  We study the effect of a time-dependent driving field with a large amplitude
on a system composed of two coupled qubits (two-level systems). Using the
rotating wave approximation (RWA) makes it possible to find simple conditions
for resonant excitation of the four-level system. We find that the resonance
conditions include the coupling strength between the qubits. Numerical
simulations confirm the qualitative conclusions following from the RWA. To
reveal the peculiarities of resonant transitions caused by the quasi-level
motion and crossing in a periodic driving field, we use Floquet states, which
determine the precise intermediate states of the system. Calculating the
quasi-energy states of the multi-level system makes it possible to find the
transition probabilities and build interference patterns for the transition
probabilities. The interference patterns demonstrate the possibility of
obtaining various pieces of information about the qubits, since the positions
of transition-probability maxima depend on various system parameters, including
the coupling strength between the qubits.
