Entanglement generation via full-resonant four-wave mixing

We present to use a full-resonant four-level atomic system to generate the steady-state entanglement. This scheme is based on the nondegenerate four-wave mixing process with the four fields being resonant with the corresponding bare atomic transitions, unlike the previous mixing schemes, where the entanglement is achieved under the condition of off-resonance. Another feature of such a mixing is that four fields are coupled to the different atomic transitions, respectively. By choosing the atoms appropriately, entanglement occurs between the fields with the large frequency difference.