Ekkehard Peik thought it would take only a few months to create the basic ingredients of a radical new clock. That was back in 2001, when he and his colleague Christian Tamm proposed a device with the potential to be even more precise and portable than the world’s best atomic clocks.
Peik’s estimate was off by more than two decades. But this year, his team and two other groups managed to finally achieve what he and Tamm had proposed, generating the first tick of a clock based on tiny energy shifts inside an atomic nucleus.
The world’s best atomic clocks rely on the energy transitions of electrons that orbit an atomic nucleus. These clocks are so accurate that they gain or lose only a second every 40 billion years.
Peik and Tamm, both physicists at the PTB, Germany’s national metrology institute in Braunschweig, came up with the idea of a nuclear approach. “We thought we could very quickly do a kind of demonstration experiment,” says Peik. For more than a year, they tried different ways to nudge nuclei of radioactive thorium-229 into an excited state. Then they would need to tune a laser to the energy of the nuclei’s transition and eventually use its frequency to mark time. “But the experiments all failed,” he says. The pair published their unproven proposal in 2003 (E. Peik and C. Tamm Europhys. Lett. 61, 181; 2003).
Tamm later retired but Peik kept at the problem. His perseverance paid off this year when his was the first of three groups to excite the nucleus to make it ‘tick’. There is still a way to go before this kind of system can replace the most precise clocks. But a starting gun has been fired. “Now everybody wants a piece,” says Thorsten Schumm, an atomic physicist and Peik’s collaborator at the Vienna University of Technology.
Peik always found precision clocks fascinating. He was thrilled by their fusion of fundamental physics and practical applications. The idea of creating an entirely new kind of clock came when his PTB colleague, metrologist Uwe Sterr, noted a quirk in the nuclear physics literature. Studies showed that the thorium-229 nucleus must have a strangely low-energy excited state — one so low that it would be possible to induce that transition with a precision laser. For nuclear physicists, this transition was a curiosity. But Peik and Tamm saw that they could use it to make a clock. “It was quite logical for us to follow up,” says Peik. “Nobody had done this.”
Immersing himself in nuclear physics, Peik soon realized that a thorium-229 clock would have a raft of advantages. Not only could a nuclear clock be more precise than atomic clocks, it would also be more robust, because the nucleus is less sensitive to electromagnetic fields than electrons are.
To make it work, they had to find the transition energy. They got creative, zapping thorium with lamps, lasers and high-powered radiation to jolt its nuclei into a higher energy state. But each method failed. Even so, they learnt something with every attempt. That was enough to keep them going, says Peik.
Schumm, who joined the effort in 2010, credits Peik for trusting his own ideas. “A different character would not have come so far.”
Triumph came thanks to the pair expanding the project into a Europe-wide consortium involving a range of approaches. Still trying ideas from the 2003 paper, Schumm’s lab set about embedding trillions of atoms of thorium-229 in a crystal, to boost the transition signal so that it would be easier to detect. Meanwhile, Peik’s lab built an ultraviolet laser that could excite the nucleus.
In 2023, consortium colleagues at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland, observed the energy transition happening naturally, by producing a short-lived radioactive element that later turned into thorium-229. Now Peik and Schumm’s team had a better idea of where to look and, by finding the right laser frequency, they succeeded in triggering the telltale transition. “We were all excited,” says Peik.
Since then, physicists in the United States have taken the first step in turning the transition into a clock. And because the tick depends on fundamental forces in the nucleus, researchers are using it to explore basic physics, such as the nature of dark matter.
But for nuclear clocks to live up to their promise, physicists need to make more-compact and precise lasers, and to better understand the sources of uncertainty, such as how the transition energy depends on the type of crystal material. Now that the field is kicking off, Peik has no plans to relax. “Once again, it’s about waiting for ideas and for experimental progress,” he says. “I’m curious what will be next.”
