A giant proposal for a new type of molecule

Researchers have predicted the existence of a new kind of gargantuan molecule, large enough to dwarf a virus, that looks weird and acts even weirder. Such a molecule, described in a paper to appear in Physical Review Letters, would have the potential to be in two configurations simultaneously, a feat that might prove useful in storing and transmitting quantum information.

Atoms in an excited state can have an electron that roams very far from its nucleus. These giant atoms, called Rydberg atoms, can form molecules that are over a thousand times larger than everyday molecules. The newly predicted molecule would be so large that a small virus — made up of its own many molecules — could fit completely inside, says study coauthor Seth Rittenhouse.

In the new study, Rittenhouse and his colleague Hossein Sadeghpour, both of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., predicted what would happen to a giant rubidium atom in the Rydberg state  if it were brought near a small molecule, composed of potassium and rubidium, with a positive electrical charge at one end and a negative charge at the other. This charge separation, called a dipole moment, wouldn’t be strong enough to rip the wandering electron away from the giant atom. But the electron would find the dipole irresistible, their calculations show. “That extra bit of charge is enough to get the electron to stick near it,” says Rittenhouse.

In this way, the giant atom and the small molecule would form a Rydberg molecule with a completely new type of chemical bond, the researchers predict. “When you talk about chemistry, you talk about bonds,” Rittenhouse says. “This type of bond is new.”

In 2000, atomic physicist Chris Greene of JILA and the University of Colorado at Boulder and colleagues predicted the existence of a Rydberg molecule made up of an excited atom and a neutral atom. The researchers calculated that the roaming electron of the excited atom would like to hover around the neutral atom and form an electron cloud that resembled, surprisingly, an ancient trilobite.

“I think it came as a surprise to a lot of people when we made our original prediction, because diatomic molecules, with just two atoms, were believed by chemists to be completely understood,” Greene says. With the addition of a molecule with a charge, the new study takes his prediction a step farther: “I view this as a really interesting extension,” he says.

One of the most interesting predictions is that the new giant molecule would exhibit a strange quantum property called superposition. The potassium-rubidium molecule can be pointed in two directions at once, the models predict, with the potassium atom on top and on the bottom of the rubidium atom at the same time. The researchers jokingly call this eerily simultaneous condition the “Rydberg cat,” in homage to the more famous Schrödinger’s cat, which is both alive and dead at the same time. Rittenhouse says that this superposition state may be able to serve as a qubit, a bit of quantum information that could store or transmit a message.

“Even though I’ve worked a lot in this field, I haven’t had this concept — and I think it is really clever — that you have two states,” Greene says. “Having seen it, it’s completely obvious, but the fact is that they were the first to see that, and I think that’s a pretty nice contribution.”

But he cautions that these proposed molecules may disappear before they can be useful. They must be kept in ultracold conditions, and the quantum properties are fragile. The lifespan of the molecules is estimated to be about 100 microseconds. Their possible usefulness in quantum computation is “a bit of a stretch to me, but it’s certainly an interesting aspect I hadn’t thought of before,” Greene says.