  One of the most striking features of quantum mechanics is the profound effect
exerted by measurements alone. Sophisticated quantum control is now available
in several experimental systems, exposing discrepancies between quantum and
classical mechanics whenever measurement induces disturbance of the
interrogated system. In practice, such discrepancies may frequently be
explained as the back-action required by quantum mechanics adding quantum noise
to a classical signal. Here we implement the 'three-box' quantum game of
Aharonov and Vaidman in which quantum measurements add no detectable noise to a
classical signal, by utilising state-of-the-art control and measurement of the
nitrogen vacancy centre in diamond.
  Quantum and classical mechanics then make contradictory predictions for the
same experimental procedure, however classical observers cannot invoke
measurement-induced disturbance to explain this discrepancy. We quantify the
residual disturbance of our measurements and obtain data that rule out any
classical model by > 7.8 standard deviations, allowing us for the first time to
exclude the property of macroscopic state-definiteness from our system. Our
experiment is then equivalent to a Kochen-Spekker test of quantum
non-contextuality that successfully addresses the measurement detectability
loophole.
