What I don't quite get about entanglement...

[Fra]

It is possible to entangle distant photons after they are created. Prior to their creation, they have no relationship or correlation whatsoever. So the correlation you mention cannot be predetermined, even as a relationship and not specific values.

Agreed. But this is not a problem to what I tried to write.

In the more composite experiments of entanglement swapping, we simply combine things to make it more complex, but the the conceptual issues are just thge same imo. I am one of those that thinkg that this experiment does not add much conceptually new. To make the entanglement swapping we need

(1) different original entangled pairs
(2) the key swapping event; which I understand as a special type of controlled interaction, where
(3) filtering the data using information from the key swapping measurments that is classically communicated

This way, yes, systems that has previously no local contact, are entangled. For this is not a mystery per see, because entanglment has to do with information, it's not some invisible "material link" between remote objects. But the understnading of how this is possible, for me at least, clearly depends on the original entangled pairs. Without them, the "construction" of the new entangled pairs would not be possible to carry out. All the details has been debated alreday in other threads. From my perspective, these "new" experiments are great and emphasise a lot of things, but adds nothing fundamentally new to my understanding.

Also, I’m not sure how you can factor in the environment in outcomes. Some elements of the environment do interact with photons, but don’t have any observable effect. Other elements of the environment, such as a polarizing beam splitter, do have an effect. But it is difficult to envision that interaction as determining the outcome due to the measurement apparatus. After all, Alice and Bob are distant and the environment always cancels out. We know that because of perfect correlations.

This is the interesting part! But also the part where I would say that we have incomplete understanding, which is a good start.

And it would be inappropriate to attempt to speculate here as it will open a can of worms we can't control. So I just settled with acknowledging this as a good question. Let's hope that published papers start to come that elaborate these things, and we can start to discuss also that.

/Fredrik

 

[Fra]

Also, I’m not sure how you can factor in the environment in outcomes. Some elements of the environment do interact with photons, but don’t have any observable effect. Other elements of the environment, such as a polarizing beam splitter, do have an effect. But it is difficult to envision that interaction as determining the outcome due to the measurement apparatus.

Not even close to an answer, but at least an interesting paper asking some small questions of how environmental noise can actively support entanglment, in that in minimizes decoherence probability.

Noise induced entanglement
"...We describe how the entanglement between the atoms arise in such a situation, and wether a noise except the white one could help preparation of entanglement."
-- https://arxiv.org/abs/quant-ph/0309202

This for me spawns or suggest many new interesting deeper foundational questions.

/Fredrik

 

[PeterDonis]

This for me spawns or suggest many new interesting deeper foundational questions.

Please note that this thread is not in the interpretations and foundations subforum, so "foundational questions" are off topic here. If you want to discuss those you need to start a new thread in that subforum.