The recent post made me fear that a lot of you are taking this "monkey looks at double-slits" meme, which was only ever supposed to be a funny monkey meme, actually seriously. Honorable mention goes to @kromem@lemmy.world, whose 12 posts on the topic, insisting that the quantum eraser experiment (but not the delayed-choice quantum eraser!) proves that the double slit is somehow bizarre, forced me to make my own meme. This meme explains the (non-delayed choice) quantum eraser paper from arXiv:quant-ph/0106078 and the figures are numbered to reference the paper.
First of all, looking at the photons, you the conscious intelligent monkey, MAKES NO DIFFERENCE. You can't actually "see" the photons going through a slit the way you could see say a bowling ball. The only way to detect a photon is to absorb or reflect it, and if the photon is getting absorbed by your eye that means it's not going through the slit or hitting the screen. The interference pattern stays visible on the screen WHETHER OR NOT YOU LOOK AT IT.
They've lied to you when they said the pattern changes when you "look" at which slit it the photon goes through. What the physicists actually do to measure the "which path" information is they put these circular polarizer filters in front of the slits, one clockwise one counterclockwise. Then the pattern disappears and you get this one single blob of density (Not even double! Figure 3). This is because light polarized in opposite directions cannot interfere with itself - wikipedia calls this the "Fresnel–Arago laws". In principle you could have put a polarization detector in place of the screen and record which way the light hitting it is polarized, which would tell you which slit the photon must have went through. The physicists DON'T EVEN BOTHER DOING IT. The fact alone that the light is polarized when it hits the screen is sufficient to destroy the interference pattern.
Well, NO SHIT. You put these giant 3D glasses in front of the slits and you still expect to see interference? This is very much a "mechanical interaction", not some "non-obtrusive conscious observation". Everything that destroys coherence will ruin your quantum experiment! Mystery solved!
So what about the quantum eraser, @kromem will ask? Popular science has created this myth that you can look at the screen and you can make the interference pattern literally shimmer in and out of existence by just flipping a switch, connected to second detector positioned elsewhere, turning it off and on. An action at a distant place (the detector POL1 observing "twinned" entangled photons created by this fancy nonlinear barium crystal before the slits, Figure 1) changes whether light over here behaves as a particle or a wave, right in front of your eyes. Spooky action at a distance, right?
THIS FUCKING DOESN'T HAPPEN. The monkey will see the single blob from Figure 3 and only single blob, no matter whether it turns the second detector on or off! The interference pattern will NEVER shimmer back into existence. The light never switches between behaving like a wave and behaving like a particle. It always behaves the same way, all the time, everywhere in the universe - like fucking light!
So what do the physicists actually fucking mean when they say the interference pattern is "restored"? If you observe the photons hitting the screen one at a time and you correlate them with simultaneous detections at detector POL1, you can mark those events as either "yes coincidence" category A or "no coincidence" category B. If you look at just all the category A events (Figure 4) you will see an interference pattern, and just category B you will see another (Figure 5). You cannot see these patterns by eye on the screen! You have to use a computer to record the events individually and separate them, you will only ever see a single blob by eye. The two interference patterns are subsets of that blob. They were always part of it, their hills and valleys mesh together into a single continuum. NO ONE EVER FUCKING EXPLAINED THIS.
The detector POL1 has a linear polarizer filter in front of it, so straight out the gate it will not see 50% of the twinned photons at all, because they will get stuck in the filter. Your category A can never match more than 50% of events. It gets worse, since the non-linear crystal in reality has very low efficiency and most photons going through are not twinned, so you cannot measure category B directly. In the experiment they do it by rotating the filter 90°, which changes the correlation to category B. In the meme I show them as if the crystal was 100% efficient.
The delayed-choice quantum eraser works similarly - you only ever see a single blob and can never see the interference pattern shimmer in and out of existence. You need the correlation data from the second detector to split the blob into two intermeshed interference patterns using a computer. The Sabine video was the first one I've ever seen that explains this correctly. Every other popular science video up to that point has lied to me!
Whatever you do, DO NOT watch the DR. QUANTUM video with an open mind! (Not even going to link to it, @manual3204 linked it in the other thread.) It's from a documentary produced by a literal UFO cult to promote their quantum woo woo, only masquerading as a quirky science video. It came out in the early days of youtube, when its production and animation quality were unusually high for its time, so it immediately became youtube's go-to video for double slit experiment. Copies of it remain highly ranked there even to present day. It's total baloney!
