Equilibrium in Periodically Time-Dependent Two-Level Systems

The Floquet solution for the evolution operator of a periodically time-dependent two-level Hamiltonian provides an effective Hamiltonian that can be used to examine the equilibrium properties of the system. In one example, calculations with a model Hamiltonian show that identical equilibrium properties are predicted in the laboratory frame as in a time-dependent interaction frame. A second example considers the changes that arise when the model Hamiltonian contains an explicit periodic time dependence.