  The interaction of flux qubits with a low frequency tank circuit is studied.
It is shown that changes in the state of the interacting qubits influence the
effective impedance of the circuit, which is the essence of the so-called
impedance measurement technique. The multiphoton resonant excitations in both
single flux qubits and pairs of coupled flux qubits are investigated. In
particular, we compare our theoretical results with recent spectroscopy
measurements, Landau-Zener interferometry, and multiphoton fringes.
