Nonclassical correlations for quantum metrology in thermal equilibrium

TL;DR

This paper investigates how nonclassical correlations, specifically quantum discord for local metrology, can enhance quantum parameter estimation in thermal equilibrium states, especially at high temperatures.

Contribution

It introduces a new measure called discord for local metrology and demonstrates its role in improving sensitivity in quantum thermometry and parameter estimation.

Findings

Quantum discord for local metrology relates to quantum Fisher information loss.

Discord for local metrology coincides with diagonal discord in certain estimation scenarios.

Nonclassical correlations can enhance quantum metrology at high temperatures.

Abstract

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.