Quantum non-demolition measurement enables macroscopic Leggett-Garg tests

TL;DR

This paper demonstrates how quantum non-demolition measurements can be used to perform macroscopic Leggett-Garg tests, showing significant quantum violations that challenge classical macrorealist views.

Contribution

It introduces measurement schemes with Gaussian states for LGI tests in macroscopic systems, highlighting quantum effects over classical invasivity.

Findings

Significant LGI violation with realistic parameters

Quantum effects confirmed over classical invasivity

QND measurements tighten macrorealist loopholes

Abstract

We show how a test of macroscopic realism based on Leggett-Garg inequalities (LGIs) can be performed in a macroscopic system. Using a continuous-variable approach, we consider quantum non-demolition (QND) measurements applied to atomic ensembles undergoing magnetically-driven coherent oscillation. We identify measurement schemes requiring only Gaussian states as inputs and giving a significant LGI violation with realistic experimental parameters and imperfections. The predicted violation is shown to be due to true quantum effects rather than to a classical invasivity of the measurement. Using QND measurements to tighten the "clumsiness loophole" forces the stubborn macrorealist to re-create quantum back action in his or her account of measurement.