Quantum mechanical violation of macrorealism for large spin and its robustness against coarse-grained measurements

TL;DR

This paper demonstrates that quantum violations of macrorealism persist for large spin systems even under coarse-grained and unsharp measurements, challenging classical explanations in macroscopic quantum systems.

Contribution

It shows that quantum violations of macrorealism remain robust in large spin systems regardless of measurement sharpness or coarse graining, using three necessary conditions of macrorealism.

Findings

Quantum violations reach algebraic maxima at large spin.

Violations persist for any degree of measurement unsharpness.

Coarse graining asymmetry influences quantum violation detection.

Abstract

For multilevel spin systems, robustness of the quantum mechanical (QM) violation of macrorealism (MR) with respect to coarse grained measurements is investigated using three different necessary conditions of MR, namely, the Leggett-Garg inequality (LGI), Wigner's form of the Leggett-Garg inequality (WLGI) and the condition of no-signalling in time (NSIT). It is shown that for dichotomic sharp measurements, in the asymptotic limit of spin, the algebraic maxima of the QM violations of all these three necessary conditions of MR are attained. Importantly, the QM violations of all these persist in that limit even for \textit{arbitrary} unsharp measurements, i.e. for any non zero value of the sharpness parameter characterizing the degree of fuzziness of the relevant measurements. We also find that when different measurement outcomes are clubbed into two groups for the sake of dichotomising the outcomes, the asymmetry/symmetry in the number of outcomes in the two groups, signifying the degree of coarse graining of measurements, has a crucial role in discerning quantum violation of MR. The results clearly demonstrate that classicality does not emerges in the asymptotic limit of spin, whatever be the unsharpness and degree of coarse graining of the measurements.