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How fast is quantum entanglement? Scientists investigate it at attosecond scale

38 points by bookofjoe 7 hours ago | 29 comments

Havoc 5 hours ago

I had assumed it is by definition instant?

Surely anything else would imply a mechanism with additional complexity and indirectly void the entire concept. Delayed entanglement effect doesn't quite have the same cleanness for lack of better word

bl0rg 5 hours ago

Reality doesn't necessarily agree with its model though, so it makes sense to verify?

nickpsecurity 3 hours ago

We model reality incorrectly all the time. A lot of math was wrong, too. So, it’s always good to test the models. In security, we also tested in normal, abnormal, and hostile environments.

contravariant 4 hours ago

Well you could imagine seeing a process that causes entanglement and just trying to halt this process somehow. Before a certain time you would simply see no entanglement at all and after some time you wouldn't see any effect on the entanglement. In between something might happen, but what exactly is a bit hard to tell.

None of this is beyond current models though, so I imagine you could predict the results quite well if the calculations were feasible. And if that can be done then that's probably exactly what they did.

mr_toad 3 hours ago

The simulations are investigating how long quantum entanglements take to form, not how long it takes for non-local action / pilot wave / wave function collapse (if you subscribe to these views of QM) to propagate.

(There is a theory in which non-local variables propagate at superluminal speeds, but not instantaneously, but this is a whole other matter).

deepfriedchokes 2 hours ago

Might I ask, what is this theory of superluminal non-local variables? I would love to read more about it.

signa11 2 hours ago

this somehow reminds me of the lovely mermin-peres-magic-square game, beautifully described by mr. greg-egan here : https://www.gregegan.net/SCIENCE/MPMS/MPMS.html

dr_dshiv 4 hours ago

Entanglement is like splitting a coin in half, so you have a tails and a head half. Then you put one in your left pocket and give the other to a friend.

Then, just by looking in your left pocket, you can learn that your friend has the heads half—instantly!

In other words, it’s not as spooky as it is made out to be.

tzs 4 hours ago

That's not correct. With entanglement you get correlations that you cannot get if the states of a pair of particles are determined at creation time.

There's a neat game called the CHSH game that illustrates this. Here's a description [1].

Here's a puzzle equivalent to CHSH but that might be easier for programmers to visualize [1].

[1] https://news.ycombinator.com/item?id=41393075

[2] https://news.ycombinator.com/item?id=35905284

drewcsillag 1 hour ago

First, I assume I’m missing some critical detail and am wrong somewhere.

Both the ERP, and the explanation of the CHSH with the difference being cos^2(theta) an isn’t that just Malus’s law? So in the case of the ERP experiment, if you fired single polarized particles at a polarizing filter at one angle or the other you still get cos^s(theta) as the difference without requiring entanglement, no?

That implies, in the case of entangled particles there is more than one dimension of “whatever” that causes the polarizing filter to “choose” whether to extinguish the particle on non-equal angles - like azimuth/elevation instead of just theta? It just seems to me that rather than disproving a “hidden variable”, it requires one?

Like I said, I assume I’m missing something and am wrong.

bawolff 2 hours ago

I mean the person you were responding to said "like". I think splitting a coin is a reasonable but not perfect analogy.

tzs 1 hour ago

The splitting a coin analogy doesn't capture anything about entanglement though. It is just capturing a property of all distinguishable persistent objects: if you have two distinguishable persistent objects and you know one is at location A and one is at location B, then if you find out which one is at A you know what the one at B must be.

To be a reasonable analogy it has to capture something this is different between entangled and not entangled particles. That's the thing that is sometimes described as "spooky" and completely missing from the split coin analogy.

bawolff 59 minutes ago

Its capturing that the two particles are correlated instead of transfering information. I feel like that is the core part to understand.

ziofill 4 hours ago

Classical analogies only go so far. What's spooky about entanglement is that it works even if you change the measurement basis (as long as the two parts are using the same basis), which you cannot do with a coin.

ozzydave 3 hours ago

Gotta love the confidence to casually think every physicist for the past 100 years is an idiot.

magicalhippo 2 hours ago

> In other words, it’s not as spooky as it is made out to be.

That's not really the spooky part of entanglement. The rabbit hole goes much deeper, like here[1] or here[2].

[1]: https://arxiv.org/abs/1203.4834

[2]: https://arxiv.org/abs/1209.4191