Detecting metrologically useful entanglement in the vicinity of Dicke   states
  We present a method to verify the metrological usefulness of noisy Dicke states of a particle ensemble with only a few collective measurements, without the need for a direct measurement of the sensitivity. Our method determines the usefulness of the state for the usual protocol for estimating the angle of rotation with Dicke states, which is based on the measurement of the second moment of a total spin component. It can also be used to detect entangled states that are useful for quantum metrology. We test our approach on recent experimental results. 
Iagoba Apellaniz, Bernd L\"ucke, Jan Peise, Carsten Klempt, Geza Toth
quant-ph cond-mat.quant-gas physics.atom-ph