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Time Warp: Delayed-choice quantum erasure

drgblackwell.substack.com

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Re: Time Warp: Delayed-choice quantum erasure

#61

Post by latenightcoding »

wasabi991011 wrote: post #17

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…

To be fair, almost everything in quantum is poorly named. That's how they attract funding.

Re: Time Warp: Delayed-choice quantum erasure

#62

Post by calny »

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.

Re: Time Warp: Delayed-choice quantum erasure

#63

Post by airstrike »

latenightcoding wrote: post #61
wasabi991011 wrote: post #17

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…

To be fair, almost everything in quantum is poorly named. That's how they attract funding.

Except for "quantum supremacy", which is the best name in the entire multiverse.

Re: Time Warp: Delayed-choice quantum erasure

#64

Post by DennisP »

nyeah wrote: post #23
yubblegum wrote: post #20

Earlier quoted context omitted.

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.

No it isn't. Please stop.

I don't know if this is what GP was talking about but Cramer's transactional interpretation of quantum mechanics has wave functions moving both forward and backward in time.

https://en.wikipedia.org/wiki/Transactional_interpretation

Re: Time Warp: Delayed-choice quantum erasure

#65

Post by goatlover »

wnmurphy wrote: post #50

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 thin…

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.

Re: Time Warp: Delayed-choice quantum erasure

#66

Post by jfengel »

nyeah wrote: post #57
ziofill wrote: post #40

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, bec…

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:

https://eksmaoptics.com/nonlinear-and-laser-crystals/nonline...

Re: Time Warp: Delayed-choice quantum erasure

#67

Post by [deleted] »

jfengel wrote: post #66
nyeah wrote: post #57

Earlier quoted context omitted.

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…

[deleted]

Re: Time Warp: Delayed-choice quantum erasure

#68

Post by nyeah »

jfengel wrote: post #66
nyeah wrote: post #57

Earlier quoted context omitted.

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…

Thanks. Somehow I thought the author was trying to explain a simple linear two-slit experiment using two photons, each with half the energy. I think it was a reading problem on my end.

Re: Time Warp: Delayed-choice quantum erasure

#69

Post by anotherpaulg »

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

Re: Time Warp: Delayed-choice quantum erasure

#70

Post by anotherpaulg »

ziofill wrote: post #40

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, bec…

Good point!

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.

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