We investigate how equilibrium entanglement is manifested in the nonlinear response of an Nqubit system. We show that in the thermodynamic limit the irreducible part of the nth-order nonlinear susceptibility indicates that the eigenstates of the system contain entangled (n + 1)-qubit clusters. This opens the way to a directly observable multiqubit entanglement signature. We show that the irreducible part of the static cubic susceptibility of a system of four flux qubits, as a function of external parameters, behaves as a global 4-qubit entanglement measure introduced in Ref. [20].We discuss the possibility of extracting purely-entanglement-generated contribution from the general multipoint correlators in a multiqubit system.
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