  We present the time evolution of the electronic two-level model in the
rotating wave approximation (RWA). We calculate the Aharonov-Anandan phase of
the vector states with cyclic evolution either within the period of an external
monochromatic electric field or the period corresponding to Rabi's frequency.
The Aharonov-Anandan phase is shown to be dependent on the initial vector
state, unless the system evolves in the adiabatic regime; in the latter case,
the Aharonov-Anandan phase recovers Berry's phase. Our results are also
discussed in the quasi-resonant regime.
