4 ms·

When you measure the spin of a particle, you can only get 2 possible answers, up or down. The up or down is with respect to the angle you measure the spin. You

When you measure the spin of a particle, you can only get 2 possible answers, up or down. The up or down is with respect to the angle you measure the spin. You could measure it with respect to the z direction, the x direction, half way in between, etc. The important part about entangled particles is if you choose to measure two entangled particles in the same direction you will always get the opposite answer. Note that once you do such a measurement, the particles are no longer entangled and you're gonna need another timmy. Teleporting the quantum state involves transferring the entanglement of a particle here to a particle over there. This is done by using another pair of entangled particles. It's not possible to just transmit quantum state by purely classical means. I'll also note that the "transmitted instantly" part depends on your interpretation of quantum physics.

umvi 7y ago
> I'll also note that the "transmitted instantly" part depends on your interpretation of quantum physics. Ok, the way I interpret it is that entangled particles are like 2 hard boiled eggs that have collided. It is well known that if you roll 2 hardboiled eggs into each other, only one will crack. So basically, if I roll two hardboiled eggs into each other and then hide them in paper bags, the eggs are now "entangled" and if I separate the bags by 1 million miles, I can observe the state of my egg and know instantly what the state of the other egg is. For example, if I peer into my bag and see my egg is not cracked, I know that the other egg is indeed cracked! However, if I change the state of my egg (crack it) then obviously nothing happens to the other egg because the eggs are no longer "entangled". There is no "instant transmission" happening here. All the properties of the eggs were set in stone the moment the eggs collided and thus it doesn't matter how far away you move them because the "entanglement" (collision) event already happened. This is not a very useful property of hardboiled eggs and so I fail to see how this could be useful by scaling down the eggs to photons.
- syrrim 7y ago
  That's the 'local hidden variable' model that was disproven by Bell in the sixties.
  - zzo38computer 7y ago
    Will the "backward in time" model work? If two experimenters far away from each other make the same measurement and have the opposite answer, then they are, from the point of view from each other, backward in time to the source and then forward to the other one, in the other direction.
    - russdill 7y ago
      This is very similar to the pilot wave interpretation of quantum mechanics.
      - zzo38computer 7y ago
        How is it similar? I tried to learn but do not quite understand how this is similar.
        russdill 7y ago
        Maybe I'm misunderstanding what you are stating.
        zzo38computer 7y ago
        Yes, I probably wrote it not very clearly, but I don't know how to write it more clearly.
  - umvi 7y ago
    Yeah, and I watched Veritasium's video on it, but I still don't really buy "spooky action at a distance". The way I understand it, particles have an "absolute orientation" in space that is impossible to determine because currently we are limited to measuring the orientation relative to another orientation and getting a yes/no probabilistic outcome (also, we can only perform this measurement once because the act of measurement changes the orientation of the particle). Nevertheless, in reality the particle has an absolute, no-guesswork spin orientation property that a theoretical godlike being running the universe simulation would be able to observe in a "read only" fashion. (We mortals have to use probability abstractions because we don't have sophisticated enough methods of getting quantum information in a read-only fashion) So basically, when you "entangle" two particles, the act of entanglement affixes their absolute spin orientation to be polar opposites. Bell's theorem doesn't disprove that, afaik, and it accounts for the "spooky action at a distance".
ksaj 7y ago
I think of it this (probably wrong) way: If a particle is sent to me, and its pair - for the sake of argument, the matching photon pair that comes from one beam of light through a prism, is sent to you, they will be continue to spin in exactly the same way since they started the exact same-but-mirrored way. So if we each look at the photon at EXACTLY the same time, and nothing has slowed down either photon's travel, the spins will match. In order for us to see it, we have to observe it - and that takes some sort of sensor that is definitely slower than the speed of light (as was the photon's travel through the prism). So the moment we see them, they'll match, but unless we know the EXACT way the spin was altered by both sensors and reconstruct that, anything beyond the initial spin we caught is forever unrelated to the spin of the other photon. Likewise an eavesdropper would have to alter one of the photon's spin in order for it to reach their eavesdropping sensor, and at that moment, it is useless. Same goes for the true recipient, because the photon passing through the eavesdropping prism changed the spin simply by reflecting it away from its original polarization, and so the "key pair" will no longer match. Think about how networking works. If your stream of photons is treated essentially like binary (but better), then your packets will all end with a checksum of sorts. If the packet contained an error, the checksum won't match, and you resend the packet so eventually the message gets there. It's not a very "spooky" factoid when you look at it purely mechanically. I believe this is also why we can coax out quantum behavior from water, electricity, and other things that move around easily. We can even demonstrate quantum weirdness using 3 polarized lenses like you get from the cinema. The randomness you can envision with all this travel and bouncing and sensing adds up to a lot of noise, making it an expensive problem to solve before things get practical. Early IP networks were pretty crappy. Look at how they go now! The information is light speed. The reading of it is simultaneous. Splitting one of the protons into another pair leaves you with 3 unrelated protons that are not read at the exact same moment (a little simplified - there is math involved to calculate when is that EXACT moment).

