Quadratic bell inequalities as tests for multipartite entanglement.

This Letter presents quantum mechanical inequalities which distinguish, for systems of n spin- 1 / 2 particles ( n>2), between fully entangled states and states in which at most n-1 particles are entangled. These inequalities are stronger than those obtained by Gisin and Bechmann-Pasquinucci [Phys. Lett. A 246, 1 (1998)] and by Seevinck and Svetlichny [quant-ph/0201046].