The project aims to see if spectroscopy in a helium bath is possible — a proof-of-concept for future experiments that will use more exotic mixed atoms.

But Sótér was curious about how mixed atoms would react to helium at different temperatures. She persuaded her partners to spend precious antimatter repeating the measurements in increasingly colder helium baths.

“It was a random thought of mine,” said Sauter, now a professor at the Swiss Federal Institute of Technology in Zurich. “People don’t believe it’s worth wasting antiprotons on it.”

The spectral lines of most atoms are completely out of control in denser and denser fluids, possibly expanding by a million times, while Frankenstein atoms do the opposite. As the researchers lowered the temperature of the helium bath to cooler temperatures, the spectral smear narrowed. Below about 2.2 Kelvin, where helium becomes a frictionless “superfluid,” they saw a line almost as sharp as the tightest they’ve seen in helium. Despite potentially taking a hit from a dense environment, the hybrid matter-antimatter atoms act in unison in uncanny ways.

Unsure how the experiment turned out, Sótér and Hori sat on the results, thinking about what might have gone wrong.

“We continued to argue for many years,” Hori said. “It’s hard for me to understand why this is happening.”

a close call

Over time, the researchers concluded that nothing went wrong. The tight spectral lines show that the mixed atoms in superfluid helium are not colliding with atoms in the billiard-ball manner typical of the gas. The question is why. After consulting with various theorists, the researchers came up with two possible reasons.

One of them involves the properties of the liquid environment. When the team cooled helium into a superfluid state, the atomic spectrum suddenly tightened, a quantum mechanical phenomenon in which individual atoms lose their identity in a way that allows them to flow together without rubbing against each other. In general, superfluidity gives atomic collisions an advantage, so researchers expect foreign atoms to experience only a slight broadening or even a limited tightening in some cases. “Superfluid helium,” Remeshko said, “is the softest thing known, and you can immerse atoms and molecules in it.”

But while superfluid helium may help mixed atoms become their most isolated selves, that alone doesn’t explain how well the atoms behave. Another key to their consistency, the researchers believe, is their unusual structure, caused by their antimatter composition.

In an ordinary atom, a tiny electron can move away from its host atom, especially when excited by a laser. On such a loose belt, electrons can easily bump into other atoms, perturbing their atoms’ intrinsic energy levels (and causing spectral broadening).

When Sótér and her colleagues swapped lively electrons for bulky antiprotons, they revolutionized the dynamics of atoms. The giant antiproton is more like a house, close to the outer electrons to protect its nucleus. “The electrons are like a force field,” Hori said, “like a shield.”