Solving the Delayed-Choice Quantum Eraser
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- Опубліковано 21 лис 2023
- This video gives a detailed explanation of how to correctly interpret the delayed-choice quantum eraser. This is followed by a full derivation of the equations that describe the problem. If you find these steps a bit rushed, I suggest pausing the video after each step.
*CORRECTION*: around 15:16, there is a small typo in the expression under “Replacing wavefunction past BS”. The term |ψ⟩ᵗᵢ at the end of the equation should not be present.
The derivation uses a combination of the following sources:
arxiv.org/abs/1707.07884
arxiv.org/abs/1908.03920
• Demystify the delayed ...
This channel is unfairly underrated
Thank you so much for your comment! 🦀 🦀 🦀
It takes twenty years for us to notice we have an interferometer instead of an eraser.
Thank you a lot! I've been following this experiment since I watch it first time back in 2015, trying to understand it thoroughly. I didn't quite understand why in the debunked videos of this year they don't explain the actual root cause of the interference. Finally I can understand it, especially in the explanation at 10:00 🙏 you are the hero man, thx for the video
Thank you for comment! Glad it was helpful.
The best video on this subject.
This is brilliant. Peeking into super-posistions always gave me a sensation that we're looking at cheat codes for ether creation 😊 Just for a while I had my own ideas just being a small lad about the functions of subatomic phenomena, just wasent able to put an idea towards the visual, growing up; these things slowly started to make sense to me through physics lectures, reviewing formulas to equations and finally seeing more demonstrations solidified. Physics is my infinatum. 🙂But not beyond my dreams.
This is an incredible video! I finally allllmost get it. Other debunking/explanatory videos have skipped over anything having to do with correlations between locations on D0 and the various other detectors, and so have been wholly unsatisfying. Your video gets me by far the closest to understanding!
The remaining confustion I have can be summed up this way. Around 4:20, you say the BBO crystal splits the waves from the top and bottom slits into two separate, isolated branches of the global wave function that cannot interfere with each other (hence no interference pattern on top screen).
Then around 9:30, you say that the information photon will still carry the probability amplitudes of having gone through the top slit or the bottom slit, and it appears that the beam splitter recombines these two wave probabilities, and the two waves interfere with each other, making certain screen locations correlate with the chances of landing at D1 or D2. But doesn't this contradict the idea that the two beams are in different branches of the wave function that can't interfere with each other? I think I'm missing something about how the information photon caries the probability information, that would make this more clear.
Oh man, this is such a good question! Well, first off, since in this explanation we are assuming that the screen photon reaches D0 first, the wavefunction reduces to one that *only* describes the information photon. Therefore, at this point we have a wave for a single particle, so it is perfectly valid to say that it interferes with itself when passed thru the beam splitter. This is why I used this language in the video.
However, consider the case in which the information photon is *not* delayed, but reaches detectors D1 and D2 first. In this scenario, we would say the info photon cannot interfere with itself and it is measured at D1 or D2 with equal probability. And then, the location where the screen photon hits D0 will correlate with whichever detector was triggered. So, if we measure at D1 first, the screen photon will be localized probabilistically in one of the regions correlated with that detector.
The bottom line is that, the information of where the screen photon hits D0 and the information of which detector the info photon triggers are correlated and stored in the global wavefunction. So, independent of which order we perform the measurements, the results are always the same: no interference on the screen, but correlations between the detector that gets triggered by the info photon and the landing location on the screen.
In retrospect, I should’ve elaborated a bit more on this. Maybe I’ll make a second video describing the scenario where the info photon is not delayed.
Hope this helps.
@@diemilio Wow! Thank you so much for this detailed response! I'll need to ponder this more--not being able to rely on the math to orient myself (I love math, but QM equations make my eyes glaze over, I'm assuming because I'm not trained in it) it always takes me a while to wrap my head around these things conceptually, but this is definitely really helpful! I can't tell yet if I can understand why the correlation creates a smeared out pattern in D3 and D4, vs a banded pattern in D1 and D2, but I think spending a little more time thinking it through might get me there. Thanks again, and thanks for the great video, I hope lots of people see it!
