Generalized π pulses

We show that the concept of {π} pulses can be extended to multilevel systems. Generalized {π} pulses selectively excite a target state via a mechanism that is closely related to the familiar excitation dynamics in a two-level system. The corresponding generalized area theorem does not refer to the ``area under the pulse envelope,'' but to an integral over a difference of instantaneous quasienergies. Nevertheless, there are the same possibilities of pulse shaping for the generalized pulses as for their two-level counterparts. A semiclassical interpretation of the resonance condition leads to an analytical approximation to the relevant quasienergies, and shows that the excitation mechanism is universal.