A novel measure, quantumness of correlations ${\mathcal{Q}}_{AB}$ is introduced here for bipartite states, by incorporating the required measurement scheme crucial in defining any such quantity. Quantumness coincides with the previously proposed measures in special cases and it vanishes for separable states—a feature not captured by the measures proposed earlier. It is found that an optimal generalized measurement on one of the parts leaves the overall state in its closest separable form, which shares the same marginal for the other part, implying that ${\mathcal{Q}}_{AB}$ is nonzero for all entangled bipartite states and it serves as an upper bound to the relative entropy of entanglement.

• Received 15 July 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.140502