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in his popular (pseudo-science) book Biocentrism ch.8 (see here, scroll down to "the most amazing experiment") Biologist Dr.Robert Lanza makes a number of assertions on quantum mechanics based on double slit experiments:

  1. the simplest case, if one passes a beam of photons through a double slit, he will get an interference pattern at the end detector. This one is obvious.

  2. If one adds quarter wave plates before each slit and a polarization detector at the end plate in such a way that he now has "which way" information on which slit each photon passed through, the wave function collapses and he will no longer see an interference pattern at the end plate. (as a side point, he claims that removal of the polarizing detector while keeping the QWPs will bring back the interference pattern. This was pointed out to be incorrect here)

enter image description here

  1. if one repeats step 2 except that he uses entangled photons with photon S going to exactly the same setup as step 2 and photon P towards an end plate but with a coincidence counter and polarization window which garbles photon P's polarization in such a way that erases the "which way" information obtainable for photon S, then here too the interference pattern shows up at the end plate of photon S (even though photon S has the exact same setup as in step 2 except for the entangled photon P). This he claims is because photon P erased the "which way" information obtainable for photon S.

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  2. Same as 3 except the length of photon P's path is lengthened so that Photon S hits its detector before Photon P gets garbled. here too the interference pattern is restored despite that photon P's erasing of "which way" information occurs after photon S hit its detector.

Are any of these assertions incorrect? for more details see above link.

From what I understand, they are all basically saying the same thing: if there is no "which way" information obtainable then the interference pattern shows up at the end detector, otherwise the wave function collapses and no interference pattern shows up.

please source with verifiable sources such as actual experiments.

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  • $\begingroup$ related: physics.stackexchange.com/q/495409 $\endgroup$ – michael Aug 6 '19 at 10:45
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Chris♦ Aug 6 '19 at 15:32
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    $\begingroup$ The OP is not asking about the clearly non-mainstream stuff in Biocentrism. He just wants to verify if Lanza is correct about the double-slit stuff, or if he's messed that up. See the chat for details. $\endgroup$ – PM 2Ring Aug 9 '19 at 13:04
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It seems an answer might have been given in the comments, but I will try to add a bit. When it comes to "which way" information and interference patterns, I find the most clear and instructive example to be the delayed choice quantum eraser experiment. It sounds similar to the case you describe in points 4 and 5.

enter image description here Diagram from Kim et al's paper.

In its most schematic form, it involves the emission of two entangled photons $\alpha$ and $\beta$ from one of two possible atoms. Photon $\alpha$ passes through a series of beam splitters until it meets a detector which belongs to one of two classes: Class 1 detectors ($D_3$ and $D_4$ in the picture) unambiguously shows which atom emitted the photon, class 2 ($D_1$ and $D_2$ in the picture) detectors can be activated by photons emitted by either atom. A detection by a class 2 detector is the same as erasure of the "which way" information of which atom emitted the photon. Photon $\beta$ is emitted towards what is essentially a photosensitive plate ($D_0$ in the picture) and has no information about which atom emitted it.

The result quite clearly demonstrates that when "which way" information has been completely erased (that is, for the cases where there was a detection by a class 2 detector) then interference patterns occur on the photo plate due to the superposition of photon $\beta$ coming from either atom. Interestingly, the interference pattern remains no matter if photon $\alpha$ or $\beta$ are detected first.

However, the general consensus in physics is that there is nothing mysterious about this, so beware before you take it as proof of anything crazier than just good old quantum mechanics.

Edit: What is the role of the order in which the photons are detected? Can we see an interference pattern in $D_0$ and thus predict which of the two classes of detectors we will detect the other photon in?

No. Here's a nice experiment where the two events are happening outside each others light cones, and so are completely causally disconnected. Thus it does not matter whether one or the other is done first with times separated by years - the two measurements do not actually influence each other. The thing is, the $D_0$ result is a mix of intereference and non-interference. You need to compare the $D_0$ results with the results of the other detectors to separate out the interference pattern. Thus you cannot use one measurement to predict the outcome of the other.

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  • $\begingroup$ good old quantum mechanics is crazy $\endgroup$ – michael Aug 7 '19 at 21:08
  • $\begingroup$ If I understand your question in its current form, the experiment I mention shows that assertion 3 and 4 are correct in that an entangled photon can erase the which-way information of its "partner", and the order in which this happens is irrelevant. And yes, quantum mechanics is indeed crazy, but at the same time pretty well-understood in a physical sense, so beware of those who would use it to argue for non-mainstream ideas. $\endgroup$ – Codename 47 Aug 8 '19 at 12:11
  • $\begingroup$ right. what if the second entangled photon was delayed for a few seconds. would the interference pattern still show up ahead of time? $\endgroup$ – michael Aug 8 '19 at 17:01
  • $\begingroup$ That's where the "delayed" part comes in. It does not matter which photon is detected when. But please not that the interference pattern cannot be explicitly found until both photons are detected. I recommend checking out the wikipedia article and reading some papers on it to fully grasp this. $\endgroup$ – Codename 47 Aug 9 '19 at 8:57
  • $\begingroup$ I read that but it's only a few nanoseconds delay. I'm asking where there are seconds delay. what will show up on the first photon's screen during those seconds. nothing? or perhaps a non-interference patttern which switches? $\endgroup$ – michael Aug 9 '19 at 10:34
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Assertion 2b is incorrect. It does not matter whether the polarisation is detected or not. If the the two slots pass orthogonal polarisation only, there is no interference.

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  • $\begingroup$ i removed that point as it seems to be a non-essential side point and you answered this already in the linked question. are all the other points correct? $\endgroup$ – michael Aug 6 '19 at 20:58

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