Strengthened Bell inequalities for orthogonal spin directions

Abstract

We strengthen the bound on the correlations of two spin-1/2 particles (qubits) in separable (non-entangled) states for locally orthogonal spin directions by much tighter bounds than the well-known Bell inequality. This provides a sharper criterion for the experimental distinction between entangled and separable states, and even one which is a necessary and sufficient condition for separability. However, these improved bounds do not apply to local hidden-variable theories, and hence they provide a criterion to test the correlations allowed by local hidden-variable theories against those allowed by separable quantum states. Furthermore, these bounds are stronger than some recent alternative experimentally accessible entanglement criteria. We also address the issue of finding a finite subset of these inequalities that would already form a necessary and sufficient condition for non-entanglement. For mixed state we have not been able to resolve this, but for pure states a set of six inequalities using only three sets of orthogonal observables is shown to be already necessary and sufficient for separability.