Though we cannot see it, we live in a quantized world where the light that illuminates our days is made up of tiny packets of energy, and the atoms that constitute matter are similarly divided into discrete energy bands.

Like coins in a slot machine, dropping the right quanta of light onto an atom can cause its electrons to shift into quantum states of higher energy bands. And as they shift back down, those 'coins' of light can be refunded.

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 making up its very core.

With a jolt of energy corresponding precisely to the gap between the nuclei's two quantum states, thorium-229 nuclei were made to 'jump' just like electrons, whole atoms, and molecules can do too.

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

Bumping 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-229 was chosen as the target because its nucleus has two very close adjacent energy states that Schumm and his collaborators at the National Metrology Institute of Germany, PTB, thought – like many scientists before them – could unlock the famed 'thorium transition'.

Scientists have been trying to precisely measure this energy gap since the 1970s, when decay experiments first revealed the closeness of thorium-229's two energy states.

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

But those measurements weren't precise enough to detect the energy difference (that's the thorium transition) and thereby know the exact energy pulse, or 'coin size', required to shift the 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 hit the right energy with a precision of one-millionth of an electron volt in order to detect the transition," Schumm says.

To increase their chances of finding the exact thorium transition, Schumm's team made crystals that housed trillions of thorium atoms, rather than placing solitary 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 built-in thorium atoms, and just 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.

Being 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-229 nuclei from a low-lying ground state into a slightly higher metastable one.

The long-awaited breakthrough shows that thorium-229 atoms embedded in solid crystals could indeed be used to make a nuclear clock that would be far more stable, precise, and practical than existing atomic clocks.

"Our measuring method is just the beginning," says Schumm of the potential applications of their work, including ultra-precise measurements of time and gravity. "We cannot yet predict what results we will achieve with it. It will certainly be very exciting."

The study has been published in Physical Review Letters.