For the first time, scientists have used an atomic nucleus as a clock.
Clocks need to keep a steady rhythm. (Think of the swinging pendulum in a grandfather clock.) The new clocks are based on how light interacts with an atomic nucleus — the collection of protons and neutrons at the center of an atom. This technique could allow scientists to make clocks that tick more precisely than any before.
The world’s most precise timepieces are already made using atoms. Existing atomic clocks rely on their electrons. Clocks based on atomic nuclei might perform even better. Now, two teams of scientists have finally made the first nuclear clocks.
This tech is still at an early stage. So nuclear clocks don’t yet tick more precisely than atomic clocks. But these new clocks can already test physics in novel ways.
A research team used one of these clocks to search for dark matter. That’s an unidentified substance that makes up much of the universe. The team describes its new clock — and the search — in a paper submitted June 3 to arXiv.org. This nuclear clock didn’t find any dark matter. But it seems to be more sensitive to some types than atomic clocks. In that search, “we’re already outperforming all of the atomic clocks,” reports Thorsten Schumm. He’s a physicist on the project at Vienna University of Technology in Austria.
“This is an outstanding result,” says Victor Flambaum, who did not take part in the work. A theoretical physicist, he works at the University of New South Wales in Sydney, Australia. The new feat should spur more progress, he says. “This is only the first step. [The] race for building super-accurate nuclear clocks just started.”
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The clocks’ laser ‘pendulum’
Nuclear clocks have the potential to weigh in on other weird physics, too. That’s why they’re “one of the most actively pursued frontiers,” says Shiqian Ding. He’s a physicist at Tsinghua University in Beijing, China. Ding’s team made a nuclear clock based on a technology similar to Schumm’s. They described this second clock in a paper submitted June 7 to arXiv.org. (Neither paper has been peer reviewed.)
Both clocks contain crystals of calcium fluoride. Embedded in those crystals are atoms of thorium-229, a radioactive isotope. That thorium is key. In the entire periodic table, it’s the only element whose atomic nucleus can be used to make a clock.
Scientists hit that thorium-229 with a laser. The wiggling electromagnetic waves of laser light acted like a clock’s swinging pendulum. Thorium’s role was to make sure that the frequency of those waves didn’t change. This kept the “ticking” of these clocks from slowing or speeding up.
The way this works is based on quantum physics, the branch of science that describes atoms and similarly tiny stuff. According to quantum physics, a nucleus can hold only certain amounts of energy. That gives each nucleus a set of energy levels. A jump between two particular energy levels of a given atom always takes the same amount of energy.
So the laser’s frequency was locked to a jump between energy levels in the thorium nucleus. Only the right frequency of light will make that jump take place. Scientists used the jump to readjust the laser. They did this over and over to keep the ticks steady.
Thorium-229 is the only atomic nucleus that has an energy jump of a size that will be initiated by a laser. So it’s the only one that can be used to make a clock.
Although scientists have previously gotten close to making nuclear clocks, this readjustment step had never been done before.
“This was the final missing step before calling it an actual clock,” says Lars von der Wense, who also was not involved with the research. A physicist, he works at Johannes Gutenberg University Mainz in Germany. With improvements to lasers and crystals on the horizon, he says, nuclear-clock technology is expected to advance rapidly.
Improving on atomic clocks
Nuclear clocks have been hotly anticipated. Compared with atomic clocks, they’re less sensitive to stray electromagnetic fields that can throw them off. And they can be made out of solid materials. The atoms in atomic clocks must be suspended in a cumbersome vacuum chamber. These traits have scientists hoping for more portable, robust clocks.
And atomic nuclei respond to different forces than electrons. Electrons mainly are subject to electromagnetic forces. In contrast, the strong nuclear force holds protons and neutrons together. That opens up new possibilities for study.
Numbers called fundamental constants determine the relative strength of those forces. Comparisons of an atomic clock to a nuclear one could be used to check if those numbers really stay constant over time.
It’s been a long wait — almost a quarter century — since scientists first dreamt of a thorium nuclear clock. But “I have always been optimistic about the success of this project,” says Ekkehard Peik. He’s a physicist at the National Metrology Institute in Braunschweig, Germany. Peik is one of the scientists who proposed the idea for such a clock and was a coauthor with Schumm on a paper describing the new clocks.
After initially slow progress, researchers have made rapid advances in recent years. Now, Peik says, “a great deal of interesting research … is only just beginning.”
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