Just as atoms exchange electrons to form molecular orbitals, an electromagnetic field can interact with a quantum system by the exchange of photons. When this interaction is strong enough to overcome decoherence effects, new hybrid light–matter states can form, separated by what is known as the Rabi splitting energy (Figure 1). This strong coupling regime is typically achieved by placing the material in an optical cavity, such as that formed by two parallel mirrors, which is tuned to be resonant with a transition to an excited state. Theory, discussed below, shows that even in the absence of light, a residual splitting always exists due to coupling to vacuum (electromagnetic) fields in the cavity. While cavity strong coupling and the associated hybrid states have been extensively studied due to the potential they offer in physics such as room temperature Bose–Einstein condensates and thresholdless lasers,[1–14] the implication for chemistry remains totally unexplored. This is despite the fact that strong coupling with organic molecules lead to exceptionally large vacuum Rabi splittings (hundreds of meV) due to their large transition dipole moments.[15–24] The molecules plus the cavity must thus be thought of as a single entity with new energy levels and therefore should have its own distinct chemistry.

We demonstrate here that one can indeed influence a chemical reaction by strongly coupling the energy landscape governing the reaction pathway to vacuum fields. In the absence of dissipation, the Rabi splitting energy h ΩR (Figure 1) between the two new hybrid light–matter states is given, for a two-level system at resonance with a cavity mode, by the product of the electric field amplitude E in the cavity and the transition dipole moment d:[13]