  Negative probability values have been widely employed as an indicator of the
nonclassicality of quantum systems. Known as a quasiprobability distribution,
they are regarded as a useful tool that provides significant insight into the
underlying fundamentals of quantum theory when compared to the classical
statistics. However, in this approach, an operational interpretation of these
negative values with respect to the definition of probability---the relative
frequency of occurred event---is missing. An alternative approach is therefore
considered where the quasiprobability operationally reveals the negativity of
measured quantities. We here present an experimental realization of the
operational quasiprobability, which consists of sequential measurements in
time. To this end, we implement two sets of polarization measurements of single
photons. We find that the measured negativity can be interpreted in the context
of selecting measurements, and it reflects the nonclassical nature of photons.
Our results suggest a new operational way to unravel the nonclassicality of
photons in the context of measurement selection.
