Time Delay Information Transfer with Entangled Particles

[jackmiller2003]

TL;DR Summary

Why can't information be sent via Quantum Entanglement through the time between the measurement of entangled particles?

Hello, my name is Jack and I'm a year 11 student in Australia. After listening to, and reading some information regarding quantum entanglement, I'm still a little unsure about the solution to a thought experiment:

Let's say that I create a situation in which multiple pairs of particles are entangled at a particular place and each partner in the pair is separated into one of two different groups. One group is sent 100 km in one direction and then other group is sent 100 km in the other direction. Couldn't you use the time delay between observation of each particle as a means of FTL communication? Say 1 second between particle measurement means a 0 in binary and 2 seconds means a 1 in binary. Hypothetically, with enough particles, couldn't you send meaningful messages faster than light?

Any help would be much appreciated!

Thanks, Jack.

P.S: The thought experiment could also work by pre-ordering the particles in a row to send information.

Sorry if I've totally missed something obvious!

 

[PeterDonis]

Couldn't you use the time delay between observation of each particle as a means of FTL communication?

No, because observers on each side don't know when the particles on the other side are measured.

 

[jackmiller2003]

No, because observers on each side don't know when the particles on the other side are measured.

Oh so the influence of measurement on one side can only be determined by measurement on the other side? Is it correct to say that if observer A has collapsed the wave function, observer B still believes there is a wave function because they haven't measured it?

 

[PeterDonis]

so the influence of measurement on one side can only be determined by measurement on the other side?

Not even that. To see the influence you have to compute the correlations between the measurement results on both sides. You can only do that when you have both sets of results communicated to you by ordinary slower-than-light means. The measurements on either side by themselves will simply look random and give no information about what's going on on the other side.

 

[jackmiller2003]

Not even that. To see the influence you have to compute the correlations between the measurement results on both sides. You can only do that when you have both sets of results communicated to you by ordinary slower-than-light means. The measurements on either side by themselves will simply look random and give no information about what's going on on the other side.

Even if the timing of the measurements are encoded? For example, if you have two particles which are entangled based on Z-spin, wouldn't the measurement of one result in the opposite result in the other?

P.S: I'm not trying to be at all combative, just trying to work out my misconceptions :)

 

[DarMM]

Even if the timing of the measurements are encoded? For example, if you have two particles which are entangled based on Z-spin, wouldn't the measurement of one result in the opposite result in the other?

P.S: I'm not trying to be at all combative, just trying to work out my misconceptions :)

Look at it this way. You measure Z-spin. You get "up", so you know the other person (if they also measure Z-spin) gets "down". You haven't really communicated anything to them or them to you. You haven't sent a message. Think of it like generating a bunch of 0s (up) and 1s. (down) You get:
010100111100000...
They'll get the same with the 0s and 1s flipped. Both of you just get related random strings, but it's not really a message from you to the other person.

It can be used to have a shared "secret" password, but not communication.

 

[DrChinese]

Even if the timing of the measurements are encoded? For example, if you have two particles which are entangled based on Z-spin, wouldn't the measurement of one result in the opposite result in the other?

P.S: I'm not trying to be at all combative, just trying to work out my misconceptions :)

Keep in mind: All that anyone sees (on either side) is a string of random outcomes. It is pretty hard* to get a message from a random string. So no matter what the relationship between them is, the actual sequence itself is still random. That is axiomatic for entanglement, where the system is in a superposition of states.*I.e. impossible.

 

[PeterDonis]

if you have two particles which are entangled based on Z-spin, wouldn't the measurement of one result in the opposite result in the other?

Yes, but you can't control in advance which one will be spin up and which one will be spin down for each pair of particles; that's random. So there's no way to encode anything in the sequence of up/down results.

 

[PeroK]

Even if the timing of the measurements are encoded? For example, if you have two particles which are entangled based on Z-spin, wouldn't the measurement of one result in the opposite result in the other?

P.S: I'm not trying to be at all combative, just trying to work out my misconceptions :)

Suppose you want to send your friend a message at a predetermined time. You and your friend have a pair of entangled things.

Your code is: up = do something; down = do nothing.

You want your friend to do something. So, you go to your entangled particle and measure it. But, you get "up", so you know your friend will get "down". Bad luck! You just sent the wrong message, if we can put it like that.

In measuring your particle you cannot control the outcome. You cannot control your friend's outcome, so you cannot send a message.

All you have is some shared data that nature has provided. Which is still something useful, but it's not a message from you to your friend.

 

[vanhees71]

Indeed, but the great thing is you can prove the correlations due to entanglement, but only after both experimenters have done their experiment and stored it in a measurement protocol by exchanging the information contained in their measusrement protocols, and that can not be done with any signal faster than light, i.e., there's no way to communicate with faster-than-light signals even when exploiting quantum entanglement. That's also by construction "encoded" in the mathematical formalism describing relativistic quantum theory, which is relativistic quantum field theory, as applied in the Standard Model of elementary particle physics.

 

[jackmiller2003]

Thanks for everyone's replies, very helpful.

Although I had understood the idea that measurement results in a random outcome, I had forgotten that both observers needed to make a measurement of the particle in order to see its result. My original thought experiment assumed that measurement on one side would allow the other side to see the influence without them measuring the particle themselves which is clearly wrong!

Once again, thanks!