Full Quantum Theory of Transient-State Electromagnetically Induced Transparency

We develop a full quantum theory of transient-stateelectromagnetically induced transparency (EIT) in the vapor ofthree-level $Lambda$-type atoms interacting with probe andcoupling lasers. As applications of the full quantum theory, weshow that transient-state EIT medium exhibits normal dispersionand find that group velocities of both coupling and probe lasersare greatly reduced. It is shown that the group velocity of theprobe laser in the transient-state EIT case is equal to that inthe adiabatic EIT case and that the coupling laser group velocityin the transient-state EIT is generally less than that in theadiabatic EIT.