5 ms·
Isn't collapse of entanglement itself useful information or is it not possible to notice it on the entangled particle?
by 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 agoYeah, 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 agoYou 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 agoI 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 agoThis 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 agoAES 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 agoThis 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 agohttps://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 agoThank you for the link and explanation.
- MAMAMassakali 4y agoFrom 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 agoYou 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.