Cirac, J. Ignacio Theory, Max Planck Institute of Quantum Optics, Max Planck Society;
Korbicz, J. K., Cirac, J. I., & Lewenstein, M. (2005). Spin Squeezing Inequalities and Entanglement of N Qubit States. Physical Review Letters, 95(12): 120502. doi:10.1103/PhysRevLett.95.120502.