Custom molecules built with optical tweezers

Scientists have hand-built a designer molecule using optical tweezers, in the process pioneering an entirely new form of chemical reaction.

A team at Harvard University in the US trapped two individual atoms in separate laser beams, then brought them together and triggered a reaction between them with a photon of light.

The achievement opens up a way to build designer molecules atom by atom, said Lee Liu, the lead author of a paper on the research published in Science.

“We’re putting atoms together more like Lego, rather than mixing chemicals together and relying on chance collisions,” Liu said. “We are doing chemistry in a way that is totally new and surprising.”

Current chemical synthesis methods typically involve often toxic solvents and elaborate procedures to separate and purify the products of reactions – and even then, random factors in the chemistry may make for unpredictable outcomes.

But the Harvard team’s technique, which triggers the reaction with a very specific pulse of light that binds the two atoms, allows very precise control.

“That selectivity alone is a huge boon to studying elementary chemical reactions,” Liu said.

The team developed the technique in their quest for new qubits, the building blocks of quantum computers. Molecules built from dissimilar atoms seem like a good contender: they interact strongly with other molecules, which is crucial for quantum processing. They also have a more complex array of energy levels available for storing quantum information.

While the combination of interactions and storage make molecules more promising than many existing qubits that are based on single atoms, they have a major drawback: molecules are hard to cool to the extremely low temperatures required for controlling quantum interactions, close to absolute zero (–273.16 degrees Celsius).

Atoms can be easily cooled with lasers but the complex energy levels of molecules make it much harder to design a simple laser cooling system for them.

Liu and the team bypassed this problem by using cold atoms to build a molecule that was already cold.

They cooled and trapped an atom of cesium in a strongly focussed laser beam, known as optical tweezers. In another laser beam they trapped a cold atom of sodium and maneuvered the two together into a single optical tweezer. There, the large cesium and the diminutive sodium atoms waited until the scientists injected a pulse of light with the requisite energy to bind the pair into a sodium cesium molecule, a process called photoassociation.

Although the reaction was successful, the resulting sodium cesium molecule was initially in too excited a state for quantum computing. However the team is exploring methods to cool it down to its ground state with another laser.

Beyond their own quantum computing experiments, the technique opens a new way to explore the quantum mechanics of chemical reactions, says Liu.

“We hope that once this technology matures, chemists will have a completely new tool to build up individual designer molecules atom-by-atom, in total isolation.”