TUNABLE BELL-INEQUALITY VIOLATIONS BY NON-MAXIMALLY-VIOLATING STATES IN TYPE-II PARAMETRIC DOWN-CONVERSION

We use a tunable quantum interference in type-II parametric down-conve rsion to construct two-photon quantum states that exhibit less-than-ma ximal, and tunable, violations of Bell-type inequalities. These states , no longer singlet-state analogs, possess an interference term of con trollable magnitude that we adjust by tuning the measurement bandwidth . We show violations of two Bell-type inequalities in polarization var iables, one more general than the other. We use these tunable violatio ns to probe the threshold degree of interference that must be present to generate a violation, using an appropriate quantitative figure of m erit.