Ground-state entanglement in coupled qubits

We study a system of qubits that are coupled to each other via only one degree of freedom represented, e.g., by $σ_z$-operators. We prove that, if by changing the Hamiltonian parameters, a non-degenerate ground state of the system is continuously transformed in such a way that the expectation values of $σ_z$ operators of at least two coupled qubits change, this ground state is entangled. Using this proof, we discuss connection between energy level anticrossings and ground state entanglement. Following the same line of thought, we introduce entanglement witnesses, based on cross-susceptibilities, that can detect ground state entanglement for any bipartition of the multi-qubit system. A witness for global ground state entanglement is also introduced.

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