We propose a technique to prepare coherent superpositions of two nondegenerate quantum states in a three-state ladder system, driven by two simultaneous fields near resonance with an intermediate state. The technique, of potential application to enhancement of nonlinear processes, uses adiabatic passage assisted by dynamic Stark shifts induced by a third laser field. The method offers significant advantages over alternative techniques: (i) it does not require laser pulses of specific shape and duration and (ii) it requires less intense fields than schemes based on two-photon excitation with non-resonant intermediate states. We discuss possible experimental implementation for enhancement of frequency conversion in mercury atoms.
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