@@diemilio OH! GOT IT! For D3 and D4, you're looking at correlations in which slit the photon came through; in D1 and D2, you're looking at correlations in the relative phases of the photons! You absolutely said this in the video, and for anyone halfway trained in QM or EM/optics, this will make instant sense. I think the only way the video was a little bit confusing for me personally is that it would have helped to have a very short description with a graphic or two about relative phase and what that means for where the beam hits DO, showing how the relative path lenghts of the beams going towards D0 create this probability. Oh! Actually I just realized that you have exactly such an explanation and graphics early in your double slit video, so even just adding a reference to that video around 1:25!
That is a small critique though, this video is, again, by far the most helpful one I've seen in terms of explaining the non-spookiness of this experiment (well, or at least an experiment where the only spookiness is enganglement/wave function collapse!)
@@erinm9445 thanks so much for the feedback! I agree, I should’ve done a better job with the relative phase explanation. Maybe I’ll clarify this on a separate video.
Thanks again!
Great presentation!
Well, the circuit that you present at the end does not correspond to the "delayed-choice quantum eraser experiement" (DCQEE); it is presented simply as the "eraser" in the literature (although you can include a "delay"). Keep in mind that in the DCQEE, the quantum system itself "decides" the type of pattern that they will become, that is, either an interference pattern or not. In the circuit that you show, YOU decide the type of pattern, that is, the circuit would need extra qubits to make sure that two types of pattern are automated by the quantum system. If that circuit that you show at the end can be called a DCQEE just because of a "delay" in the measurement of one of the qubits, then you can also call a Bell-type of experiment a DCQEE when a "random" strategic measurement is performed after the other, by a the researcher, faster than the time that it would have taken the information to travel between the detectors, that "erases" whatever information was detected in the first, such that the outcomes "violate" a Bell-type inequality. Notice that in the latter case, it is not as simple to explain the outcomes in terms of "instructions" that can be set from the start, which is really what the DCQEE should have been about given that it was an attempt to show the "spookiness" of quantum systems where retrocausality is just another interpretation among so many others.
If you really want to make the analysis more intuitive so that one does not have to resort to retrocausality, you could just say that the experiment results in correlations of measurement, not causations of one on the other, given that "coincidences" is what is being measured at two space-like separated locations.
Thanks for the comment.
All of what you just mentioned for the circuit will be explained in detail in the next video. Notice how the gate in the bottom right flips between a Hadamard and an Identity. This can be controlled by another qubit, just like I explained in the double-slit video for the screen qubits. The “delay” portion will also be clarified; it’s just a matter of applying the last gate after the measurement for the first qubit, as shown in the code below, However, due to the “deferred measurement principle”, this does not affect the results.
github.com/diemilio/streams/blob/main/delayed-choice-quantum-eraser/dcqe-quantum-eraser.ipynb
As for your comment on correlation vs causation, you’re right. It is definitely important to use that language.
@@diemilio Yes, more qubits would have to added in order to offer something comparable to the DCQEE. What I am saying is that in the setup you present, where a researcher can make the choice of basis, one may as well use three of the settings required to make a Bell-type test, which is even a better way to present what was intended with the DCQEE. Just offering an experiment where the researcher can opt to see an inference pattern or not, after one of the two measurements is done, does not capture the "spookiness" that was intended in the DCQEE. The experiment was "misunderstood" because some couldn't conceive it in terms of "instruction" set from the start with an automated mechanism, allowing "randomly" an interference and non-interference pattern to take place, even if the measurements at two space-like separate locations were not done simultaneously, but nevertheless relatively fast. You see, if you add more qubits to your circuit to automate the interference/non-interference pattern, where the researcher can only select a phase value, like in the DCQEE, to observe a smooth pattern, then it becomes more challenging to offer a "classical" explanation for the "erasure" (which is not to imply that retro-causality is an explanation).
Thanks @@user-jw7vc4ll2b. I see what you’re saying.
My experience is in solid-state physics and optoelectronics, so I'm not an expert in quantum computation and am hoping to learn more. In the actual photon experiment, an essential component is the Glan-Thompson (G-T) prism straight after after the BBO. Surely that prism forms a type of quantum gate that processes the photon pairs into orthogonally polarized states. Nobody seems to talk about the implications of this in discussions of this experiment, and yet it seems to be a crucial part of discussing how it works in quantum terms. Before the G-T we don't know the relative polarization along each path, after, we do. Surely it adds another Q-bit to each path.
Interesting observation! Thanks for sharing.
