Addressing the clumsiness loophole in a Leggett-Garg test of macrorealism

TL;DR

This paper proposes a revised Leggett-Garg testing protocol that reduces the clumsiness loophole, enabling more reliable distinction between macrorealistic and non-macrorealistic behaviors in quantum systems.

Contribution

It introduces an explicit measurement invasiveness check, significantly narrowing the loophole that allows macrorealists to evade conclusive testing.

Findings

The protocol effectively reduces the collusion loophole.

It enables more definitive tests of macrorealism.

Provides a practical method to verify measurement invasiveness.

Abstract

The rise of quantum information theory has lent new relevance to experimental tests for non-classicality, particularly in controversial cases such as adiabatic quantum computing superconducting circuits. The Leggett-Garg inequality is a "Bell inequality in time" designed to indicate whether a single quantum system behaves in a macrorealistic fashion. Unfortunately, a violation of the inequality can only show that the system is either (i) non-macrorealistic or (ii) macrorealistic but subjected to a measurement technique that happens to disturb the system. The "clumsiness" loophole (ii) provides reliable refuge for the stubborn macrorealist, who can invoke it to brand recent experimental and theoretical work on the Leggett-Garg test inconclusive. Here, we present a revised Leggett-Garg protocol that permits one to conclude that a system is either (i) non-macrorealistic or (ii) macrorealistic but with the property that two seemingly non-invasive measurements can somehow collude and strongly disturb the system. By providing an explicit check of the invasiveness of the measurements, the protocol replaces the clumsiness loophole with a significantly smaller "collusion" loophole.