Chemistry is not always as quantum mechanical as it could be. Although electrons need to be described by their quantum wavefunctions, atomic nuclei at ambient temperatures are often well approximated as classical billiard balls. Only at sub-microkelvin temperatures does the full spectrum of quantum chemical effects—such as the requirement that when two identical fermionic molecules react, their collective wavefunction must be antisymmetric—reveal itself.

Adapted from M.-G. Hu et al., Science 366, 1111 (2019)

But experiments on such ultracold reactions have been severely limited in what they can measure. Researchers prepare a gas of cold molecules—such as potassium–rubidium, or KRb—in an optical trap, monitor the rate at which the molecules disappear, and infer that the disappearance must be due to a chemical reaction. The presumed reaction products, K 2 and Rb 2 , are immune to the effects of the trap and undetectable by the experiment.

The invisibility of the products means the kinetic energy released in the reaction and other important properties are unobservable. And disconcertingly, other similar molecules that theory predicts should be energetically unable to react, such as rubidium–cesium, also disappear when trapped, so it’s not completely clear that the disappearance is due to a reaction at all.

Now, for the first time, Kang-Kuen Ni and her colleagues at Harvard University have directly detected the reaction products of ultracold KRb molecules. Around their optical trap (the blue-green region at the center of the figure), they shine a second, ring-shaped laser beam with energy chosen to ionize any molecules that might cross its path. Applied electric and magnetic fields then sweep the newly formed ions toward a detector and sort them by mass.