I will look into this more carefully; however, I don’t think it is that crucial for this experiment to mention that the photons are orthogonally polarized (same as for mentioning that they have opposite momenta in the vertical direction). We don’t use their relative polarization at any point, so don’t think this will change any of the conclusions.
As for how the actual physical setup works, it is definitely important to keep these details in mind. I did find this “physics stack exchange” post mentioning that the job of the GT prism is actually done by the BBO, so there’s an error in the Wikipedia version of the diagram that labels what is a regular prism right after the BBO as a GT:
physics.stackexchange.com/questions/18605/variation-of-delayed-choice-quantum-eraser
On second thought, polarization can play a role in ensuring that the two branches of the information photon are orthogonal to each other. I do mention this around 13:20, but it might have been good to explicitly say that the states are orthogonal because they constitute equal parts of a superposition of a vertically-polarized and a horizontally-polarized state. So, even though the relative polarization between the two photons is not critical, it is important for the two branches of the info photon.
Thanks again for your feedback.
@@diemilio Hey, thanks; I'm not able to comment on the quantum logic way of analyzing this yet. Just for my own sake I did more checking on the BBO/GT. I'm coming from a different field so this is more how I might think of the setup. One aspect is that the photon pairs are generated in phase but at half the frequency of the source (otherwise no phase coherence with the source then interference is scrambled). As noted in Kim's et al's paper, the regions of pair emission are like a pair of phased antennas, the slits serve to localise the antenna regions.
Think of a normal laser two slit and add in a phase delay on one slit or the other to steer the pattern left and right. In the case of the entangled pair, each has both polarizations i.e. each might be thought of as effectively circularly polarized. One in one direction the other in the opposite to conserve angular momentum. The GT acts as polarizing beamsplitter, converting the pair into complimentary linear polarizations each 90 deg out of phase with the other, either forward or back relative to the source laser. If so, this would then allow the interferometer output to be correlated with, and separate out, the overlapping interference pattern at the D0 detector as it moves across the screen.
@@JohnKNMurphy-nz thank you for all the detailed explanation. This is very helpful.
This is actually an important experiment not because of the pattern on the screen but because of the statistical interpretation that arouses based on the questioning bias
Interesting. I’ve watched a lot of these kinds of videos, and this one does the best job so far I feel. I feel like if I go through that math, it’ll be a bunch of alg and trig identities. Which kinda helps explain it, but it that “cause the math says so” kind of explanation.
I’m still confused why one pair sorts into a interference pattern, and the other doesn’t. I’ll need to watch this a bunch more times.
Thank you so much for your comments, @PsychoMuffinSDM.
In summary, the reason particles correlated with D3 and D4 don’t show a pattern is because the top-slit and bottom-slit branches of the info are directly sent into these detectors, not allowing these branches to interfere with each other. In the case of D1 and D2, the beam splitter combines these two branches causing them to constructively and destructively interfere depending on where their partner screen photon landed on D0.
It is also worth mentioning that, the reason I spend 12 out of the 16 minutes trying to provide an explanation that uses no equations, is precisely because I want to avoid the “the maths say so” justification. I only added a super quick explanation of the equations to back up the claims previously made.
Hope this helps!
@@diemilio I'm still trying to wrap my head around this. It is so unintuitive!
The D3 and D4 make sense, if you think of it as a particle, however, you do make it sound odd by saying "because ... the info are directly sent into these detectors" making it sound like the pattern looks that way because there are detectors, which is what I guess you are trying to talk about with the misconception. But I thought that is true, that detectors get rid of the interference pattern, right? But then how/why does one entangled partner interact with the other? I find this especially hard to grasp, especially given that it seems hidden variable hypothesis has been disproven.
I think the other thing that is still weird is not just the fact the not only D1 and D2 are so different from D3/4, but D1 is shifted to the left, and D2 is shifted to the right. Although this is what you'd normally expect, the fact that the detectors are after a 50/50 mirror makes it seem like the points from D1 and D2 should all be mixed together. Like, I picture it as if I have 500 red ping pong balls, 500 blue, and 10 buckets lined up, and I randomly grabbed balls, without out looking at color, and randomly threw them into buckets (my aim is random, it could go into any bucket), I would think each buck should be 50/50 red and blue, but I find the majority of reds are in the odd buckets, and the majority of blues in the even. Maybe I am think too much like particles and not enough like waves...
