Nonlinear response and observable signatures of equilibrium entanglement

We investigate how equilibrium entanglement is manifested in the nonlinear response of an $N$-qubit system. We show that in the thermodynamic limit the irreducible part of the $n$th-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.\cite{Love}. We discuss the possibility of extracting purely-entanglement-generated contribution from the general multipoint correlators in a multiqubit system.