Amplification without population inversion in a three-level Λ-type system with an external coherent magnetic field

In this Letter, we investigate the amplification without inversion (AWI) in a new scheme, a three-level Λ-type system which the two lower states are coupled by an external magnetic field. We find that, owing to the presence of the external magnetic field, the absorption of the probing field is very sensitive to the relative phase of the probing and the pumping fields. By appropriately choosing the Rabi frequency of the pumping field, the Lamor frequency of the external magnetic field and the relative phase, AWI can be realized.