  It is possible to prepare classical optical beams which cannot be
characterized by a tensor product of vectors describing each of their degrees
of freedom. Here we report the experimental creation of such a non-separable,
tripartite GHZ-like state of path, polarization and transverse modes of a
classical laser beam. We use a Mach-Zehnder interferometer with an additional
mirror and other optical elements to perform measurements that violate Mermin's
inequality. This demonstration of a classical optical analogue of tripartite
entanglement paves the path to novel optical applications inspired by
multipartite quantum information protocols.
