  The threshold between classical and nonclassical two-qubit states is drawn at
the place when these states can no longer be described by classical
correlations, i.e., quantum discord or entanglement appear. However, to check
if the correlations are classical (in terms of quantum discord and
entanglement) it is sufficient to witness the lack of quantum discord because
its zero value implies the lack of entanglement. We explain how the indicator
of quantum discord introduced by Girolami and Adesso [Phys. Rev. Lett. 108,
150403 (2012)] can be practically measured in linear-optical systems using
standard beam splitters and photon detectors. We study the efficiency of the
setup assuming both ideal and real components and show that the efficiency of
the proposed implementation is better than the full two-photon quantum
tomography. Thus, we demonstrate that a class of experiments previously
available on NMR platform can be implemented in optical systems.
