THE RESONANT 2-LEVEL SPIN SYSTEM IN A MONOCHROMATIC MAGNETIC-FIELD

A detailed study of the resonant two-level spin system driven by a monochromatic magnetic field is accomplished using both the theory of quantum resonance as proposed in M. Frasca, Nuovo Cimento B, 107, 915 (1992) and the theory of the strong perturbations in quantum mechanics. The main parameter is the ratio between the strength of the external magnetic field and its oscillation frequency. If the ratio is less than unity, we obtain improved results on this system, compared with the rotating-wave approximation, when the initial conditions are properly accounted for. On the other side, when the ratio becomes greater than unity, it is shown that the dynamics of the model is driven just by the perturbation, irrespective of resonance, as expected on the basis of the theory of the strong perturbations. The problem of the transition region between the two regimes is pointed out.