4 ms·

GP and all sibling comments are unfortunately incorrect. The measurement of one _does_ instantaneously affect the other. No useful information can be transmitt

GP and all sibling comments are unfortunately incorrect. The measurement of one _does_ instantaneously affect the other. No useful information can be transmitted, since the outcomes are random, but that does not falsify the original statement. In fact "affect the other" has no implication of "information is transferred" - someone would need to be an expert on classical information theory and simultaneously a novice of quantum information theory to draw that conclusion.

tauwauwau 4y ago
Isn't collapse of entanglement itself useful information or is it not possible to notice it on the entangled particle?
- MAMAMassakali 4y ago
  Yeah, I too want to know this. If we know that the collapse has happened, then it can be used to infer that someone has tried to measure the other entangled particle. Can this be be leveraged or my understanding is wrong?
- frutiger 4y ago
  You don’t know the collapse happened. Locally you can just perform a measurement and you will concretely see one of the possible outcomes.
  - spiddy 4y ago
    I had an understanding that quantum entanglement can be used to for encryption and guaranty there are no man-in-the-middle attacks by making sure the entanglement is not broken. if we can’t know the collapse happened how can we make such claim (mitm can meassure stolen briefcase put it back and no one would know)
    - fsh 4y ago
      This is incorrect. Quantum key distribution ("quantum cryptography" is a misnomer) is vulnerable to man-in-the-middle attacks. Security relies on having a pre-shared key which makes it essentially useless in practice (just use AES if you have a PSK).
      - willis936 4y ago
        AES 256 only requires millions of qubits to crack. What do we do once we think that's attainable behind close doors? It'd be better to be prepared. https://www.fierceelectronics.com/electronics/aes-256-joins-quantum-resistance https://www.fierceelectronics.com/electronics/aes-256-joins-...
        fsh 4y ago
        This is a wildly optimistic estimate. Even if we had an error corrected quantum computer that could evaluate an AES key (complete science fiction for the forseeable future), running Grover's algorithm would take millenia assuming extremely fast gate times (1 ps). In any case, doubling the key length would be infinitely simpler than using QKD.
- thorum 4y ago
  https://www.askamathematician.com/2018/03/q-with-entangled-particles-can-you-tell-when-how-the-other-particle-is-measured/ https://www.askamathematician.com/2018/03/q-with-entangled-p... > Tricks that use entanglement to instantly send information generally involve collapsing the state of the distant particle (by measuring your own), and assuming that the person in charge of that particle will somehow notice. Unfortunately (and this is the answer), there’s no “new-particle-smell” for particles that are still in superpositions and there’s no big flash when that superposition collapses. If you measure an electron’s spin the result is simply “spin up” or “spin down”. That’s all you get. But that doesn’t tell you what the state was before, or if the original state was a superposition of several states, or even if the particle was entangled with something else. > If you only have access to one particle, then the two situations, 1) the other particle is inaccessible or 2) it has been quietly measured, are indistinguishable in every physical sense. Regardless of whatever else is involved, you can never notice the weirdness of entangled pairs until you directly compare them with each other.
  - tauwauwau 4y ago
    Thank you for the link and explanation.
  - MAMAMassakali 4y ago
    From the complementary states of the entangled particles we know entanglement happens. But then how do we know that the collapse and "de-entaglement" happens instantaneously if we can't determine whether superposition collapsed or not?
    - Jweb_Guru 4y ago
      You can't determine it at the exact moment it happens, but you can meet up later (traveling well below the lightspeed limit) and compare notes.
nlitened 4y ago
What does “instantaneously affect the other” mean when there’s no absolute simultaneity? https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity “Instantaneously” kind of means that one event does not happen before or after the other, but something happening before or after should depend on the reference frame, right?
- Winsaucerer 4y ago
  My recollection when I was once studying physics and looking into this is that some interpretations of QM do entail or allow for absolute simultaneity. Some do not, such as the prevalent Copenhagen interpretation. This article from 2009 discusses these kinds of matters, and near the end it talks about absolute simultaneity: "The kind of nonlocality one encounters in quantum mechanics seems to call for an absolute simultaneity, which would pose a very real and ominous threat to special relativity." https://www.scientificamerican.com/article/was-einstein-wrong-about-relativity/ https://www.scientificamerican.com/article/was-einstein-wron...

tauwauwau 4y ago

Isn't collapse of entanglement itself useful information or is it not possible to notice it on the entangled particle?

