Amplification without inversion with two coherent coupling lasers in a four-level atom

Abstract

We show that amplification without inversion (AWI) can be realized with two coherent driving lasers in a four-level atom. With the help of quantum-jump theory, two transition processes are found to contribute to the absorption or gain behaviour. It is explicitly shown that an asymmetry between the three-photon stimulated emission and the single-photon absorption process leads to AWI. Furthermore, it is demonstrated that AWI can occur in this model with or without quantum interference. The origin of AWI without interference is explained by introducing weights for dressed-state transitions. The asymmetry between the weights for a dressed-state transition in absorption and its counterpart in emission leads to the possibility of AWI.