Cross phase modulation in a five--level atomic medium: Semiclassical   theory
  The interaction of a five-level atomic system involving electromagnetically induced transparency with four light fields is investigated. Two different light-atom configurations are considered, and their efficiency in generating large nonlinear cross-phase shifts compared. The dispersive properties of those schemes are analyzed in detail, and the conditions leading to group velocity matching for two of the light fields identified. An analytical treatment based on amplitude equations is used in order to obtain approximate solutions for the susceptibilities, ehich are shown to fit well with the numerical simulations of the full Bloch equations in a large parameter region. 
Carlo Ottaviani, Stojan Rebic, David Vitali and Paolo Tombesi