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Many fundamental processes in nature, such as photosynthesis or human vision, are due to chemical reactions that are triggered by light. In a simplified picture of such processes we assume that photons merely provide the necessary energy to the chemical system to overcome a certain reaction barrier. Besides this energy contribution, the photons supposedly do not influence the system any further. Such a perturbative picture of the interaction between light and matter is reasonable in many situations of photochemistry and justifies what is known as the Stark-Einstein law [1]: for each absorbed quantum of light, no more than one molecule undergoes a chemical reaction.

Nature, however, does not need to adhere to this law and can violate it in specific cases. Javier Galego from the Autonomous University of Madrid and co-workers [2] consider such a case, in which a handful of molecules are strongly coupled to a confined light mode in an optical cavity [3]. The authors predict that a single photon entering the cavity can set off a reaction involving many more than just one molecule. What makes this example so special is that on the one hand it is simple—not needing specialized resonance conditions as do normal chain reactions—and on the other hand it highlights the complexity of chemical reactions, which are defined by an intricate interplay among electrons, nuclei, and photons. If such multimolecule triggering can be realized, it could have important practical implications in solar energy storage. Molecules that under normal conditions efficiently absorb solar energy could be brought into strong coupling so that a single photon could release the stored energy of the whole ensemble on demand.

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