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I understood that, after the photon passes through the double slit, and before it lands on the screen, if the Quantum eraser is activated, there will appear the interference pattern due to the loss of knowledge; otherwise no interference pattern if the eraser is not activated.

my question is, what if the photon has already landed on the screen, before we decide to turn on or off the quantum eraser?

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    $\begingroup$ The outcome has nothing to do with what a human mind is thinking, seeing or acknowledging. The photons will go through one of the two slits and do what they’re going to do regardless of your consciousness. $\endgroup$ – Bill Alsept Aug 26 '18 at 17:18
  • $\begingroup$ It's very important to understand that you do not "see the interference pattern" in one run! You only see it if you have a very large number of runs of the same experiment. Therefore, one may not think that somehow magically a "interfering photon" is turned into an "non-interfering photon", no, these are only statistical effects. All these are well explained without paradox. $\endgroup$ – Luke Aug 27 '18 at 11:47
  • $\begingroup$ i am thinking, if, before the photon hits the screen, we don’t activate the quantum eraser, there will be the “clump pattern” on the screen. and after that, no matter quantum eraser is activated or not, but the knowledge has already been marked on the screen, it can’t be really erased by quantum eraser any more. $\endgroup$ – eyemooneye Aug 27 '18 at 18:49
  • $\begingroup$ The real creepy part is, after the photon had already passed the slits(either one or both), before it lands on the screen, if we turn on the quantum eraser, there will appear the interference pattern; if we don’t turn on the quantum eraser, there will appear the clump pattern. How does the photon know whether i activate the quantum eraser or not? it’s mind-boggling. $\endgroup$ – eyemooneye Aug 27 '18 at 18:55
  • $\begingroup$ Not sure you’ll ever see a clump pattern. Check this out physics.stackexchange.com/q/423613/129433 $\endgroup$ – Lambda Aug 28 '18 at 0:58
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For your first question the wave function is destroyed as well as the entanglement when the photon is detected, you can't go back in time.

For the creepy question, a way to understand is that the wave function for a photon with the eraser quickly switched on (diagonal polarizer on) while the other paired photon is still in flight affects the overall wave function, which includes the state of the electrons in the polarizers as well. I believe this affect is limited in distance, also the photon in the non-slit path must not have been destroyed or measured.

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