential for cell phone network performance, and many other parts of our day-to-day lives. The most accurate timekeeping method in use right now is called an atomic clock.
“This nucleus is a quirk of nature, a precious gift."
Atomic clocks take advantage of the natural oscillations of an atom. To measure the oscillation of the atom, researchers probe it with energy from a laser, and they can track the oscillation over the duration of the laser pulse, using the oscillations as “ticks” to mark time passing. The best atomic clocks are guaranteed to neither gain nor lose a second over the estimated age of universe, 14 billion years.
Andrei Derevianko, a theoretical physicist at the University of Nevada, Reno, has contributed theoretical work over the past two decades to help reach an inconceivable level of accuracy in timekeeping through studies of atomic clocks.
Recently, Derevianko, the Sara Louise Hartman Endowed Professor in Physics, was part of two large teams of theoretical and experimental physicists working on nuclear clocks. He performed some of the calculations that allowed researchers to understand the observed clock signal and to inform paths forward in the development of nuclear clocks.
“Nuclear clocks, when fully realized, can be unimaginably precise, more accurate than atomic clocks,” Derevianko said.
Nuclear clocks work similarly to atomic clocks, Derevianko said, but they probe the nucleus rather than the atom itself. This makes them almost immune to external forces, which can throw off an atomic clock, improving accuracy by orders of magnitude.
“It’s much more difficult to probe an atom’s nucleus than it is to probe an atom, which is why we have atomic clocks and not nuclear clocks,” Derevianko explained.
Probing the nucleus requires lasers that humans have yet to develop, except in the case of a unique and rare isotope, Thorium-229, also written 229Th, a promising timekeeping candidate. 229Th comes from decaying Uranium-233, which is used in nuclear reactors.
“This nucleus is a quirk of nature, a precious gift,” Derevianko said.
He helped develop a new method of manipulating Thorium-229 into a thin layer for use in a clock, using less of the rare and radioactive material and thus making it much cheaper and safer.
“It is going to open up this new field of research,” Derevianko said.
The thinness of the films also leads to new quantum effects, which Derevianko is interested in studying further. The decay of 229Th may also provide new ways to study thorium compounds, with the potential to aid studies of nuclear power generation.
Derevianko’s latest work, published in Nature, expands on the efforts to develop the clock by successfully demonstrating the use of a previously theoretical laser spectroscopy method for the first time.
Physicists have commonly used Mossbauer spectroscopy to monitor an atom’s nucleus and its sensitivity to the environment around it, which is a crucial piece of information for understanding limitations of the clock. In Derevianko’s most recent publication, he again turns to the Thorium-229 atom, this time paired with two oxygen atoms, to use laser Mossbauer spectroscopy for the first time. Previously, the materials had to transmit light to allow recording of the nuclear transition, but with laser Mossbauer spectroscopy, they can be opaque, allowing a whole new class of materials to be investigated and studied with this technology. This method improves the ability of the spectrometer to detect the nuclear transition in various materials.
The new spectroscopy technique also provides a greater understanding of the decay of Thorium-229, potentially opening doors to study nuclear power generation with new types of materials.
Over the past two decades, Derevianko has worked on inventing, improving and developing several classes of clocks. In 2012, he published an article proposing a nuclear clock, co-authored by Alex Kuzmich, now at the University of Michigan, and several other collaborators (this work, alongside much of his other work, recently earned Derevianko the title of Fellow of the American Association for the Advancement of Science). They showed that the nuclear clock would offer vast improvements in clock accuracy, and they proposed a specific platform that can realize that accuracy. The 2012 paper has served as a major motivation for the search of the elusive narrow “needle in a haystack” nuclear clock spectral line which can excite the Thorium-229 nucleus. This line was finally found, fortunately within the ultraviolet part of the light spectrum, in 2024 as reported in two independent papers in Physical Review Letters, a banner year in the development of nuclear clocks.
More recently, Derevianko has been working on the solid-state nuclear clock theory in close collaboration with the UCLA and JILA experimental groups. This pioneering work has led to observing the nuclear clock signal for the first time in 2024 and has developed ideas that may shape the field for years to come.
“If you understand the complex mechanisms, you can not only invent the clockwork but also optimize its performance,” Derevianko said.
Theory has helped to improve the clock precision by magnitudes and make the clocks smaller.
“One of the dreams in this community is to have super-precise clocks that are transportable,” Derevianko said. “Currently the best transportable clocks are the size of a large cabinet. Our paper points a way to a smartphone-sized, super-accurate clock that one can hold in your hand.”
There are many applications for more precise clocks. Nuclear clocks could be used in satellites for GPS and communications, where many atomic clocks are used, as well as in geodetic measurement of gravity, which varies around the planet. Geodetic measurements can also identify potential mineral resources based on the unique densities of minerals. Nuclear clocks could also aid in the search for dark matter, which is of primary interest for Derevianko.
