Keeping time with an atomic nucleus

Atomic clocks measure time so precisely that they gain or lose less than a second every 30 billion years. With so-called nuclear clocks, it would be possible to measure time even more accurately. Furthermore, they would enable scientists to delve deeper into fundamental physical phenomena. "We're talking about the forces that hold the world together at its core," says LMU physicist Professor Peter Thirolf, who has been researching nuclear clocks for many years. In contrast to conventional atomic clocks, this type of clock would register forces inside the atomic nucleus. "This would open up a whole range of research fields that could never be investigated with atomic clocks," adds Thirolf's colleague Dr. Sandro Kraemer, who played a major role in driving the project forward while completing his doctorate at KU Leuven in Belgium.

In the race for nuclear time, Thirolf and Kraemer are in the leading pack. Working at the Chair of Experimental Physics in Garching, the two scientists have now made an important advance on the road to the first nuclear clock as part of an international team. As they report in the journal Nature, they have managed to characterize the excitation energy of thorium-229 with great precision thanks to a new experimental approach. This atomic nucleus is to be used as the timekeeping element of nuclear clocks in the future. Precise knowledge of what frequency it needs for excitation is crucial for the feasibility of the technology.

The innermost clock

For a clock, you need something that periodically oscillates and something that counts the oscillations. A grandfather clock has a mechanical pendulum, the oscillations of which are registered by the clock's mechanism. In atomic clocks, the atomic shell functions as the timekeeper. Electrons are excited and switch back and forth between high and low energy levels. Then it is a matter of counting the frequency of light particles emitted by the atom when the excited electrons fall back into their ground state.

In nuclear clocks, the basic principle is very similar. In this case, we penetrate to the nucleus of the atom, where various energy states can also be found. If we managed to excite them precisely with a laser and measure the radiation emitted by the nucleus when falling back into its ground state, then we would have a nuclear clock. The difficulty is that of all atomic nuclei known to science, there is only one that could lend itself to this purpose: thorium-229. And even that was purely theoretical for a long time.

A nucleus like no other

What makes thorium-229 so special is that its nucleus can be put into an excited state using a relatively low light frequency -- a frequency just about obtainable with UV lasers. Research stalled for 40 years, because although scientists suspected that an atomic nucleus with the right characteristics exists, they were unable to experimentally confirm this hypothesis. And then in 2016, Thirolf's research group at LMU made a breakthrough when they directly confirmed the excited state of the nucleus of thorium-229. This fired the starting gun on the race for the nuclear clock. In the meantime, many groups worldwide have taken up the topic.

To get a clock going, the timekeeping element and the clockwork need to be perfectly attuned to each other. In the case of the nuclear clock, this means that you need to know at what exact frequency the atomic nucleus of thorium-229 oscillates. Only then can you develop lasers that excite exactly this frequency. "You can picture it as being like a tuning fork," explains Kraemer. "As a musical instrument tries to match the frequency of the tuning fork, so the laser tries to hit the frequency of the thorium nucleus."

If you were to try out all possible frequencies with different lasers, it would take forever. Not to mention that lasers would have to be laboriously developed first in the corresponding UV light spectrum. To narrow down the range in which the oscillation frequency of thorium-229 lies, the researchers therefore took a different tack. "Nature is sometimes merciful and offers us various routes," says Thirolf. As it happens, lasers are not the only way of producing the excited state of the thorium nucleus. It also occurs when radioactive nuclei decay into thorium-229. "So we start with the grandparents and great-grandparents of thorium, as it were."

ISOLDE is forging new paths

These ancestors are called francium-229 and radium-229. As neither are found readily in nature, they have to be manufactured synthetically. Currently, there are very few places in the world that are capable of doing this. One of them is the ISOLDE laboratory at the European Organization for Nuclear Research (CERN) in Geneva, which has made possible the old dream of the alchemists -- of transforming one element into another. To accomplish this, scientists bombard uranium nuclei with protons accelerated to extremely fast speeds, thereby producing various new nuclei -- including francium and radium. These elements decay rapidly into the radioactive parent nucleus of thorium-229: actinium-229.

Kraemer, Thirolf, and their international colleagues embedded this elaborately manufactured actinium into special crystals, where the actinium decays into thorium in an excited state. When the thorium jumps back into its ground state, it emits the light particles whose frequency is so crucial for the development of the nuclear clock. Demonstrating this is no trivial task, however. "If the nuclei do not sit in exactly the right place in the crystal, we've got no chance," says Kraemer. "The electrons in the environment absorb the energy and nothing that we can measure makes it outside."

Previous attempts that inserted uranium into the crystal lattice instead of actinium fell at this hurdle. "When uranium-233 decays into thorium-229, a recoil is produced that wreaks havoc in the crystal," explains Thirolf. The decay of actinium into thorium, by contrast, causes much less damage, which is why the researchers chose this laborious path for the new study in collaboration with CERN.

The hard work and patience have paid off: With their new method, the team was able to determine the energy of the state transition very precisely. They also demonstrated that a nuclear clock based on thorium embedded in a crystal is feasible. Such solid-state-based clocks would have the advantage over other approaches in that they would yield measurement results much more quickly, because they work with a larger number of atomic nuclei.

A matter of time

"We now know the approximate wavelength we need," says Thirolf. Building on the new findings to progressively narrow down the exact transition energy will be the next task. First, the researchers will create an excitation with a laser. And then they can keep homing in on the frequency with increasing accuracy with more precise lasers. So that this does not take too long, they do not use tweezers to find the needle in a haystack, so to speak, but a rake. This 'rake' is called a "frequency comb" and was developed by Thirolf's LMU colleague Professor Theodor Hänsch, who received the Nobel Prize in Physics in 2005 for the achievement. Scientists can use the comb to scan hundreds of thousands of wavelengths simultaneously until they find the right one.

Some challenges remain on the road to nuclear clocks. Scientists must understand the thorium isomer better, develop lasers, work out theories. "But it's worth sticking the course," reckons Thirolf. "The project opens up such a plethora of new application possibilities in the long run that it's worth all the experimental effort," adds Kraemer. These new possibilities encompass not only fundamental physics research, but also practical applications. With a nuclear clock, scientists could detect the tiniest changes in the Earth's gravitational field, such as occur when tectonic plates shift or ahead of volcanic eruptions. With the new successes, the prize is within arm's reach. The first prototypes could be here in less than ten years. "We might even have them ready in time for the redefinition of the second in 2030," the two physicists hope. They are referring to plans to come up with a new, more precise standard definition of a second, for which scientists will use state-of-the-art atomic clocks -- and perhaps even the first nuclear clocks.