1 Department of Physics and Astronomy, Science and Technology, Aarhus University2 Institut für Quantenoptik, Leibniz Universität Hannover, 30167 Hannover3 Department of Theoretical Physics, The University of the Basque Country, 48080 Bilbao4 Institut für Theoretische Physik, Leibniz Universität Hannover, 30167 Hannover5 Department of Physics and Astronomy, Science and Technology, Aarhus University
Recent experiments demonstrate the production of many thousands of neutral atoms entangled in their spin degrees of freedom. We present a criterion for estimating the amount of entanglement based on a measurement of the global spin. It outperforms previous criteria and applies to a wider class of entangled states, including Dicke states. Experimentally, we produce a Dicke-like state using spin dynamics in a Bose-Einstein condensate. Our criterion proves that it contains at least genuine 28-particle entanglement. We infer a generalized squeezing parameter of −11.4(5) dB.