And I do like the math, and I hope you expand on it, I'm just having trouble connecting it to the graphic. Is there a way to make that math look more like Alg2 and trig? Can you use more simple variables and nomenclature? I'll probably try doing that myself, it's just that I am really rusty on my identities.
Anyways, thanks again for making this video! This is such a fascinating topic, and so frustratingly hard to understand!
@@PsychoMuffinSDM I think the confusion comes from thinking of the information photon as a localized object. It isn’t. It is always a wave. So when passed thru the beam splitter, it can cause interference. Don’t think of them as ping-pong balls being randomly selected to go one way or the other, but like water waves that can interfere
@@PsychoMuffinSDM Not sure if this helps you, but the reason I was still struggling with where the interference pattern comes from is that I totally missed the shift from correlations in which slit the photon goes through in the first half of the video (D3 and D4), vs the correlations in the relative phases of the photons in the second half of the video (D1 and D2), and the way those phase shift probabilities map onto the screen IS the double slit interference pattern.
D3 and D4 keep sort based on slit location, and destroy information about relative phase. D1 and D2 sort based on relative phase, but destroy slit location information. This youtube channel actually has a great little overview of this with graphics at the 1:25 mark of the recent-ish video on the double slit experiment "The Quantum Double-Slit Experiment". I'm used to thinking about this kind of interference in the context of the double slit, but for whatever reason I wasn't able to translate that knowledge to the phase shift correlations of this experiment. Hope that helps!
I like the video and I think all you say is perfectly fine, but you seem to downplay the interesting part: the D0 photon must "know" where to land based on something that will only happen in the future, even though nobody can ever use this "knowledge" because there is no communication nor causation (only the correlation with a future choice of the setup).
Thanks for your comment!
The point of the video is to show precisely the opposite. The screen photon does not need to “know” anything about the information photon or any future events at all. Its landing position is fully determined by the wavefunction of having gone thru the top slit OR the bottom slit. That’s it. It’s completely independent of anything else.
After the photon hits D0, the statistics of which detector the info photon triggers are determined by its updated wavefunction, which will have certain correlations with where the photon hit the screen.
There’s no need to know anything about future events. Everything is predetermined by the global wavefunction.
@@diemilio yes, everything is predetermined by the global wavefunction, that is why this experiment is completely uninteresting/unsurprising from a mathematics/physics point of view (likewise EPR btw). The interesting bit is that it elucidates that what photons do in our present depend (even if we can only ascertain this dependency or correlation after comparing the outcomes) on future events, otherwise you cannot explain the mere existence of a correlation in the first place.
Also there is no objective "After the photon hits D0": the eraser can be light years apart and you can find another inertial observer for whom the eraser event happens first.
This whole delayed quantum eraser is in fact pretty much the same thing as EPR except that the "spooky" stuff happens mainly because of time separation instead of space.
Best video out there explaining the Delayed-Choice Quantum Eraser experiment. Thanks a lot! However I still don't understand some elements of it. What does it mean that the corresponding plane waves emitted from each of the two-slits are orthogonal to each other? Why the coherence is lost after SPDC and than recombining the two waves onto D0 screen yet at the same time the beam splitter before D1 and D2 can lead "magically" once again to coherence and therefore to interference happening at D1 and D2 respectively. I guess I am missing something at this point. Otherwise it is quite clear that there is no backward action, just phase information that somehow is preserved at the D0 measurement that correspond to the interference happening after the beamspliter on the other side.
YES, I think I finally understood it !!! It is all about a simple concept not fully explained in this video: the phase shift relation between the two entangled photons and the pump photon that creates them !!! The phase shift of each of the idler and signal photons (the two entangled photons) looks random and that's why there is no interference at D0, but in reality due to the nature of SPDC the phase of the initial pump photon should match exactly the imposed waves of the generated photons, thus there is a strict correlation between the phases of the two entangled photons. Each place of the screen at D0 correspond to different phase shift between these two photons and thus the partner going to the beam splitter have its phase related to the position of the partner at D0 and the created interference (constructive or destructive) is related to the x-axis position on DO!!!! That's it.
Hi, @tiamnik I am glad you found the video helpful! Also, these are great questions; they help me understand where to improve in my explanations. Let me try to address them.
