Floquet-Landau-Zener interferometry: Usefulness of the Floquet theory in pulse-laser-driven systems

We develop the Landau-Zener transfer matrix theory for the instantaneous Floquet states (IFSs) for quantum systems driven by a strong pulse laser. Applying this theory to the pulse excitation probability in two-level quantum systems, we show unexpectedly good quantitative agreement for few-cycle pulses. This approach enables us to qualitatively understand the probability's peculiar behaviors as quantum path interference between IFSs. We also study the pulse-width dependence, finding that this Floquet-state interpretation remains useful for shorter pulses down to two-cycle ones in the present model. These results imply that the Floquet theory is meaningful in experimental few-cycle lasers if applied appropriately in the sense of IFSs.