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.Comment: 7 pages, 3 figure
