  We develop the Landau-Zener transfer matrix theory for the instantaneous
Floquet states (IFSs) for quantum systems driven by strong pulse lasers.
Applying this theory to the pulse excitation probability in two-level quantum
systems, we show unexpectedly good quantitative agreements 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 2-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.
