Efficient population transfer by delayed pulses despite coupling ambiguity

In traditional schemes of multilevel multilaser excitation, each laser puls e interacts with only one pair of states, and the rotating wave approximati on (RWA) is applicable. Here pie study the population transfer process in a three-state system when each of the two lasers interacts with each of the pair of states and when the Rabi frequencies characterizing tl-le interacti on strengths of che system are comparable to or larger than the difference of the transition frequencies. We show that complete and robust population transfer is possible under conditions more general than those hitherto cons idered necessary for stimulated Raman adiabatic passage (STIRAP) or for suc cessive pi pulses. Using adiabatic Floquet theory we show that successful p opulation transfer can be interpreted as adiabatic passage by means of a tr ansfer state which collects the initial and final states. The Floquet pictu re offers a convenient interpretation of the population transfer as accompa nied by multiple absorption of photons from or emission into the laser fiel ds.