4 ms·

I think OP is asking, can you entangle, measure, discuss, and then go travel to another galaxy with real time communications? Edit: or can the communication on

I think OP is asking, can you entangle, measure, discuss, and then go travel to another galaxy with real time communications? Edit: or can the communication only happen once and then you need to remeasure over classic communication?

magicalhippo 6y ago
> travel to another galaxy with real time communications? As I said, no. How would you relay your measurement results in real-time? Without those measurement results, the receiver would in essence just hear white noise. That's the perplexing part of entanglement. Somehow it feels like information is instantaneously transferred from A to B, yet the information content is somehow zero so can't be used for proper communication.
- lemonspat 6y ago
  So you have to measure every time you want to transmit?
  - rodiger 6y ago
    There is no "transmission." When you measure you learn the state of one of the particles, and you can use that information to deduce the state of the other (which until measurement is indeterminate). You can see that this information is useless unless communicated classically.
  - magicalhippo 6y ago
    No, you have to transmit every time you want to transmit. When measuring one particle of an entangled pair and you get say "spin up", you know immediately that if someone measures the other particle (with the same measurement settings) they'll get "spin down", and vice versa. The chance that you get "spin up" or "spin down" is 50/50 and, as far as we know, cannot be affected or determined in advance. So, on the receiving end, they measure some random combination of "spin up" and "spin down". Without anything else, this information is for all intents and purposes noise. What you can do however, is to send a message using regular means with what you measured: "up, down, down, up, down". Ok, at least now they can check that what they got was the exact opposite. However that still doesn't tell them anything. So instead what you do is that you change the measurement settings, and send via regular means not just your measurement results but also your measurement settings. So you'll send "H left, V down, V up, H left, H right". The recipient will then take the measurement settings (H or V) and measure the entangled particles in the same way you did, and then note down that they get the opposite. Note now that you suddenly got a way to communicate some actual information. By agreeing in advance that a Vertical measurement means 0 and a Horizontal measurement means 1, you can send information to the recipient. However also note that you had to make a measurement of your particles and then send the results using regular means, limited by the speed of light. So why bother with this complicated setup? Why not just send the data without all this entangled stuff? And indeed, for just sending plain messages it makes no sense to use entangled pairs. However as I noted in my other post, the inability to clone entangled states means an eavesdropper can be detected using the entangled setup.
    - lemonspat 6y ago
      That was a very clear explanation, thank you. So all my grand ideas about how quantum communication might work are now sadly dead
      - magicalhippo 6y ago
        Yeah, it seems we're doomed to pay the price of time.

magicalhippo 6y ago

> travel to another galaxy with real time communications? As I said, no. How would you relay your measurement results in real-time? Without those measurement results, the receiver would in essence just hear white noise. That's the perplexing part of entanglement. Somehow it feels like information is instantaneously transferred from A to B, yet the information content is somehow zero so can't be used for proper communication.

lemonspat 6y ago
So you have to measure every time you want to transmit?
- rodiger 6y ago
  There is no "transmission." When you measure you learn the state of one of the particles, and you can use that information to deduce the state of the other (which until measurement is indeterminate). You can see that this information is useless unless communicated classically.
- magicalhippo 6y ago
  No, you have to transmit every time you want to transmit. When measuring one particle of an entangled pair and you get say "spin up", you know immediately that if someone measures the other particle (with the same measurement settings) they'll get "spin down", and vice versa. The chance that you get "spin up" or "spin down" is 50/50 and, as far as we know, cannot be affected or determined in advance. So, on the receiving end, they measure some random combination of "spin up" and "spin down". Without anything else, this information is for all intents and purposes noise. What you can do however, is to send a message using regular means with what you measured: "up, down, down, up, down". Ok, at least now they can check that what they got was the exact opposite. However that still doesn't tell them anything. So instead what you do is that you change the measurement settings, and send via regular means not just your measurement results but also your measurement settings. So you'll send "H left, V down, V up, H left, H right". The recipient will then take the measurement settings (H or V) and measure the entangled particles in the same way you did, and then note down that they get the opposite. Note now that you suddenly got a way to communicate some actual information. By agreeing in advance that a Vertical measurement means 0 and a Horizontal measurement means 1, you can send information to the recipient. However also note that you had to make a measurement of your particles and then send the results using regular means, limited by the speed of light. So why bother with this complicated setup? Why not just send the data without all this entangled stuff? And indeed, for just sending plain messages it makes no sense to use entangled pairs. However as I noted in my other post, the inability to clone entangled states means an eavesdropper can be detected using the entangled setup.
  - lemonspat 6y ago
    That was a very clear explanation, thank you. So all my grand ideas about how quantum communication might work are now sadly dead
    - magicalhippo 6y ago
      Yeah, it seems we're doomed to pay the price of time.