Characterizing multiparticle entanglement in symmetric N-qubit states   via negativity of covariance matrices
  We show that higher order inter-group covariances involving even number of qubits are necessarily positive semidefinite for N qubit separable states, which are completely symmetric under permutations of the qubits. This identification leads to a family of sufficient conditions of inseparability based on the negativity of 2kth order inter-group covariance matrices of symmetric N-qubit systems. These conditions have a simple structure and detect entanglement in all even partitions of the symmetric multiqubit system. The observables involved are feasible experimental quantities and do not demand full state determination through quantum state tomography. 
A. R. Usha Devi (1), R. Prabhu (1)(Department of Physics, Bangalore   University.), and A. K. Rajagopal (2)(Department of Co...