  In this letter, we propose and experimentally test a quantumness criterion
for single systems. The criterion is based on the violation of an already
reported classical inequality. This inequality is expressed in terms of joint
probabilities that we identify as realizations of preparation-and-measurement
schemes. Our analysis points out that superposition is the key notion to grasp
the quantum nature of physical systems. We verify our criterion in an
all-optical experimental setup with heralded single photons generated via
spontaneous parametric down conversion and use exclusively projective
preparations and projective measurements.