MAMAMassakali 4y ago
Yeah, I too want to know this. If we know that the collapse has happened, then it can be used to infer that someone has tried to measure the other entangled particle. Can this be be leveraged or my understanding is wrong?
frutiger 4y ago
You don’t know the collapse happened. Locally you can just perform a measurement and you will concretely see one of the possible outcomes.
- spiddy 4y ago
  I had an understanding that quantum entanglement can be used to for encryption and guaranty there are no man-in-the-middle attacks by making sure the entanglement is not broken. if we can’t know the collapse happened how can we make such claim (mitm can meassure stolen briefcase put it back and no one would know)
  - fsh 4y ago
    This is incorrect. Quantum key distribution ("quantum cryptography" is a misnomer) is vulnerable to man-in-the-middle attacks. Security relies on having a pre-shared key which makes it essentially useless in practice (just use AES if you have a PSK).
    - willis936 4y ago
      AES 256 only requires millions of qubits to crack. What do we do once we think that's attainable behind close doors? It'd be better to be prepared. https://www.fierceelectronics.com/electronics/aes-256-joins-quantum-resistance https://www.fierceelectronics.com/electronics/aes-256-joins-...
      - fsh 4y ago
        This is a wildly optimistic estimate. Even if we had an error corrected quantum computer that could evaluate an AES key (complete science fiction for the forseeable future), running Grover's algorithm would take millenia assuming extremely fast gate times (1 ps). In any case, doubling the key length would be infinitely simpler than using QKD.
thorum 4y ago
https://www.askamathematician.com/2018/03/q-with-entangled-particles-can-you-tell-when-how-the-other-particle-is-measured/ https://www.askamathematician.com/2018/03/q-with-entangled-p... > Tricks that use entanglement to instantly send information generally involve collapsing the state of the distant particle (by measuring your own), and assuming that the person in charge of that particle will somehow notice. Unfortunately (and this is the answer), there’s no “new-particle-smell” for particles that are still in superpositions and there’s no big flash when that superposition collapses. If you measure an electron’s spin the result is simply “spin up” or “spin down”. That’s all you get. But that doesn’t tell you what the state was before, or if the original state was a superposition of several states, or even if the particle was entangled with something else. > If you only have access to one particle, then the two situations, 1) the other particle is inaccessible or 2) it has been quietly measured, are indistinguishable in every physical sense. Regardless of whatever else is involved, you can never notice the weirdness of entangled pairs until you directly compare them with each other.
- tauwauwau 4y ago
  Thank you for the link and explanation.
- MAMAMassakali 4y ago
  From the complementary states of the entangled particles we know entanglement happens. But then how do we know that the collapse and "de-entaglement" happens instantaneously if we can't determine whether superposition collapsed or not?
  - Jweb_Guru 4y ago
    You can't determine it at the exact moment it happens, but you can meet up later (traveling well below the lightspeed limit) and compare notes.

nlitened 4y ago

What does “instantaneously affect the other” mean when there’s no absolute simultaneity? https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity “Instantaneously” kind of means that one event does not happen before or after the other, but something happening before or after should depend on the reference frame, right?

Winsaucerer 4y ago
My recollection when I was once studying physics and looking into this is that some interpretations of QM do entail or allow for absolute simultaneity. Some do not, such as the prevalent Copenhagen interpretation. This article from 2009 discusses these kinds of matters, and near the end it talks about absolute simultaneity: "The kind of nonlocality one encounters in quantum mechanics seems to call for an absolute simultaneity, which would pose a very real and ominous threat to special relativity." https://www.scientificamerican.com/article/was-einstein-wrong-about-relativity/ https://www.scientificamerican.com/article/was-einstein-wron...