Observation of a resonant four-body interaction in cold cesium Rydberg   atoms
  Cold Rydberg atoms subject to long-range dipole-dipole interactions represent a particularly interesting system for exploring few-body interactions and probing the transition from 2-body physics to the many-body regime. In this work we report the direct observation of a resonant 4-body Rydberg interaction. We exploit the occurrence of an accidental quasi-coincidence of a 2-body and a 4-body resonant Stark-tuned Forster process in cesium to observe a resonant energy transfer requiring the simultaneous interaction of at least four neighboring atoms. These results are relevant for the implementation of quantum gates with Rydberg atoms and for further studies of many-body physics. 
J. H. Gurian, P. Cheinet, P. Huillery, A. Fioretti, J. Zhao, P.L.   Gould, D. Comparat, and P. Pillet
physics.atom-ph