Chemical bond makes giant butterfly molecules

Physicists built a new, supersized molecule made of atoms held together by a far-roaming electron – like a flock of sheep being herded by a sheepdog. 

Reporting in Nature Communications, the team from Germany and America created fleeting “butterfly” Rydberg molecules they predicted on paper 14 years ago – and which could find a place in quantum computers.

The new kind of molecule is bound by a lone electron ranging extremely far from its nucleus and whizzing around another atom, herding it close like a sheepdog does a stray sheep.

“It’s a whole new way an atom can be bound by another atom,” says Chris Greene a physicist at Purdue University, who co-authored the research.

Back in 1888, when most scientists didn’t believe in atoms, Swedish physicist Johannes Rydberg found a formula that reproduced colours of light emitted by different chemical elements.

Some 25 years later, Danish physicist Niels Bohr built on Rydberg’s ideas when he described the ‘solar system’ model of the atom, with the nucleus at the centre orbited by electrons.

One of Bohr’s central ideas was that if you give an electron a kick of energy, you can promote it to a higher energy level, meaning it orbits further, on average, from the nucleus.

Rydberg atoms are extreme examples of this. The outermost electron, promoted to an extremely high energy, can roam up to 1,000 times further from the nucleus than normal.

Rydberg atoms are also atomic monstrosities. They can be up to a millionth of a metre in diameter. That might seem small, but it’s about the size of an Escherichia coli bacterium, which is built from about 90 billion regular atoms.

Scientists have been studying Rydberg atoms for decades – even using them to build a qubit, the basic element of a quantum computer.

In 2002, Greene and his team predicted that the free-ranging electron of a Rydberg atom might be used to form a new kind of chemical bond.

They worked out the shape of the atomic orbitals (describing the probability of finding an electron at a particular position around the nucleus) and found it looked like a butterfly – hence the name.

Now they’ve made one.

Since the molecule would be bound by only the “tiniest conceivable” force, Greene knew their only hope using ultracold, almost motionless atoms. His team used rubidium, an element chosen for cold atom experiments because it’s easy to manipulate with lasers.

Greene’s team cooled rubidium gas to just 10 millionths of a degree above absolute zero. Using a laser, they gave an electron a kick of energy, knocking it from its usual orbit out into a super-excited state and creating a Rydberg atom.

They then used the laser again to corral another rubidium atom into just the right distance nearby. That’s when the excited electron took over.

“This electron is like a sheepdog,” says Greene. This herding creates a tiny force of attraction holding the two atoms together in the very fragile butterfly state.

Though the molecule lasted only about five millionths of a second, it was long enough to study.

The butterfly state caused changes in the frequency of light that the Rydberg molecule absorbed. By detecting these changes, the team could measure the energy of binding between the two atoms. 

This is not the first kind of Rydberg molecule created. Back in 2007, scientists managed to coax two Rydberg atoms together, each with a herding electron, to form a molecule that looked a little like an extinct marine animal called a trilobite.

The butterfly Rydberg is different because only one atom needs to be in a super-excited state. The other is passively herded.

From a practical point of view, Rydberg molecules have a very high electric dipole moment (in essence, the separation of charge within the molecule) coming from the large distance between the negative electron and positive nucleus.

This means they can be moved around with electric fields 100 times weaker than those needed for regular atoms – useful for setting up the long-range interactions between atoms needed for quantum computing.

For now, Greene plans to see if the ranging electron can herd more than one atom. A three-winged butterfly, perhaps?