Figure 1: Two-body and three-body FRET processes.

(a) Schematic energy diagram as a function of the applied electric field for the different three-atom states involved: the dotted lines represent the energy of each state in the abscence of interactions, for example, for atoms far apart. Interactions turn the different resonances into avoided crossings marked with dash-dotted lines. They correspond to two-body resonances ((II), (III) and (II′)) and three-body resonances ((I) and (I′)) where the resonant energy transfers detailed in (b) to (e) can take place. (d) Describes how the two-body FRET (II) (resp. (II’)) from the p (resp. ) state occurs through the exchange of a single virtual photon symbolized with two blue arrows. (e) Describes the similar two-body FRET from a p and mixture. (b) (resp. (c)) Describes how the three-body FRET transfers the starting ppp (resp. ) state to the end state through the exchange of two different virtual photons. The first virtual photon, symbolized with blue arrows crossed with a single mark, leads the system to a virtual intermediate state before the second, symbolized with red arrows crossed with two marks, leads it resonantly to the end state. For reference, an horizontal dashed line at the energy represents when possible the two-body resonance condition.