  The state disturbance induced by locally measuring a quantum system yields a
signature of nonclassical correlations beyond entanglement. Here we present a
detailed study of such correlations for two-qubit mixed states. To overcome the
asymmetry of quantum discord and the unfaithfulness of measurement-induced
disturbance (severely overestimating quantum correlations), we propose an
ameliorated measurement-induced disturbance as nonclassicality indicator,
optimized over joint local measurements, and we derive its closed expression
for relevant two-qubit states. We study its analytical relation with discord,
and characterize the maximally quantum-correlated mixed states, that
simultaneously extremize both quantifiers at given von Neumann entropy: among
all two-qubit states, these states possess the most robust quantum correlations
against noise.
