Scientists thought that even with all the complexity and strangeness quantum mechanics involves, they at least had a good grasp of the simplest chemical reactions, those involving very few electrons and protons. Further investigation showed that's not really the case, confirming physicists' rueful observation that no matter how weird you think quantum behavior might be, the reality will always out-weird you.
Hydrogen has the simplest atom there is, just one proton and electron, followed by its isotope deuterium, which has the same pair and a neutron as well. So when a hydrogen atom meets a molecule made of up hydrogen and deuterium, and replaces the deuterium (H + HD → H2 + D in chemistry notation), there are not a lot of moving parts – three protons, three electrons, and a neutron. Compared to most chemical reactions, this is about as simple as it gets. As a result, the reaction has been studied intensely as a gateway to greater complexity.
However, a team from the Dalian Institute of Chemical Physics have reported in Science that a form of quantum interference occurs during the course of this reaction. Not only has this been missed in previous studies of hydrogen/deuterium displacement, it's never been seen before at all.
The authors report that when they crossed a beam of hydrogen atoms with another of hydrogen/deuterium molecules, the hydrogen could displace deuterium in two different ways. The first method involves the incoming hydrogen atom colliding with the hydrogen part of the molecule, knocking it out and the two going off together. In the second case, the incoming atom causes the bond between the existing hydrogen and deuterium atoms to stretch, at which point it inserts itself inside.
One might think of this like a single person seeking to break up a couple and marry one of the partners. They might sweep one partner off their feet, carrying them away before the jilted lover knows what is happening. Alternatively, the interloper may hang around creating stress within the relationship until the previous closeness loosens and they can form a new bond instead.
In a classical (that is non-quantum) situation, one or another of these would occur. However, at the heart of quantum mechanics is that sometimes particles can do two apparently incompatible things at once, most famously a photon going through two slits simultaneously. We know this happens when the two states created interfere with each other.
Professor Xueming Yang and colleagues observed something very similar. The incoming hydrogen atoms displaced the deuterium in both ways at once (although the first method was dominant). The waveform created by the different methods interfered with itself, creating distinctive oscillations in the newly formed molecules that allowed the researchers to know both events had happened.
And you thought your love life was complicated.