Coherent population transfer in four-level ladder systems by single frequency-chirped laser pulse

Adiabatic passage induced by a frequency-chirped laser pulse in four-level ladder systems is investigated. Two different strategies for efficient population transfer (intuitive and counterintuitive laser pulse) are analyzed. For the larger detuning, the four-level ladder system can be reduced to a two-level system with which we are familiar. For the smaller detuning, the main conditions for realization of population transfer are the following: the width of the transform-limited laser pulse envelope frequency spectrum (without chirp) must be smaller and the peak Rabi frequency of the pulse must be larger than the detuning ε₂₁ and ε₂₁∼ε₃₂ in the case of the three-photon resonance. With this laser pulse, it is possible to achieve complete population via the intermediate levels by three successive adiabatic passages. Complete inversion is also obtained by a counterintuitive direction of the frequency sweep.