  We show how polarisation measurements on the output fields generated by
parametric down conversion will reveal a violation of multi-particle Bell
inequalities, in the regime of both low and high output intensity. In this case
each spatially separated system, upon which a measurement is performed, is
comprised of more than one particle. In view of the formal analogy with spin
systems, the proposal provides an opportunity to test the predictions of
quantum mechanics for spatially separated higher spin states. Here the quantum
behaviour possible even where measurements are performed on systems of large
quantum (particle) number may be demonstrated. Our proposal applies to both
vacuum-state signal and idler inputs, and also to the quantum-injected
parametric amplifier as studied by De Martini et al. The effect of detector
inefficiencies is included.
