Representation of entanglement by negative quasiprobabilities

Any bipartite quantum state has quasiprobability representations in terms of separable states. For entangled states these quasiprobabilities necessarily exhibit negativities. Based on the general structure of composite quantum states, one may reconstruct such quasiprobabilities from experimental data. Because of ambiguity, the quasiprobabilities obtained by the bare reconstruction are insufficient to identify entanglement. An optimization procedure is introduced to derive quasiprobabilities with a minimal amount of negativity. Negativities of optimized quasiprobabilities are necessary and sufficient for entanglement; their positivity proves separability.