At 11:30 one night a discovery that scientists have waited for, for 50 years was finally made...

A team in the lab of Jun Ye, led by graduate student Chuankun Zhang saw a peak rise from the static on his monitor at the research institute JILA in Boulder, Colorado. Their team was in disbelief. Was what they were seeing real? They checked again, and saw a signal from a thorium-229 nucleus switching between two states, known as the “nuclear clock” transition. Time stamp: 3:42 a.m.

During their weekly meeting they announced their discovery to Jun Ye. The team tried to play it cool. “They were all poker-faced,” Ye said, until Zhang shared a slide displaying the long-sought peak. Tears flooded Ye's eyes as he took in the information.

The team published their findings in a Nature journal, after the 3rd observation of the thorium-229 transition. The new measurement is millions of times more precise than the others, and it marks the end of a marathon search for the exact laser frequency needed to induce the nuclear clock transition. 

“This paper is an incredible technical achievement,” said Hannah Williams, a physicist at Durham University in the United Kingdom who was not involved in the work.

This discovery can help physicists with understanding many mysteries of the universe. Measuring this thorium-229 transition at different times could reveal any variability in the fundamental constants of physics.

A Special Isotope

While studying the Cold War nuclear weapons, the byproduct of these weapons formed the isotope thorium-229 in 1976, which scientists found were unique.  

"Atoms are normally in what’s called the ground state, in which all the electrons orbit the nucleus in a stable manner. But an electron can also absorb energy from the outside world in the form of a photon, which causes it to become excited, move about the atom more quickly for a moment, then re-emit the photon and return to the ground state. The photon has to have just the right amount — or “quantum” — of energy to excite the electron.

The modern notion of time is actually defined by this process. Scientists use a laser to bathe a cesium atom with photons. Then they vary the laser’s wavelength until its photons each have just the right energy to excite an electron. This ultra-precise wavelength then defines the international standard for a second, which is the time it takes for 9,192,631,770 of those wavelengths to pass a given point in space.

Nuclei, the tight balls of neutrons and protons at every atom’s core, also have ground and excited states, in which one of their constituent protons or neutrons absorbs a photon and briefly swirls about more energetically. But these particles are packed much more tightly than electrons, so it takes much more energetic photons — gamma rays — to excite them. Those are much harder to produce in large quantities or with precise energy.

The thorium-229 nucleus, however, is different.

From the 1950s to the 1970s, the United States produced about two tons of uranium-233, a weapons-grade fissile material that was being investigated as a possible alternative to uranium-235 and plutonium-239 in atomic weapons research. The program was eventually scrapped, leaving only some tanks of radioactive liquid behind. But when the nuclear physicists Larry Kroger and Charles Reich at Idaho National Laboratory studied the radiation emanating from that liquid in 1976, they found indirect evidence that uranium-233’s “daughter” nucleus (the product of its radioactive decay), thorium-229, had a mysterious excited nuclear state that involved far less energy than expected.

Every nucleus lives in a tense tug-of-war between two of nature’s forces. The electromagnetic force between its positively charged protons tries to rip it apart, while the strong force holds the bundle together. Exciting a neutron or proton causes the nucleus to settle into a new, more energetic equilibrium between the two forces.

The Idaho researchers observed that reversing the intrinsic angular momentum, or “spin,” of thorium-229’s outermost neutron seemed to take 10,000 times less energy than a typical nuclear excitation. The neutron’s altered spin slightly changes both the electromagnetic and strong forces, but those changes happen to cancel each other out almost exactly. Consequently, the excited nuclear state barely differs from the ground state. Lots of nuclei have similar spin transitions, but only in thorium-229 is this cancellation so nearly perfect" (Howlett).

The End Result

Following Hudson’s lead, groups began building solid crystal compounds with the thorium embedded inside — an approach mentioned in Peik and Tamm’s original proposal. The crystals can hold quadrillions of atoms instead of just a few, so a laser could rule out wavelengths at a rapid clip.

A breakthrough last year at CERN kicked the race into overdrive. As in the older Idaho studies, the CERN team produced excited thorium-229 through radioactive decay, then looked at the photons coming out. But they found a way to do so in a much quieter environment, which enabled them to directly measure the faint rays of ultraviolet light coming from the nuclear clock transition and put a tighter estimate on the transition energy.

The CERN team’s updated estimate narrowed the wavelength hunters’ search from an entire forest to a small copse of trees, which they immediately began scouring. In April of this year, a European team became the first to report that they had probed the state with a laser. Peik contributed his laser expertise, and the collaboration made use of a crystal-growing powerhouse built by the physicist Thorsten Schumm at the University of Vienna.

Hudson’s group was right on their heels — a paper reporting their discovery ran in Physical Review Letters in July.

Ye’s group at JILA had also obtained one of Schumm’s crystals and was racing to excite the thorium-229 transition as well. For years, the group has been using its clock-building acumen to engineer a special ultraviolet laser with the sole purpose of turning thorium-229 into a nuclear clock. The laser allows Ye and his group to test many wavelengths at once to close in on any transition he seeks. His team’s new paper caps this trio of parallel discoveries with what will likely be the most precise measurement of the state’s energy for years to come.

“These results have all come out in a very short period of time,” Williams said, “so that is very exciting as to what they’re going to do next.”

The result starts the clock on thorium’s test of nature’s forces. “Now the fun starts,” Hudson said, excited to put the new tool to use studying fundamental constants. “We can actually do this stuff.”

The thorium nuclear state’s energy is far more sensitive to variations in the fundamental constants than that of any atomic state. But scientists will need to improve the precision of their measurements even further to notice changes more subtle than those already ruled out by conventional atomic clocks. Currently, Ye can measure the nuclear clock transition with a precision of one part in a trillion, but possible variations would be as small as one part in 10 trillion. “It’s many years down the road,” he said.

Eventually, though, some old Cold War byproducts could yield the first evidence for deeper, still undiscovered physics that underlies the universe we see. 

“We call them constants, but why?” Hudson asked. “Nothing is ever that simple when you zoom in and look at it.”

Citations:

Howlett, Joseph. “The First Nuclear Clock Will Test if Fundamental Constants Change.” Quantumagazine, Quantumagazine, 4 September 2024, https://www.quantamagazine.org/the-first-nuclear-clock-will-test-if-fundamental-constants-change-20240904/#:~:text=An%20ultra%2Dprecise%20measurement%20of,forces%20that%20bind%20the%20universe. Accessed 4 September 2024.