  Dissimilar notions of quantum correlations have been established, each being
motivated through particular applications in quantum information science and
each competing for being recognized as the most relevant measure of
quantumness. In this contribution, we experimentally realize a form of quantum
correlation that exists even in the absence of entanglement and discord. We
certify the presence of such quantum correlations via negativities in the
regularized two-mode Glauber-Sudarshan function. Our data show compatibility
with an incoherent mixture of orthonormal photon-number states, ruling out
quantum coherence and other kinds of quantum resources. By construction, the
quantumness of our state is robust against dephasing, thus requiring fewer
experimental resources to ensure stability. In addition, we theoretically show
how multimode entanglement can be activated based on the generated,
nonentangled state. Therefore, we implement a robust kind of nonclassical
photon-photon correlated state with useful applications in quantum information
processing.