Please correct if I am wrong, but I don’t think anywhere in the video I said that “the corresponding plane waves emitted from each of the two-slits are orthogonal to each other”. I believe you might be referring to what I say around 13:00, which was: “…since the purpose of the information photon is to identify which slit the source photon passed through, its corresponding states are orthogonal to each other”. What I mean by this is that, in this experiment the paths of the two branches of the information are routed to not overlap by design. Otherwise, you could not definitively say that, when you get a detection at D4 (or D3), the photon emerged from the top (or bottom) slit, respectively. Therefore, the inner product of the wavefunctions is 0 (i.e., they’re orthogonal).
Regarding your other question, coherence is lost whenever there’s entanglement, therefore the screen photon cannot interfere with itself (since it is maximally entangled with the info photon). Now, the reason you do get the waves going towards D1 and D2 to coherently interfere with each other is because, in this version of the experiment, we are assuming that the screen photon reaches D0 before the info photon reaches any of the other detectors (I mention this around 13:55). This means that the screen photon is now localized in one particular place in the screen, which causes a partial “collapse” of the global wavefunction. This results in the loss of entanglement, allowing the info photon to regain its individual coherence. It is worth noting that if we were to allow the info photon to reach detectors D1 thru D4 before the screen photon reaches D0, the results will be exactly the same! It is just that our analysis will change a bit.
Let me know if this helps.
@@diemilio Thank you very much for your quick reply! Yes, about the coherence you are right, I think I misunderstood what you said. About the lost coherence at D0 and regaining coherence at D1 and D2 your explanation seems reasonable, however I found even a better explanation which I pointed in another comment here. In brief here is what I found - when the entangled pair is created it keeps not only the energy and polarization of the photon that created it, but also it keep the phase as a whole, which mean that the phase of each individual photon can be different, but the sum of both should always match that of the original source photon. Equipped with this knowledge we can now find out what is happening. Because the screen photon has "semi-random" phase it appear on the screen without interference pattern visible from its second possible path through the lower slit. However the distribution of the screen photons along the x-axis is still phase dependent, although without visible interference pattern. Now we know that the information photon on the other side of the setup is phase complementary to the screen photon, in such a way that all the information photons with screen photon partners coming from specific position or angle on the screen will have same phases, and thus interfere either constructively or destructively on the screen. If you look deeper this explanation is even more intuitive and clear for explaining why the interference on D1 and D2 can recreate an interference-looking pattern on D0.
Even more, there is no interference pattern at D1 and D2 as 50% goes to D1 and 50% to D2. However all the photons that goes to D1 correspond to specific positions on D0 and all that goes to D2 are correlated with screen photons that goes to different but complementary positions on D0. Thus we have interference in both cases on a single photon level, but due to the randomization of the phase between the two entangled photons there is no interference pattern unless we correlate phase information from D1 and D2 and position information from D0.
That means the quantization it dependent on the particles orientation on the wave
This thing makes me feel like we live in a dream. Also about the simulation theory, could we ever make a virtual universe? I dont think so
Great video! The first I heard about this experiment was around 2016 and it has haunted me ever since. For someone with zero academic background on quantum mechanics, all of the debunking videos I've watched only added more to my confusion. Thanks to you I've cleared my misconceptions.
This might be outside of the scope of this video, but one thing caught my mind. So far what I understand is that the beam splitter "decides" whether the photon goes into D1 or D2 based on their relative phase difference hence the interference pattern. And that a "fair" beam splitter would not results an interference pattern. But what's the mechanism behind this beam splitter? Why does the constructive interference go one way while the destructive goes the other way?
Great question! I think the Wikipedia article on beam splitters actually offers a great answer. To the point where I even included their diagram on phase shift in my next video.
en.m.wikipedia.org/wiki/Beam_splitter#Phase_shift
Here's the part of that video that might help with your question:
ua-cam.com/video/t_1jozBgXZE/v-deo.htmlsi=AxriZ78W5AmI_ffZ&t=397
so how come the wave doesn't collapse when it hits the space between the slits?
Think of the photoelectric effect happening on the plate. If the photon does not have the right frequency, it won’t have enough energy to interact with the plate. If it does, then some measurement will happen at the plate, we just don’t consider these as part of the experiment.
