Finetuning chemistry by quantum interference

Friday, March 11, 2022

Atoms and molecules are the building blocks of chemistry and are important to our understanding of the world. By cooling atoms and molecules to ultracold temperatures it opens up a new understanding of quantum chemistry. Working with nanokelvin temperatures, one billion times colder than Antarctica during winter, researchers can observe and control particles in ways not possible at room temperature. When really cold, particles behave in strange and exciting ways. Researchers are discovering unexpected results by looking at particles from a quantum perspective.

Alan Jamison
New research by Professor Alan Jamison in the Department of Physics and Astronomy at the University of Waterloo, in collaboration with the MIT-Harvard Center for Ultracold Atoms, has demonstrated magnetic control of chemical reactions by quantum interference. The team successfully finetuned chemical reactions between an atom and a molecule to advance new insights for future applications in chemistry.

In the lab, the researchers cooled sodium atoms and sodium-lithium molecules. The ultracold temperature creates an environment where the particles slow down and are easier to observe. Researchers can then use quantum states to control the particles.

By studying chemical reactions in specific quantum states, we can learn how an effect like interference can direct reactions. Particles must collide at short range for a chemical reaction to occur, but at ultracold temperatures, particles usually reflect away at longer distances. When a reaction is guaranteed to happen every time, the particles collide at short range. This reaction rate can be calculated and is referred to as the “universal limit.”

Surprisingly, when a reaction is unlikely to happen at short range, it becomes possible to tune the reaction rate to be faster than the universal limit. This becomes possible through quantum interference. In the quantum world, particles behave like waves and can have constructive or destructive interference depending on whether the waves build off each other or cancel each other out.

If two particles can collide at short range and move apart again without reacting, the returning part of the wavefunction can interfere with the part of the wavefunction that reflected at long range, cancelling some of it out. If the collision can be tuned just right, this destructive interference at long distances becomes strong, and the particles collide at short range much more often.

The team demonstrated, for the first time, the ability to modify the reaction rate from far below the universal limit to far above. Using a Feshbach resonance, they were able to tune this destructive interference to demonstrate control of chemical reactions. Feshbach resonances occur when two or more particles collide and are bound together for a short period of time, allowing researchers to modify how collisions occur.

“We can tune the chemical reaction rate and try to get an intuitive sense of what’s going on with these reactions,” said Jamison. “With a Feshbach resonance we can measure in either direction, to slow reactions down or speed them up.”

You can think of this like tuning a radio dial. If you tune it just right, you can hear your favourite station but if you tune it elsewhere, you will hear static. The Feshbach resonance allowed the team to finetune and direct the reactions between the atoms and molecules. They tuned the resonance between a point where a reaction was unlikely to occur to a point where reactions were very likely.

Quantum control of chemistry is unlocking exciting streams of fundamental research with new directions for researchers to explore.

“From a physicist’s perspective, chemistry is complicated. We each have our own tools to understand our own fields of study. When we approach chemistry from a physics perspective we can explore and discover new fundamental insights,” said Jamison.

Control of reactive collisions by quantum interference was published in Science on March 4, 2022.