Intensity-dependent dispersion under conditions of electromagnetically induced transparency in coherently prepared multistate atoms

Interest in lossless nonlinearities has focussed on the dispersive properties of $\ensuremath{\Lambda}$ systems under conditions of electromagnetically induced transparency (EIT). We generalize the $\ensuremath{\Lambda}$ system by introducing further degenerate states to realize a ``chain $\ensuremath{\Lambda}''$ atom where multiple coupling of the probe field significantly enhances the intensity-dependent dispersion without compromising the EIT condition.