Proposed realization of the Dicke-model quantum phase transition in an   optical cavity QED system
  The Dicke model consisting of an ensemble of two-state atoms interacting with a single quantized mode of the electromagnetic field exhibits a zero-temperature phase transition at a critical value of the dipole coupling strength. We propose a scheme based on multilevel atoms and cavity-mediated Raman transitions to realise an effective Dicke system operating in the phase transition regime. Output light from the cavity carries signatures of the critical behavior which is analyzed for the thermodynamic limit where the number of atoms is very large. 
F. Dimer, B. Estienne, A.S. Parkins, H.J. Carmichael