Two-, few- and many-body effects in frozen Rydberg gases
Prof. Dr. Pierre Pillet (Laboratoire Aimé Cotton, CNRS, Université Paris-Sud und ENS, Cachan, Frankreich)

Cold Rydberg atoms provide nice opportunities for considering and for simulating many physical problems. In Förster Rydberg resonant couplings (in analogy with the Förster Resonance Transfer, FRET, in biology), two Rydberg atoms exchange internal energy through long-range dipole-dipole interaction by a resonant way. An example of Förster resonance is for Cs atom: np+np → ns+(n+1)s, where the exchange of internal energy between the two atoms is reached in resonance by adding a weak static electric field, F0, which Stark-shifts the energy of the p-states, leading to a substantial transfer of population towards the s states. The saturation of the process can be observed.

In a cold gas of Rydberg atoms in mutual dipole-dipole or Förster coupling, few-body processes can occur and can be separated from the two-body ones [1]. Three-body Förster resonances has been observed with a reaction: 3 x np → ns + (n+1)s + np’, corresponding to an efficient transfer toward the s states. The resonance occurs for a value of the static electric field different from the one associated to the two-body process, F1 ≠ F0. The two-body Förster coupling is out of resonance, and the missing exchanged internal energy between two Rydberg atoms is compensated by the change of level by a third atom, p → p’ [2]. Such a three-body process presents a Borromean character, where the three-body processes become the elementary coupling in absence of two-body ones.
Ensembles of Rydberg atoms in a Borromean three-body Förster coupling can be studied with many implications in the understanding of the many-body problem, in the route for the formation of trimmers Rydberg molecules or Rydberg clusters and in many various domains from quantum physics to biology.

[1] Observation of a Resonant Four-Body Interaction in Cold Cesium Rydberg Atoms, J.H. Gurian, P. Cheinet, P. Huillery, A. Fioretti, J. Zhao, P.L. Gould, D. Comparat, P. Pillet, Phys.Rev.Lett. 108, 023005 (2012).

[2] Borromean three-body FRET in frozen Rydberg gases, R. Faoro, B. Pelle, A. Zuliani, P. Cheinet, E. Arimondo, and P. Pillet (submitted)