Observation of the Borromean three-body F\"orster resonances for three interacting Rb Rydberg atoms

Three-body F\"orster resonances at long-range interactions of Rydberg atoms were first predicted and observed in Cs Rydberg atoms by R.Faoro et al., Nature Comm. 6, 8173 (2015). In these resonances, one of the atoms carries away an energy excess preventing the two-body resonance, leading thus to a Borromean type of F\"orster energy transfer. But they were in fact observed as the average signal for the large number of atoms $N\gg1$. In this Letter we report on the first experimental observation of the three-body F\"orster resonances ${\rm 3}\times nP_{3/2} (|{\rm M}|)\to nS_{1/2} +(n+1)S_{1/2} +nP_{3/2} (|{\rm M}^{*} |)$ in a few Rb Rydberg atoms with $n=36, 37$. We have found here clear evidence that there is no signature of the three-body F\"orster resonance for exactly two interacting Rydberg atoms, while it is present for $N=3-5$ atoms. This demonstrates the assumption that three-body resonances can generalize to any Rydberg atom. As such resonance represents an effective three-body operator, it can be used to directly control the three-body interactions in quantum simulations and quantum information processing with Rydberg atoms.