I appreciate this article, as I agree with the author that the delayed-choice quantum eraser is a misnomer due to ignoring what we now know of quantum states. It's really frustrating learning modern quantum mechanics but then reading about the delayed-choice quantum eraser making conclusions from an older understanding.
However, I still haven't seen anyone do the math about it. It shouldn't be too hard to keep track of a photon's state through Kim et al.'s experiment, and I think it would be clearer than relying on words alone (as done by the author here). I have attempted this myself, but I am particularly terrible at quantum optics. If anyone has seen such a derivation before please let me know.
Our understanding of the world is overfit to the macro level, where we project concepts onto experience to create the illusion of discrete objects, which is evolutionally beneficial.
However, at the quantum level, identity is not bound to space or time. When you split a photon into an entangled pair, those "two" photons are still identical. It's a bit like slicing a flatworm into two parts, which then yields (we think) two separate new flatworms... but they're actually still the same flatworm.
Experiments like this are surprising precisely because they break our assumption that identity is bound to a discrete object, which is located at a single space, at a single time.
Quantum physicist here. My PhD back in the day was about the entanglement between downconverted photons. I've thought about this more than I like to admit.
While I appreciate the blog post, it seems a bit disingenuous. I hope everyone understand that if you take two entangled photons A and B and detect A before B, then the outcome of the measurement of B must depend on the outcome of the earlier measurement of A, because measuring A causes the collapse of the joint state and determines the wavefunction of B undergoing the later measurement.
The MAGIC about delayed choice measurements is that they work even when the temporal order is UNDETERMINED. By this I mean that the two measurements of A and B can be set up to occur so close in time to each other that there is no time for a signal travelling at the speed of light to travel between the two events. Under this condition, you can witness both orderings (A measured before B and B measured before A) just by changing your reference frame. Under these conditions, the delayed choice experiment STILL WORKS!
In this case, there cannot be any argument like "but the idler was measured first", because "first" does not make any sense.
A highly visual explanation of the same topic:
Physics Videos by Eugene Khutoryansky: Delayed Choice Quantum Eraser - Quantum Physics https://www.youtube.com/watch?v=SzAQ36b9dzs (26m31s) [2015-07-16]
I am I crazy or do Feynman diagrams not explicitly show interactions of particles moving back in time?
PBS Spacetime did an interesting video on DCQE, but it tripped me up trying to fully understand what was happening: https://www.youtube.com/watch?v=8ORLN_KwAgs&t=601s ... Later Sabine Hossenfelder did a video debunking the proposition that DCQE somehow showed that the past was being rewritten. https://www.youtube.com/watch?v=RQv5CVELG3U And Matt from PBS Spacetime acknowledged she was right in this respectful comment:
> Sabine, this is amazing. You are, as usual, 100% right. The delayed choice quantum eraser is a prime example of over-mystification of quantum mechanics, even WITHIN the field of quantum mechanics! I (Matt) was guilty of embracing the quantum woo in that episode 5 years ago. Since then I've obsessed over this family of experiments and my thinking shifted quite a bit.
Apparently the author writes picture poetry books and has no degree in physics.
I like to give people the benefit of the doubt, can anyone speak to his credibility on this topic?
I did some actual research in making a delayed choice quantum erasure system and the TLDR of why any of these "time travel" results is that what you have to keep in mind is that it is that the entanglement you create you have to still use a classical channel of information to transmit the information as a result which is still bound the speed of light which is what the quantum erasure systems must also show that it doesn't occur.
I always enjoy reading about quantum optics, and the concept of entanglement is mind-blowing (I mean after all, no less than Albert Einstein went to his grave thinking we must be missing something in the theory.)
However, the quantum erasure experiments are really just a variation on other Bell Violation experiments (which also appear to violate causality at first glance.) At their heart the wave functions appear to say that particles are in multiple locations at the same time (so "touching" one particle must affect the state of the other particle instantly.) This information, though, isn't useful until all the results are brought back to one location (i.e., all the strangeness is buried in _some combination_ of the lists of results in the different locations, which can only be combined in a way that obeys causality.) Each list of results by itself looks random. So in this sense, "when" exactly you get your result won't tell you anything.
So no, you can't retake that picture you really wanted but flubbed on your last vacation.
It would be nice if photons from the past could be captured to take pictures of millennia ago.
No, and Yes, because there is no "back in time", nor "forward in time". Time is just a useful illusion we create to navigate space.
This idea is set up on a false premise.
But I was extremely happy to read; "There’s no such thing as wave-particle duality" "Light only ever travels as a wave".
Everything is only fundamentally a wave.
Please take a look at "The end of time : the next revolution in physics" by Julian Barbour. Or here are some YouTube videos:
https://www.youtube.com/watch?v=K49rmobsPcY
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I literally posted about this topic a few days ago! But for some reason it was flagged as a “Dupe” and buried by someone:
https://news.ycombinator.com/item?id=43173195
It links here:
https://chatgpt.com/share/67bde29f-a56c-800a-8e26-44a5a3ad23...
I will summarize by saying that I think our current understanding of Faster-than-Light communication is wrong, and the no-go theorem about no information transmission faster than light will be debunked (in very specific but slight ways I describe in the link) soon as quantum error-correction gets better. Before you say it’s preposterous, skim the above chat, maybe looking at my side of it for instance. This is an interesting format I often share ideas in these days.
This is a very interesting, novel take on explaining the delayed choice quantum eraser. I think I can summarize it as follows:
The signal photon hits the screen, which is a measurement. The entangled idler's wave function is thereby constrained by that measurement, influencing the probabilities of later detecting it at each of the D1-4 detectors. It's not that the fate/erasing of the idler changes the already committed path(s) of the signal. It's that the measurement of the signal constrains the subsequent detection of the idler.
Two other high quality discussions of eraser experiments are by Sean Caroll [0] and Sabine Hossenfelder [1]. Like the OP, both Sabine and Sean demystify/debunk these experiments.
These three discussions all use different language to explain the outcome, which is clearly predicted by the QM math. Sean's article includes the gist of the math.
[0] https://www.preposterousuniverse.com/blog/2019/09/21/the-not...
[1] https://www.youtube.com/watch?v=RQv5CVELG3U