Ground-state entanglement in coupled qubits
Anatoly Yu. Smirnov, M. H. S. Amin · Physical Review A · 2013
We study a system of qubits that are coupled to each other via only one degree of freedom, represented, e.g., by ${\ensuremath{\sigma}}_{z}$ operators. We prove that if, by changing the Hamiltonian parameters, a nondegenerate ground state of the system is continuously transformed in such a way that the expectation values of ${\ensuremath{\sigma}}_{z}$ 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 multiqubit system. A witness for global ground-state entanglement is also introduced.