  Testing quantum theory on macroscopic scales is a longstanding challenge that
might help to revolutionise physics. For example, laboratory tests (such as
those anticipated in nanomechanical or biological systems) may look to rule out
macroscopic realism: the idea that the properties of macroscopic objects exist
objectively and can be non-invasively measured. Such investigations are likely
to suffer from i) stringent experimental requirements, ii) marginal statistical
significance and iii) logical loopholes. We address all of these problems by
refining two tests of macroscopic realism, or `quantum witnesses', and
implementing them in a microscopic test on a photonic qubit and qutrit. The
first witness heralds the invasiveness of a blind measurement; its maximum
violation has been shown to grow with the dimensionality of the system under
study. The second witness heralds the invasiveness of a generic quantum
operation, and can achieve its maximum violation in any dimension -- it
therefore allows for the highest quantum signal-to-noise ratio and most
significant refutation of the classical point of view.
