Nonclassicality, entanglement, and beyond
Prof. Dr. Werner Vogel (AG Theoretische Quantenoptik, Universität Rostock)

We have introduced a complete characterization of nonclassical states by the concept of the nonclassicality quasiprobability. Unlike any other known phase-space distribution, it displays the nonclassicality -- in the spirit of the quantum optics notion -- of any quantum state via negativities of a regular distribution. Our quasiprobabilities are accessible in experiments, which is demonstrated for photon-added and squeezed states. Entanglement can be fully characterized by the negativity of the entanglement quasiprobability. This requires to solve an optimization procedure of the given quantum state -- the so-called separability eigenvalue problem -- leading to its optimal representation with respect to the property entanglement. As an example, the analytical solution of this problem has been derived for a phase-randomized two-mode squeezed vacuum state. In a next step, our concept is further generalized to characterize multipartite quantum correlations. We demonstrate that quantum correlations may exists, even when the state is not entangled and has zero quantum discord.