Abstract
We discuss the transient effects of the four-state system proposed by Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)]. In this system, a weak probe laser and a coupling laser form a three-state A-type configuration of electromagnetically induced transparency; a switching laser drives a transition between the fourth state and the ground state of the coupling field. The presence of the switching field causes absorption of the probe. We show that the rise time of the probe absorption can not be shorter than 2/Γ and is equal to 2Γ/(Ω c 2 + Ω s 2 ) in the low-intensity limit, where Γ is the spontaneous decay rate of the excited states, and Ω c and Ω s are the Rabi frequencies of the coupling and switching lasers. A simple picture based on dark and non-dark states is provided to explain these results.