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Quantum eraser experiment is explained in this youtube video in a basic way.

Correct me if I am wrong but I understand it like this:

Detectors A and B are placed further from the splitting crystal than the interference screen. C and D in the eraser part are placed even further. So:

  1. If detectors A and B are active, then we will not see an interference pattern on the screen even if the idler photon has not reached to A or B yet.

  2. If we deactivate detectors A and B, then just by looking at C and D, we will not be able to deduce which slit the photon went through; therefore, we will observe an interference pattern on the screen.

Now, there are some limitations in the experiment like the conversion rate of the entanglement-crystal. Only a very small amount of incoming photons are converted into two entangled photons. Therefore we must use a coincidence circuit to distinguish them from the non-entangled majority. This implies that it is not possible to detect the interference pattern instantly. We must collect data over time and take the superposition of the relevant photons with the help of the coincidence circuit.

But, is it theoretically possible to affect the interference pattern instantly by activating /deactivating the eraser detectors? Assuming the efficiency of the entanglement-crystal is 100% and the eraser detectors are positioned 1 light-year away from the interference screen, does the interference pattern on the screen change at the same instant that we switch the detectors on /off? Do you know anything on this?

P.S. I edited the question considerably. @benrg's answer was for the previous version.

Xfce4
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1 Answers1

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Since only a very small amount of incoming photons are converted into two entangled photons we must use a coincidence circuit to distinguish them from the non-entangled majority. This rarity implies that it is not possible to detect the interference pattern instantly.

No, you always have to use the coincidence circuit, even in an idealized version of the experiment. No interference pattern is ever visible on the screen, even in principle.

Nothing that happens in the "lower" path of the experiment (containing detectors A-D) affects what happens in the "upper" path (the screen). The detections in the two halves are only correlated; there's no causal connection between them.

"Backward in time correlation" is possible even classically. Here's an example from another answer:

Suppose you have a bowl containing two red slips and two black slips. You draw a slip. If it's red, you draw another one. The second one will be black in about 2/3 of the trials, because there are two black slips and one red one left in the bowl when you draw it.

Now consider a variation of this experiment with the draws reversed. You draw a slip and set it aside. Then you draw a second one . If the second one is red, you look at the first one. Even though there were equally many red and black slips in the bowl when you made the first draw, if you actually try this experiment, you'll find that the first slip will be black in about 2/3 of the trials.

Is this an example of retrocausality in classical physics? No, it's an example of the inherent atemporality of probabilistic reasoning, whether classical or quantum. If A is correlated with B, then B is correlated with A; it makes no difference which one happens first.

In the DCQE experiment, you "see" a pattern in the earlier measurement because you're postselecting on the result of the later measurement, just as in the experiment with the colored slips. The time separation of the measurements doesn't matter.

In fact, because the detections are timelike separated, it's possible to reproduce the results of a DCQE experiment with a classical local hidden variable theory – unlike the EPR experiment, where the measurements are spacelike separated and Bell's theorem shows that it's impossible to explain the results that way.

benrg
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  • Thank you. But assume that detectors A and B are 1 light-hour away. In this 1 hour, would we be observing the interference or not? Normally, I would expect to observe an interference pattern until the idler photon ends up in the detector A or detector B, but what I understand from the video is the opposite. – Xfce4 Sep 14 '21 at 00:11
  • @Xfce4 I didn't watch the video before writing the answer. I've now watched it, and it's just a wrong description of the experiment. It says (at 2:52) that you see an interference pattern when the lower detectors are switched off. In reality, there's never an interference pattern on the screen in any circumstances in this family of experiments. There are only correlations which you can't calculate until the lower measurements have happened, 1 hour later in your case. – benrg Sep 14 '21 at 02:14
  • benrg, What if we change the setup? Assuming the crystal converts 100% of the photons into entangled pairs, what would happen if we moved the double slit after the crystal (instead of before) on the upper part? Would passing through the double slit break the entanglement? If not, would this setup allow us to transfer data faster than the speed of light? – Xfce4 Sep 14 '21 at 17:20