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.
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.
I don't think it has anything to do with what we know "now". It's just paying attention to the fact that the signal photon hitting the screen causes a collapse that affects the state of the idler photon. Which then explains the data via the collapsed state depending on the position of the hit, and one of the possible idler measurements being in a basis perpendicular to those variations. All quantum interpretations give the right answer for this experiment, and very few of them invoke retrocausation, therefore the experiment clearly doesn't require retrocausation.
I don't even think the delayed choice eraser is a "quantum" paradox. It involves quantum particles, but they're really just there for flair. They're not crucial. You can apply the same confusion to a classical experiment. Set up some basic correlation between A and B, with A revealed first and then a choice to reveal B or an unrelated C. Then describe the situation so badly that it sounds like choosing to measure B vs C is changing the probability distribution of A backwards in time (since if you condition on B you'll see the correlation vs A, but conditioning on C shows no correlation).
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.
Depends on your interpretation of quantum mechanics. In Bohmian Mechanics, there is a discrete particle guided by a wave described by the wave function. Also, macro discrete objects are not illusions, they're the result of decoherence. The superposition is suppressed from view, assuming the wave function isn't collapsed or just a mathematical prediction tool.
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.
I was fascinated by this experiment when I first learned about it. At one point I thought of a modified version where you let the 'B' go across a event horizon of a Black-hole. in this case there would be a clear before and after right? will DCQE still work across singularity boundary?
This is easy to picture if you imagine widely spreading out the equipment used for the eraser experiment. If the signal hitting the screen and idler hitting one of the detectors are space-like separated events... the OP's explanation no longer seems to apply.
The delayed choice experiment doesn't contain a bell inequality, so spacelike seperation doesn't really mean much here. You can reproduce the results with local classical models.
> 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.
This bakes in an assumption that collapse happens, which I don't believe everyone agrees with...
Sure sure, you can ignore that wording. The point is that the first measurement determines the state of the particle undergoing the second measurement.
The author talks about "two downconverted photons" each at half the energy, in that simple linear experiment. Is that mainstream physics? If so ... big ask, but do you happen to have a butt-simple reference at the undergrad QM level? It feels like I need more equipment than a fixed pair of slits to downconvert frequencies.
Yes, that's the main way we produce entangled photons. You put in a photon; you get out two photons, each with half the energy. To follow conservation laws, they must have complementary values of things like polarization: if one is up-down polarized, the other must be left-right polarized.
Those values are linked: if you measure one, the other will have the opposite. You can tell that it's not just pre-set values by measuring at a 45 degree angle, so you get some up-down and some left-right in each measurement. Take a bunch of those, and you'll see that the expected correlation between your measurements follows what quantum mechanics predicts, and not classical mechanics.
It's usually done with beta barium borate. You can buy it at optical suppliers:
A time reversed photon is still a photon, and as such photons in Feynman diagrams aren't given a direction. They're equally valid to view traveling forward or backward in time.
This is as opposed to an electron, which is given a direction, because reversing it in time produces an anti-electron.
Not in the science-fiction sense. It's just a convenient way to express anti particles in the diagrams but the travelling back in time should not be taken literally.
They do. In fact i always thought this was the answer to the matter/antimatter imbalance.
Feynmann diagrams literally show anti matter as the same particle as a matter particle, just travelling back in time (see election/positron interactions).
So what happens when matter and antimatter are created in a big boom? Well the antimatter is in the past, we're here in the future.
Not sure how we could ever prove something like that but it's certainly an amusing and symmetrical view of the universe: big bangs create two universes, moving in opposite time directions from each other. Each seeing the other's particles as anti-particles.
Thinking about it though: photos are their own antiparticle. So I'd expect to see a lot more cosmic microwave background than we should because at least in the early days the antimatter universe would have been visible to us?
Side note: how can photos be their own antiparticle? Same reason they move at the speed of causality. They have no mass thus do not experience the flow of time themselves. So they do not annihilate with themselves. From a photon's POV a trip across the universe is instant.
Yes they do and that is the secret to the wave-particle duality manifested in the double-slit experiment. Information from future completely illuminates (pi) the paradox.
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.
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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
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.
I don't even think the delayed choice eraser is a "quantum" paradox. It involves quantum particles, but they're really just there for flair. They're not crucial. You can apply the same confusion to a classical experiment. Set up some basic correlation between A and B, with A revealed first and then a choice to reveal B or an unrelated C. Then describe the situation so badly that it sounds like choosing to measure B vs C is changing the probability distribution of A backwards in time (since if you condition on B you'll see the correlation vs A, but conditioning on C shows no correlation).
Deleted Comment
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.
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.
https://physics.stackexchange.com/questions/318967/can-bells...
Does that mean quantum calculations are just a fancy way of describing correlated probabilities, and have nothing to do with spooky action?
This is easy to picture if you imagine widely spreading out the equipment used for the eraser experiment. If the signal hitting the screen and idler hitting one of the detectors are space-like separated events... the OP's explanation no longer seems to apply.
This bakes in an assumption that collapse happens, which I don't believe everyone agrees with...
Those values are linked: if you measure one, the other will have the opposite. You can tell that it's not just pre-set values by measuring at a 45 degree angle, so you get some up-down and some left-right in each measurement. Take a bunch of those, and you'll see that the expected correlation between your measurements follows what quantum mechanics predicts, and not classical mechanics.
It's usually done with beta barium borate. You can buy it at optical suppliers:
https://eksmaoptics.com/nonlinear-and-laser-crystals/nonline...
Physics Videos by Eugene Khutoryansky: Delayed Choice Quantum Eraser - Quantum Physics https://www.youtube.com/watch?v=SzAQ36b9dzs (26m31s) [2015-07-16]
This is as opposed to an electron, which is given a direction, because reversing it in time produces an anti-electron.
Feynmann diagrams literally show anti matter as the same particle as a matter particle, just travelling back in time (see election/positron interactions).
So what happens when matter and antimatter are created in a big boom? Well the antimatter is in the past, we're here in the future.
Thinking about it though: photos are their own antiparticle. So I'd expect to see a lot more cosmic microwave background than we should because at least in the early days the antimatter universe would have been visible to us?
Side note: how can photos be their own antiparticle? Same reason they move at the speed of causality. They have no mass thus do not experience the flow of time themselves. So they do not annihilate with themselves. From a photon's POV a trip across the universe is instant.
Either way, the article does just fine elucidating the delayed-choice quantum eraser without quantum field theory.
> 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.
I like to give people the benefit of the doubt, can anyone speak to his credibility on this topic?