title: Spin squeezing inequalities and entanglement of N qubit states.
abstract: We derive spin squeezing inequalities that generalize the concept of the spin squeezing parameter and provide necessary and sufficient conditions for genuine 2-, or 3-qubit entanglement for symmetric states, and sufficient condition for N-qubit states. Our inequalities have a clear physical interpretation as entanglement witnesses, can be easily measured, and are given by complex but elementary expressions.
authors.affiliation: Institut für Theoretische Physik, Universität Hannover, D-30167 Hannover, Germany.
authors.forename: J K
authors.initials: JK
authors.lastname: Korbicz
authors.forename: J I
authors.initials: JI
authors.lastname: Cirac
authors.forename: M
authors.initials: M
authors.lastname: Lewenstein