Ground-state cooling of a nanomechanical resonator coupled to two interacting flux qubits

We discuss cooling of a nanomechanical resonator to its mechanical ground state by coupling it to a collective system of two interacting flux qubits. We find that the collectivity crucially improves cooling by two mechanisms. First, cooling transitions proceed via subradiant Dicke states, and the reduced linewidth of these subradiant states suppresses both the scattering and the environmental contribution to the final phonon number. Second, detrimental carrier excitations without change in the motion of the resonator are suppressed by collective energy shifts.