Odd “butterfly” molecule could lead to new parts of the quantum realm

A large, cold molecule that resembles a butterfly, with “wings” made from electrons, has been made for the first time, completing the search for a “zoo” of similar molecules. The result could provide a gateway to completely new parts of the quantum realm.

Herwig Ott at RPTU University Kaiserslautern-Landau in Germany and his colleagues made the molecule by cooling rubidium atoms to a few millionths of a degree above absolute zero by using lasers and electromagnetic forces. The researchers then used lasers again to make some atoms very large by pushing their outermost electron very far from their nuclei. The quantum properties of atoms that have been cooled and enlarged in this way can be precisely manipulated with lasers, which the team leveraged to move a giant atom’s electron towards a normal-sized rubidium atom, binding them together to create a new type of molecule with extreme properties.

Each molecule was about 25 nanometres in size – bigger than the diameter of a DNA strand that contains billions of atoms – and thousands of times more responsive to electric fields than most molecules. The shape of the new molecule was determined by its electrons, with the outermost being spread out in space in a shape that resembles the wings of a butterfly.

Ott says that tuning the lasers just right to get molecules with this exact configuration was tricky – it took weeks of tweaking before the team successfully created them in the lab. Ott compares the process to searching for an object on a road by inspecting 1 millimetre at a time while standing a kilometre away.

Team member Matthew Eiles at Purdue University in Indiana says the work is a culmination of several past mathematical and experimental studies that helped narrow down this search. Based on mathematical models, researchers have spent 20 years searching for a “zoo” of giant ultracold molecules ought to exist, and the new butterfly is the last one to be discovered, he says.

At the same time, this experiment opens a path towards creating more exotic and elusive ultracold molecules, including those that could be very heavy in addition to being very large and charged, says Michał Tomza at the University of Warsaw in Poland. Weibin Li at the University of Nottingham in the UK says the butterfly molecule was difficult to create but could now be used as precursor for making something even more difficult – ultracold atoms with negative charge, or anions. If made ultracold, anions could be used in tests of fundamental laws of particle physics or antimatter studies, but standard cooling methods have failed to chill them so far.

Eiles and Ott have already mathematically investigated exactly how to use the butterfly molecule to create ultracold anions and hope to see the first signs of them within just a few years. “The theory is already written,” says Eiles.