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We present a method to estimate entanglement measures in experiments. We show how a lower bound on a generic entanglement measure can be derived from the measured expectation values of any finite collection of entanglement witnesses. Hence witness measurements are given a quantitative meaning without the need of further experimental data. We apply our results to a recent multiphoton experiment M. Bourennane, Phys. Rev. Lett. 92, 087902 (2004), giving bounds on the entanglement of formation and the geometric measure of entanglement in this experiment.
Gühne et al. (Fri,) studied this question.