Thanks! Looks like the famous Quantum Eraser Experiment is a kind of failure. But what about retrocausality, demonstrated in John Wheeler's Delayed Choice thought experiment? Is it valid? According to Australian National University web article, they made a version of it, that was tested in real life and it works as described by Wheeler (retrocausality works). If this IBM Quantum Computer would be also able to simulate this experiment, and you, Diego, someday decide to make a video of it, that would be great. "At the quantum level, reality does not exist if you are not looking at it" (ANU Professor Andrew Truscott)
Thanks @andruss2001. The concept of Wheeler's delay experiment is pretty much identical to what we describe in here, just using photons from a source millions of light years away. Any article claiming that "retrocausality" has been demonstrated is demonstrably wrong :)
Hey everybody, can someone explain why the two branches of the wavefunction hitting D1 and D2 have opposite relative phases? In other words: why is there a plus at the top branch and a minus at the bottom branch? I mean, all ways to D1 and all ways to D2 are the same. I do not see any difference in the geometry of this experiment
Great question. This is because these beam splitters are designed to induce a 0 degree phase shift in the reflected beam coming from one direction, and a 90 degree phase shift on the beam coming from the other direction.
I realized I did not explain this after posting the video, so I added a short explanation in the next video. Sorry about that:
ua-cam.com/video/t_1jozBgXZE/v-deo.htmlsi=IDGyFStryCWbdK5X&t=397
You can also read more about this here:
en.wikipedia.org/wiki/Beam_splitter#Phase_shift
thats right. i watch that later?
Thanks for this, im so tired of people mystifying the post selection of data based on physical correlations
They aren't changing the past, they're changing the dataset they're looking at. The key to revealing the illusion seems to be recognizing that the "interference patterns" aren't identical
The same physical nature that determines where the photon lands on d0 is what determines if its entangled partner lands at d1 or d2. End states are correlated because the properties of the entangled pair are correlated
Yes! You are 💯% right!
Yes! This is completely true! But for some reason all of the other debunking videos say "they aren't changing the past, they're changing the dataset they're looking at", and then they say nothing else at all about why that dataset has any meaning! First they tell you the photons are randomly sorted into D1 or D2 with 50/50 probability, then they imply (without even stating it explicitly) that there is a correlation between the D1 vs D2 photons and D0 screen location. This, on its face, is nonsense without understanding that there is a "hidden variable" here that is actaully influencing how D1 vs D2 sorts each individual photon, and that was determined for both entangled photons when the first photon landed on the D0 screen.
Furthermore, they imply that it is corrlated in a way that recretes the double slit interference pattern without giving a clue as to why on earth this would be--or why there would be any interference pattern at all. Plenty of viewers seem happy to just trust the word of the presenters and call it a day. But for anyone who thinks through the logic of it (but doesn't have a deep understanding of all of the quantum processes and relationships involved), especially for people who understand layman's QM basics but lack that deeper understanding, it's just maddeningly incoherent, and feels no less magical than retrocausality would.
Now that I understand all of this thanks to this fabulous video and the generous willingness of the creator to answer some questions I had, it all seems pretty trivial, but it took me a LOT of work to get there! The bonus is that I feel like I now understand superposition a LOT better than I did before, and better understand how it is related to determination/measurment and entanglement too. Not to mention an improved understanding of the geometry of light waves and phase interaction, which I had previously understood well enough for purposes of the double slit experiment, but had glazed over enough of the details that I couldn't apply it to other contexts.
@@erinm9445 I think it's more accurate to say that the hidden variable is set at entangled pair production in the bbo, not by landing at d0. The exact mechanics of the pair production process determines both where one lands on d0 and which detector it's compliment arrives at.
I think one of the biggest misconceptions about this experiment and ones like it comes from the idea that the beam splitters operate by pure probability instead of acting in some physically mechanical manner. They're baking the magic thinking into the apparatus when really it's the state of the photon, its properties and trajectory in interacting with the splitter, that determines which path will be taken.
@@erinm9445 I still don't understand some of the details. For me it looks that the "hidden variable" is the phase information which is kept for each photon at D0 and it is associated with its entangled partner going to the beam splitter so that the probability of interference and hitting either D1 or D2 is determined by this x-axis position on the screen D0. If this is correct it explain a lot! However I still don't get how the coherence is lost at D0 and than magically it is achieved again at D1 and D2. I really lost this part. Another thing I am confused is that it is obvious that SPDC is a process where the initial wavefunction collapses as the act is discrete at specific atom, but on the other hand it looks like we continue to follow this wave function propagation as there was no collapse at all at BBO crystal.
So it doesnt change the past.
It reveals which branch of reality, which path, was taken.
I like that
Schrödinger's cat was always either alive or dead.