Comment on `A scattering quantum circuit for measuring Bell's time inequality: a nuclear magnetic resonance demonstration using maximally mixed states'

TL;DR

This paper critiques a recent experiment claiming to violate a Leggett-Garg inequality using maximally mixed states, arguing that their method does not convincingly demonstrate non-invasive measurements and thus the conclusions are unjustified.

Contribution

The paper provides a critical analysis of SOS's experimental approach, highlighting its shortcomings in convincingly establishing non-invasive measurements in testing macrorealism.

Findings

The method used by SOS is unconvincing to skeptics of macrorealism.

The experiment's conclusions about quantum superposition are unjustified.

Critique emphasizes the difficulty of truly non-invasive measurements in such tests.

Abstract

A recent paper by Souza, Oliveira and Sarthour (SOS) reports the experimental violation of a Leggett-Garg inequality (sometimes referred to as a temporal Bell inequality). The inequality tests for quantum mechanical superposition: if the inequality is violated, the dynamics cannot be explained by a large class of classical theories under the heading of macrorealism. Experimental tests of the LG inequality are beset by the difficulty of performing the necessary so-called 'non-invasive' measurements (which for the macrorealist will extract information from a system of interest without disturbing it). SOS argue that they nevertheless achieve this difficult goal by putting the system in a maximally mixed state. The system then allegedly undergoes no perturbation during their experiment. Unfortunately the method is ultimately unconvincing to a skeptical macrorealist, and so the conclusions drawn by SOS are unjustified.