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My understanding of quantum entanglement is that when you measure the state of an entangled particle, its counterpart will measure a correlated state, i.e. we know for sure that if for example Particle A is measured to be in state A, then particle B will definitely be measured to be in a correlated state B at the same instance.

So my question is, can we not exploit this property for communication? The way this could be done is that we fix two frequences, say 10 times/second and 20 times/second. At the destination, we always measure 20 times/sec, at the source we measure 10 times/second or 20 times/second depending on whether we want to transmit a 0 or a 1. Then at the destination, based on the measured probability of states, we can decide whether the source was transmitting a 0 or a 1.

Would this work?

glS
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pragman
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  • "measurements of entangled systems", i.e. quantum correlations, can be used for communication purposes in several ways. They cannot be used to achieve superluminal communication though, see e.g. https://en.wikipedia.org/wiki/No-communication_theorem – glS Jun 18 '21 at 08:22

2 Answers2

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The problem with this method is that entanglement is a one-shot process. Once either particle is measured the entanglement is broken, and the outcomes of any future pairs of measurements on the two particles are independent.

gandalf61
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  • That's not the reason, otherwise you would just need a large enough supply of entangled states!! – Norbert Schuch Jun 18 '21 at 08:26
  • @NorbertSchuch I think it is clear that the method the questioner is describing involves repeatedly measuring the same pair of particles. But even if you use a different pair of particles for each measurement then there is certainly no correlation over time between measurements at the destination and the method still does not work. – gandalf61 Jun 18 '21 at 09:25
  • Of course it still doesn't work. My point is that your argument in the answer misses the point: It is not due to the entanglement being "used up", otherwise a large/infinite supply of entangled states would solve the problem. – Norbert Schuch Jun 18 '21 at 09:38
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The simple fact that special relativity forbids information to travel faster than light forbids this. No information is sent to the other detector. If you measure the state of the entangled system, then there is no information sent to the entangled other parts of the system. You know what the other side is like instantaneously, but you can't even send a zero or one in this way because you don't know if the measured state is gonna be a one or zero. So all further connections, depending on your initial measurement, will be arbitrary ones or zeros, useless for sending information.