The video that you watched on YouTube is full of gross errors and cannot be relied on to get an accurate picture of the quantum eraser experiment.

The underlying error in the video is that it claims there's detectors at the slits detecting which slit each photon traveled through. Nothing at the slits is detecting anything. What's placed before the slits in the experiment are nothing more than *polarizing lenses,* typically one horizontal and one vertical. *That's it!* 

*Almost two hundred years ago, Fresnel and Arago got together with a bunch of double-slit tools and accessories and discovered a remarkable fact about light: that exactly perpendicularly polarized light, when brought together via the double-slit, shows no interference.* 

Well, according to the quantum explanation, what's really going on when such polarizers are placed over the slits in the double-slit experiment (i.e. which setup constitutes the quantum eraser experiment) is that each polarizer is, not detecting, but *marking* or imbuing each photon itself with distinctive information. (This is important because, in every instance except this, it's held by quantum mechanics that no two photons of the same wavelength can be distinguished.) But this information doesn't allow you to distinguish each and every photon; in particular, it allows you to mark each photon with only enough information to be able to tell, in theory, *through which slit it traveled* (i.e. because every photon traveling through each slit is imbued with the *same* information, that is, either its vertical or horizontal polarization.)

And you might be surprised to hear this, but never during the experiment does that "which-slit" information given to the photons have to be attempted to be read or deciphered. The point is solely that the photons were *marked* with this information. *That,* according to the quantum explanation, is what causes the absence of interference on the screen behind the slits. By *marking* the photons in this manner, the experimenter *changes* the behavior of the photons, causes them to relinquish their wave-like behavior, to choose only one slit through which to travel, and not to interfere on the other side.

It's also helpful to know that the *"eraser"* in the video is in reality nothing other than another polarizing lens, rotated at 45 degrees from either of the two polarizing lenses already present. Interposing this third lens, and as a result restoring the interference pattern on the screen behind the slits, is something that was also discovered by Fresnel and Arago. The point of this "eraser" in the quantum eraser experiment is to show that one can change the behavior the photons, i.e. restore their wave-like behavior by effectively erasing their which-way information, even *after* they've traveled through the slits exhibiting particle-like behavior.

Don't hesitate to let me know if you want any of the above clarified, if your questions based on the YouTube video still exist, or if you have any additional questions.