In everything I've read about the Delayed Choice Quantum Eraser Experiment, the descriptions talk about the BBO crystal splitting of the photon into two entangled parts. The signal photon of the pair goes to D0 while the idler photon goes down various divergent paths.

You can see the "which-path information" of the photons at D0, by only showing the photons at D0 that are associated with the paired photons at D3 and D4 (which has their own "which-path information" maintained). That association always talks about the particles at D0 and D3/D4 being entangled, which they are by the nature of the BBO, but does the experiment require entanglement for us to detect the particle pattern at D0?

My understanding is that you isolate the "D3/D4"-paired photons at D0 by looking at the photons that hit D3 or D4 and then isolating the photons that hit D0 4 nano-seconds before the photons at D3 or D4. This seems to indicate that we aren't relying on the fact that the particles are entangled to detect the "which-path information." Is that accurate?


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Depends on the experiment; more recent versions usually choose to use entangled photons because they are completely non-classical, but earlier versions did not.

Experiments that don't depend upon entanglement instead usually depend upon superposition of states.

See https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser


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