Quantum effects in chemical reactions

Chemical reactions are usually modelled as random, statistical events, with particles bumping into each other and rearranging according to energy and probability. But at extremely low temperatures, atoms and molecules can behave as coherent matter waves, meaning many particles share a common quantum phase. In this setting, a reaction can look less like classical chemistry and more like wave mixing in optics.

In order to learn more about this phenomenon, a team of researchers from the University of Chicago studied a gas of caesium atoms, cooled down to 11nK to form a Bose–Einstein condensate.

Their approach was to use a Feshbach resonance to initiate the chemical reaction. This resonance is a way of using a magnetic field to tune how strongly ultracold atoms interact with each other.

At this point, two atoms can couple strongly to a bound molecular state, so pairs of atoms can be converted into weakly bound caesium diatomic molecules. Using this technique, they were able to transform an atomic Bose–Einstein condensate into a molecular Bose–Einstein condensate, consisting of about 10,000 molecules.

Their key result is the observation of phase doubling. This is the matter-wave version of phase matching in second harmonic generation, where two red photons in a nonlinear medium combine to produce one blue photon with twice the frequency and twice the momentum. Here, two atomic matter waves combine into a molecular matter wave whose phase is twice the atomic phase.

They also showed that molecules formed from atoms in different momentum states were enhanced beyond a classical expectation. This allowed the team to identify non-separable, entangled two-atom momentum states created during the reaction.

This work is one example of quantum many-body chemistry, where reactions are governed by the shared quantum phase of many atoms and molecules. This is also similar to how the direction of a Josephson supercurrent is determined by the phase difference across its junction.

Read the full article

Observation of phase doubling and entanglement in coherent matter-wave reactions – IOPscience

Shu Nagata et al 2026 Rep. Prog. Phys. 89 060501

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