  Non-classical correlations denote the ways in which quantum systems can be
correlated beyond what is possible in classical physics. They include but are
not limited to entanglement: non-entangled states can also exhibit forms of
non-classical correlations. We here review the literature on metrological
measures of non-classical correlations and show how they relate to increased
probe sensitivity in quantum metrology protocols such as interferometric
parameter estimation and state discrimination.
