Characterizing Nonclassical Correlations via Local Quantum Uncertainty

TLDR

Quantum mechanics predicts a fundamental minimum uncertainty for incompatible observables, yet a single quantity can in principle be measured with arbitrary precision. The study demonstrates that intrinsic quantum uncertainty on a single observable is unavoidable in many situations and explores its implications for quantum metrology. The authors analyze discord‑type correlations arising from local quantum uncertainty and their impact on quantum metrology. They find that local quantum uncertainty yields a class of nonclassical correlation measures, with a unique closed‑form measure for 2×d systems, and that discord in mixed probe states guarantees a minimum precision in optimal phase estimation.

Abstract

Quantum mechanics predicts that measurements of incompatible observables carry a minimum uncertainty which is independent of technical deficiencies of the measurement apparatus or incomplete knowledge of the state of the system. Nothing yet seems to prevent a single physical quantity, such as one spin component, from being measured with arbitrary precision. Here, we show that an intrinsic quantum uncertainty on a single observable is ineludible in a number of physical situations. When revealed on local observables of a bipartite system, such uncertainty defines an entire class of bona fide measures of nonclassical correlations. For the case of 2 × d systems, we find that a unique measure is defined, which we evaluate in closed form. We then discuss the role that these correlations, which are of the "discord" type, can play in the context of quantum metrology. We show in particular that the amount of discord present in a bipartite mixed probe state guarantees a minimum precision, as quantified by the quantum Fisher information, in the optimal phase estimation protocol.