Linking Quantum Discord to Entanglement in a Measurement

We show that a von Neumann measurement on a part of a composite quantum system unavoidably creates distillable entanglement between the measurement apparatus and the system if the state has nonzero quantum discord. The minimal distillable entanglement is equal to the one-way information deficit. The quantum discord is shown to be equal to the minimal partial distillable entanglement that is the part of entanglement which is lost, when we ignore the subsystem which is not measured. We then show that any entanglement measure corresponds to some measure of quantum correlations. This powerful correspondence also yields necessary properties for quantum correlations. We generalize the results to multipartite measurements on a part of the system and on the total system.

Figure 1

A measurement apparatus $M$ is used for a von Neumann measurement on $A$ (green colored area), which is part of the total quantum system $AB$. The measurement implies a unitary evolution on the system $MA$, which can create entanglement $E^{M|AB}$ between the apparatus and the system. The partial entanglement $P_E=E^{M|AB}-E^{M|A}$ quantifies the part of entanglement which is lost when ignoring $B$.