  We have studied the low-frequency magnetic susceptibility of two inductively
coupled flux qubits using the impedance measurement technique (IMT), through
their influence on the resonant properties of a weakly coupled high-quality
tank circuit. In a single qubit, an IMT dip in the tank's current--voltage
phase angle at the level anticrossing yields the amplitude of coherent flux
tunneling. For two qubits, the difference (IMT deficit) between the sum of
single-qubit dips and the dip amplitude when both qubits are at degeneracy
shows that the system is in a mixture of entangled states (a necessary
condition for entanglement). The dependence on temperature and relative bias
between the qubits allows one to determine all the parameters of the effective
Hamiltonian and equilibrium density matrix, and confirms the formation of
entangled eigenstates.
