  Coincidence detection is a key technique used in nonlocal quantum-correlation
measurements to test Bell inequality violation between remotely separated local
detectors. With individual uniform intensity of local measurements, the
nonlocal correlation fringe is a mysterious quantum feature that cannot be
achieved classically. Here, the coincidence detection is coherently
investigated to understand the fundamental physics of the nonlocal correlation
fringe via second-order quantum superposition between selected nonlocal
measurement events. Because of the coherence feature of paired photons, the
coincidence technique modifies the measurement events for the rule of thumb of
indistinguishability between selected measurement bases of paired photons. This
indistinguishability is quantum superposition between nonlocally detected
events resulting from a selected time slot of coincidence, where coherence
between individually measured photons is an absolute condition.
