  In analogy with Bell's inequality for two-qubit quantum states we propose an
inequality criterion for the non-separability of the spin-orbit degrees of
freedom of a classical laser beam. A definition of separable and non-separable
spin-orbit modes is used in consonance with the one presented in Phys. Rev.
Lett. 99, 160401 (2007). As the usual Bell's inequality can be violated for
entangled two-qubit quantum states, we show both theoretically and
experimentally that the proposed spin-orbit inequality criterion can be
violated for non-separable modes. A discussion on the classical-quantum
transition is also presented.
