Nonclassical correlations for quantum metrology in thermal equilibrium. (arXiv:1810.02378v2 [quant-ph] UPDATED)

Nonclassical correlations 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, to which
entanglement belongs as a subset. 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
correlations. In our recent work [Phys. Rev. A, 98, 012115 (2018)], we
considered a protocol---which we call the greedy local thermometry protocol---
for estimating the temperature of thermal equilibrium states from local
measurements, elucidating the role of diagonal discord in enhancing the
protocol sensitivity in the high-temperature limit. 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.

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