Nonclassical correlations for quantum metrology in thermal equilibrium

Nonclassical correlation beyond entanglement might provide a resource in quantum information tasks, such as quantum computation or quantum metrology. Quantum discord is a measure of nonclassical correlations beyond entanglement. Exploring the operational meaning of quantum discord as a resource in quantum information processing tasks, such as quantum metrology, is of essential importance to our understanding of nonclassical correlation. In our recent work [Phys. Rev. A, 98, 012115 (2018)], we have demonstrated that the diagonal discord plays a role in enhancing the high-temperature sensitivity of the greedy local thermometry scheme, where one measures the subsystems sequentially with the local optimal measurement. In this paper, we extend our results to a general greedy local parameter estimation scenario. In particular, we introduce a quantum discord, which we call discord for local metrology, to quantify the nonclassical correlations induced by the local optimal measurement on the subsystem. We demonstrate explicitly that discord for local metrology plays a role in sensitivity enhancement in the high-temperature limit by showing its relation to loss in quantum Fisher information. In particular, it coincides with diagonal discord for estimating a linear coupling parameter.