  According to the world view of macrorealism, the properties of a given system
exist prior to and independent of measurement, which is incompatible with
quantum mechanics. Leggett and Garg put forward a practical criterion capable
of identifying violations of macrorealism, and so far experiments performed on
microscopic and mesoscopic systems have always ruled out in favor of quantum
mechanics. However, a macrorealist can always assign the cause of such
violations to the perturbation that measurements effect on such small systems,
and hence a definitive test would require using non-invasive measurements,
preferably on macroscopic objects, where such measurements seem more plausible.
However, the generation of truly macroscopic quantum superposition states
capable of violating macrorealism remains a big challenge. In this work we
propose a setup that makes use of measurements on the polarization of light, a
property which has been extensively manipulated both in classical and quantum
contexts, hence establishing the perfect link between the microscopic and
macroscopic worlds. In particular, we use Leggett-Garg inequalities and the
criterion of no-signaling in time to study the macrorealistic character of
light polarization for different kinds of measurements, in particular with
different degrees of coarse-graining. Our proposal is non-invasive for coherent
input states by construction. We show for states with well defined photon
number in two orthogonal polarization modes, that there always exists a way of
making the measurement sufficiently coarse-grained so that a violation of
macrorealism becomes arbitrarily small, while sufficiently sharp measurements
can always lead to a significant violation.