umvi 7y ago

> I'll also note that the "transmitted instantly" part depends on your interpretation of quantum physics. Ok, the way I interpret it is that entangled particles are like 2 hard boiled eggs that have collided. It is well known that if you roll 2 hardboiled eggs into each other, only one will crack. So basically, if I roll two hardboiled eggs into each other and then hide them in paper bags, the eggs are now "entangled" and if I separate the bags by 1 million miles, I can observe the state of my egg and know instantly what the state of the other egg is. For example, if I peer into my bag and see my egg is not cracked, I know that the other egg is indeed cracked! However, if I change the state of my egg (crack it) then obviously nothing happens to the other egg because the eggs are no longer "entangled". There is no "instant transmission" happening here. All the properties of the eggs were set in stone the moment the eggs collided and thus it doesn't matter how far away you move them because the "entanglement" (collision) event already happened. This is not a very useful property of hardboiled eggs and so I fail to see how this could be useful by scaling down the eggs to photons.

syrrim 7y ago
That's the 'local hidden variable' model that was disproven by Bell in the sixties.
- zzo38computer 7y ago
  Will the "backward in time" model work? If two experimenters far away from each other make the same measurement and have the opposite answer, then they are, from the point of view from each other, backward in time to the source and then forward to the other one, in the other direction.
  - russdill 7y ago
    This is very similar to the pilot wave interpretation of quantum mechanics.
    - zzo38computer 7y ago
      How is it similar? I tried to learn but do not quite understand how this is similar.
      - russdill 7y ago
        Maybe I'm misunderstanding what you are stating.
        zzo38computer 7y ago
        Yes, I probably wrote it not very clearly, but I don't know how to write it more clearly.
- umvi 7y ago
  Yeah, and I watched Veritasium's video on it, but I still don't really buy "spooky action at a distance". The way I understand it, particles have an "absolute orientation" in space that is impossible to determine because currently we are limited to measuring the orientation relative to another orientation and getting a yes/no probabilistic outcome (also, we can only perform this measurement once because the act of measurement changes the orientation of the particle). Nevertheless, in reality the particle has an absolute, no-guesswork spin orientation property that a theoretical godlike being running the universe simulation would be able to observe in a "read only" fashion. (We mortals have to use probability abstractions because we don't have sophisticated enough methods of getting quantum information in a read-only fashion) So basically, when you "entangle" two particles, the act of entanglement affixes their absolute spin orientation to be polar opposites. Bell's theorem doesn't disprove that, afaik, and it accounts for the "spooky action at a distance".

ksaj 7y ago

I think of it this (probably wrong) way: If a particle is sent to me, and its pair - for the sake of argument, the matching photon pair that comes from one beam of light through a prism, is sent to you, they will be continue to spin in exactly the same way since they started the exact same-but-mirrored way. So if we each look at the photon at EXACTLY the same time, and nothing has slowed down either photon's travel, the spins will match. In order for us to see it, we have to observe it - and that takes some sort of sensor that is definitely slower than the speed of light (as was the photon's travel through the prism). So the moment we see them, they'll match, but unless we know the EXACT way the spin was altered by both sensors and reconstruct that, anything beyond the initial spin we caught is forever unrelated to the spin of the other photon. Likewise an eavesdropper would have to alter one of the photon's spin in order for it to reach their eavesdropping sensor, and at that moment, it is useless. Same goes for the true recipient, because the photon passing through the eavesdropping prism changed the spin simply by reflecting it away from its original polarization, and so the "key pair" will no longer match. Think about how networking works. If your stream of photons is treated essentially like binary (but better), then your packets will all end with a checksum of sorts. If the packet contained an error, the checksum won't match, and you resend the packet so eventually the message gets there. It's not a very "spooky" factoid when you look at it purely mechanically. I believe this is also why we can coax out quantum behavior from water, electricity, and other things that move around easily. We can even demonstrate quantum weirdness using 3 polarized lenses like you get from the cinema. The randomness you can envision with all this travel and bouncing and sensing adds up to a lot of noise, making it an expensive problem to solve before things get practical. Early IP networks were pretty crappy. Look at how they go now! The information is light speed. The reading of it is simultaneous. Splitting one of the protons into another pair leaves you with 3 unrelated protons that are not read at the exact same moment (a little simplified - there is math involved to calculate when is that EXACT moment).