Observable Entanglement measures

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Although entanglement constitutes one of the most remarkable differences between classical and quantum mechanics, and entanglement does have directly observable consequences, entanglement is not a regular observable like momentum or energy. It is rather a non-linear functional of a typically large set of such observables.
Therefore, one often needs to perform many different measurements, in order to determine the degree of entanglement of a given quantum state. We show, how the number of measurement can be reduced -- sometimes to a single one -- if collective observables are measured, that respect the natural invariance properties of entangled states. Finally, we discuss the implementation of this approach in a laboratory experiment.
Nature 440, 1022 (2006)
PRL 79, 050501 (2006)