Tight bounds for the Pearle-Braunstein-Caves chained inequality without the fair-coincidence assumption

Abstract

In any Bell test, loopholes can cause issues in the interpretation of the results, since an apparent violation of the inequality may not correspond to a violation of local realism. An important example is the coincidence-time loophole that arises when detector settings might influence the time when detection will occur. This effect can be observed in many experiments where measurement outcomes are to be compared between remote stations because the interpretation of an ostensible Bell violation strongly depends on the method used to decide coincidence. The coincidence-time loophole has previously been studied for the Clauser-Horne-Shimony-Holt (CHSH) and Clauser-Horne (CH) inequalities, but recent experiments have shown the need for a generalization. Here, we study the generalized "chained" inequality by Pearle-Braunstein-Caves (PBC) with two or more settings per observer. This inequality has applications in, for instance, Quantum Key Distribution where it has been used to re-establish security. In this paper we give the minimum coincidence probability for the PBC inequality for all N and show that this bound is tight for a violation free of the fair-coincidence assumption. Thus, if an experiment has a coincidence probability exceeding the critical value derived here, the coincidence-time loophole is eliminated.