The conditions for quantum violation of macroscopic realism

TL;DR

This paper investigates the specific conditions under which quantum systems can violate macroscopic realism, highlighting the role of non-classical Hamiltonians and the effects of decoherence and measurement coarse-graining.

Contribution

It identifies sufficient conditions for classical evolution in spin systems and introduces non-classical Hamiltonians that can cause quantum violations of macroscopic realism.

Findings

Classical evolution occurs under decoherence or coarse measurements.

Existence of non-classical Hamiltonians that violate macroscopic realism.

Such Hamiltonians are likely rare in nature due to their complexity.

Abstract

Why do we not experience a violation of macroscopic realism in every-day life? Normally, no violation can be seen either because of decoherence or the restriction of coarse-grained measurements, transforming the time evolution of any quantum state into a classical time evolution of a statistical mixture. We find the sufficient condition for these classical evolutions for spin systems under coarse-grained measurements. Then we demonstrate that there exist "non-classical" Hamiltonians whose time evolution cannot be understood classically, although at every instant of time the quantum spin state appears as a classical mixture. We suggest that such Hamiltonians are unlikely to be realized in nature because of their high computational complexity.