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Local unitary operations allow for a unifying approach to the quantificationof quantum correlations among the constituents of a bipartite quantum system.For pure states, the distance between a given state and its image underleast-perturbing local unitary operations is a bona fide measure of quantumentanglement, the so-called entanglement of response, which can be extended tomixed states via the convex roof construction. On the other hand, when defineddirectly on mixed states perturbed by local unitary operations, such a distanceturns out to be a bona fide measure of quantum correlations, the so-calleddiscord of response. Exploiting this unified framework, we perform a detailedcomparison between two-body entanglement and two-body quantum discord ininfinite XY quantum spin chains both in symmetry-preserving andsymmetry-breaking ground states as well as in thermal states at finitetemperature. The results of the investigation show that in symmetry-preservingground states the two-point quantum discord dominates over the two-pointentanglement, while in symmetrybreaking ground states the two-point quantumdiscord is strongly suppressed and the two-point entanglement is essentiallyunchanged. In thermal states, for certain regimes of Hamiltonian parameters, weshow that the pairwise quantum discord and the pairwise entanglement canincrease with increasing thermal fluctuations.

Entanglement and quantum correlations in many-body systems: a unified approach via local unitary operations