Demonstration of the Jaynes-Cummings ladder with Rydberg-dressed atoms
  We observe the nonlinearity of the Jaynes-Cummings (JC) ladder in the Autler-Townes spectroscopy of the hyperfine ground states for a Rydberg-dressed two-atom system. Here the role of the two-level system in the JC model is played by the presence or absence of a collective Rydberg excitation, and the bosonic mode manifests as the number $n$ of single atom spin flips, symmetrically distributed between the atoms. We measure the normal-mode splitting and $\sqrt{n}$ nonlinearity as a function of detuning and Rabi frequency, thereby experimentally establishing the isomorphism with the JC model. 
Jongmin Lee, Michael J. Martin, Yuan-Yu Jau, Tyler Keating, Ivan H.   Deutsch, and Grant W. Biedermann
quant-ph