In principle, a nuclear clock could be even more precise than the world’s current best timekeepers, known as optical clocks, and less sensitive to disturbances.
A nuclear timekeeper could also allow physicists to study fundamental forces of nature in new ways. “We will be able to probe scenarios of dark matter and of fundamental physics that are currently inaccessible to other methods,” says Elina Fuchs, a theoretical physicist at Leibniz University Hannover, Germany.
The long-sought breakthrough — made by a collaboration between the Vienna University of Technology and Germany’s national metrology institute, the PTB, in Braunschweig — involved using an ultraviolet laser to prompt a nucleus of the radioactive metal thorium-229 to switch between energy states. The frequency of light absorbed and emitted by the nucleus functions as the clock’s tick.
“It is a culmination of nearly a half a century of effort of many scientific groups,” says Olga Kocharovskaya, a physicist at Texas A&M University in College Station.
Precision timing
Optical clocks keep time so well that they waver by just 1 second every roughly 30 billion years. Their ticks are governed by the frequency of the visible light needed to shift an electron orbiting an atom such as strontium between energy states.
But a nuclear clock could do even better. It would use the more energetic transition of boosting the nucleus’s protons and neutrons to a higher energy state. This would use slightly higher frequency radiation, meaning that time could be sliced even more finely to create a more precise clock. More importantly, such a clock would be much more stable than an optical clock, because particles in the nucleus are less sensitive than electrons to
But finding a material with a suitable nucleus has proved difficult. Energy transitions in most nuclei tend to be huge, requiring much more than the nudge of a tabletop laser. In the 1970s, physicists discovered that thorium-229 is an anomaly — its first energy state is extremely close to its lowest, ground state. And in 2003, physicists proposed using thorium-229 as the basis of a super-stable clock, but they needed to find the precise energy of the transition and its corresponding laser frequency, which would have been impossible to predict with any accuracy using theory. Since then, experimentalists have used a range of methods to narrow down the figures.
To observe the transition, researchers placed radioactive thorium atoms into crystals of calcium fluoride that were a few millimetres wide. Scanning across the expected region with a purpose-built laser, they eventually hit on the right frequency — around 2 petahertz (1015 oscillations per second) — which they detected by spotting the photons emitted as the nuclei returned to the lower energy state. Co-author Thorsten Schumm, an atomic physicist at the Vienna University of Technology, recalls scrawling “found it” in large red letters across his lab book at a meeting convened the next day to discuss the promising-looking signal. “It was crystal clear,” he says.
The team pinpointed the frequency with a resolution 800 times better than the next best attempt. A team at the University of California, Los Angeles, has since reproduced the result using a different crystal, but the same frequency, says co-author Ekkehard Peik, a physicist at the PTB. It’s “a very nice confirmation”, he says.
Fundamental physics boost
To turn the system into an actual clock, physicists will need to markedly reduce the resolution of the laser, so that it stimulates the nucleus at almost exactly the right frequency to be read off reliably, says Peik. Building such a laser “remains a big challenge, but there are little doubts that it will be achievable in the near future”, adds Kocharovskaya.
If all goes well, the team says that a thorium-based nuclear clock could end up being around ten times more accurate than the best optical clocks. “It’s the robustness with respect to external perturbations that will make this a better clock,” says Schumm. Hosting the nuclei in a solid crystal could also help to make the clock more compact and portable than optical systems.
Scientific methods that were made possible by super-precise optical clocks, such as probing Earth’s gravitational field by measuring differences in clock speed, “could get a major boost”, says Kocharovskaya
