  We investigate the quantum dynamics of a system of two coupled
superconducting qubits under microwave irradiation. We find that, with the
qubits operated at the charge co-degeneracy point, the quantum evolution of the
system can be described by a new effective Hamiltonian which has the form of
two coupled qubits with tunable coupling between them. This Hamiltonian can be
used for experimental tests on macroscopic entanglement and for implementing
quantum gates.
