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

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-jum p theory, two transition processes are found to contribute to the absorptio n or gain behaviour. It is explicitly shown that an asymmetry between the t hree-photon stimulated emission and the single-photon absorption process le ads to AWI. Furthermore, it is demonstrated that AWI can occur in this mode l with or without quantum interference. The origin of AWI without interfere nce 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.