Separate particle detectors measure the quantum states of each particle and send the resulting signal to a coincidence counter. In any experiment studying entanglement, the entangled particles are vastly outnumbered by non-entangled particles which are also detected; patternless noise that drowns out the entangled signal.
In particle detection, separate instruments measure the quantum properties of each particle and relay the findings to a counter. In studies of entanglement, the particles involved are vastly outnumbered by non-entangled counterparts, which are also detected; background noise that obscures the entangled signal.