Long-awaited breakthrough: laser excites atomic nuclei for precision measurement

Although we can’t see it, we live in a quantized world where the light that illuminates our days is made up of tiny bundles of energy, and the atoms that make up matter are similarly divided into discrete bands of energy.

Like coins in a slot machine, dropping the right quanta of light on an atom can cause its electrons to jump into quantum states of higher energy bands. And as they go down again, those “coins” of light can be redeemed.

Now, researchers in Austria and Germany have achieved a decades-long goal of using lasers to excite an isotope of thorium, not its electrons, but the tightly bound bundle of protons and neutrons that make up its nucleus.

With a jolt of energy corresponding precisely to the gap between the two quantum states of the nuclei, the thorium-299 nuclei were made to “jump” just as electrons, whole atoms and molecules can.

“Normally atomic nuclei cannot be manipulated with lasers. The energy of the photons is simply not enough,” explains physicist Thorsten Schumm of the Vienna University of Technology.

Moving atomic nuclei from one quantum state to another requires at least a thousand times more energy than electrons making the jump between orbital shells, Schumm continues. The researchers also needed to know precisely what that energy gap is, so they could fine-tune their lasers.

Thorium-299 was chosen as a target because its nucleus has two very close adjacent energy states that Schumm and his collaborators at Germany’s National Institute of Metrology, PTB, thought like many scientists before them they could unlock the famous “thorium transition”.

Scientists have been trying to precisely measure this energy gap since the 1970s, when decay experiments first revealed the proximity of thorium-299’s two energy states.

Over decades, different teams have steadily refined their estimates, from less than 100 electron volts to about 8. This is the amount of energy released (as radiation) when a thorium nucleus drops from an energy state to another.

But these measurements were not precise enough to detect the difference in energy (this is thorium’s transition) and therefore know the exact pulse of energy, or “coin size”, needed to switch nuclei between two states

In fact, because the thorium transition is so difficult to observe, its existence was only confirmed in 2016 and directly measured (not deduced) for the first time last year.

“You have to get the energy right to within a thousandth of an electronvolt to detect the transition,” says Schumm.

To increase their chances of finding the exact thorium transition, Schumm’s team made crystals that housed trillions of thorium atoms, rather than placing single thorium atoms in electromagnetic traps and zapping them individually, as many previous teams had done.

The crystals had to be completely transparent so that the laser only affected the embedded thorium atoms, and only a few millimeters in size to minimize any interference.

In November 2023, they finally found it: a clear signal from their experiments gave them a much improved measurement for the thorium transition of 8.355743 0.000003 electron volts.

At a fraction of the transition energies of other atomic nuclei that researchers have studied, Schumm’s team was able to use benchtop lasers, rather than high-energy X-ray light from a synchrotron, to shift the thorium-299 nuclei of a low place. ground state in a slightly higher metastable.

The long-awaited breakthrough shows that thorium-299 atoms embedded in solid crystals could be used to make a nuclear clock that would be far more stable, accurate and practical than existing atomic clocks.

“Our measurement method is just the beginning,” Schumm says of the potential applications of his work, including ultra-precise measurements of time and gravity. “We can’t yet predict what results we’ll get with this. It’s definitely going to be very exciting.”

The study was published in Physical review letters.