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Suttonian t1_isq04t5 wrote

Reply to comment by StuckinbedtilDec in Experimental demonstration of entanglement delivery using a quantum network stack by matpompili

As far as I'm aware, that is not thought to be possible - no information can be transmitted faster than light.

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StuckinbedtilDec t1_isq18m7 wrote

Bell tests have been performed where the locations were sufficiently separated that communications at the speed of light would have taken longer—in one case, 10,000 times longer—than the interval between the measurements.[7][6]

https://en.m.wikipedia.org/wiki/Quantum_entanglement

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matpompili OP t1_isq1p6z wrote

Yes, entanglement and its effect propagate instantaneously, but that cannot be used to send information faster than the speed of light.

Check out this video from Veritasium, it explains very nicely why that is the case: https://www.youtube.com/watch?v=ZuvK-od647c

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StuckinbedtilDec t1_isq4it1 wrote

As long as the effect can be detected then information could be sent.

Left spin = zero

Right spin = one

Time to go binary... 01101000101011... all over again.

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matpompili OP t1_isq5v6a wrote

The problem with your argument is that you cannot force the system to go "left" or "right".
All that entanglement gives you is: either the two spins go "left-right", or they go "right-left", but you cannot force the system in one of the two options.
So when you measure your spin, and you get "left", you know that the other spin will be right, but you have no way to imprint a message on this process.
You can only use this as a correlated source of random events (which is why it is so useful for cryptography)

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StuckinbedtilDec t1_isq6vwf wrote

Won't the other particle be effected (in a detectable way) when the first one is measured?

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matpompili OP t1_isq7f52 wrote

Without communication between the two sides, no: imagine you are in a closed room, no communication to the outside, and have one of the spins.
You measure it repeatedly, and you get random outcomes: 0, 1, 1, 0, 0.
The other side got 1, 0, 0, 1, 1, but since neither you nor the other side has a way to change what those bits are going to be, all you have after measuring is a correlated list of random numbers.
The fundamental problem, is that you have no way to tell whether the other side has done anything to their spin. If the other side had not done anything, you could have totally gotten the same string of random bits!

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StuckinbedtilDec t1_isq96ja wrote

Would the other side know exactly when the measurement was being conducted?

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matpompili OP t1_isqbrmy wrote

They could, it would not change the result. The measurement of the two spins gives correlated results, it does not change the result.

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StuckinbedtilDec t1_isqc9ia wrote

Then it is possible to send information by knowing exactly when the measurement was being conducted. As long as both groups have clocks that are synchronized then a 1/0 could be transferred based on the timing of the measurement.

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Suttonian t1_isqdtu3 wrote

Let's do the experiment.

  • Team A and Team B.
  • They agree that at exactly 10 am they will take measurements.
  • Team A gets a left spin, they know Team B has the opposite.
  • Team B gets a right spin, they know Team A has the opposite.

But what information has actually been transmitted? How could a message be transmitted?

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StuckinbedtilDec t1_isqgegx wrote

If team A takes a secret measurement at 10 am there's no way team B would know what time it was taken?

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Suttonian t1_isqryyy wrote

No there is no way for them to know if team A took a secret measurement.

If either side measures the entangled particle it will break the entanglement, the 'connection' is lost.

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StuckinbedtilDec t1_isqsxdp wrote

Could B team detect the moment a connection was lost?

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Suttonian t1_isqzspi wrote

No. Any detection/measurement results in the entanglement breaking.

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warplants t1_isr24mq wrote

No. The only possible way to know there was a connection in the first place is to compare the measurements of A and B teams; if their measurements are strongly correlated, there was a connection.

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