Science
In reply to the discussion: Quantum Entanglement, Dark Counts, Coincidence Detection [View all]caraher
(6,276 posts)A better way to put it might be this: what you measure at the distant detector is determined by what measurement you've already made at the near detector (or system), when they have timelike separation. My answer presumed you measure the interference pattern up close, but you could also imagine instead doing a which-way measurement.
The quality of the measurement made second probably depends on things like the detection efficiency of the first measurement. That's because if you measure photons only half the time, you actually get a mix of eigenstates and the original wavefunction for the distant photon. Measuring in coincidence cleans up the signal. You can always gate the detection system for the second measurement with a suitable delay to clear out the cases where the first measurement did not detect a photon.