Why FTL communication via entanglement doesn't work

[bascule]

Run a laser through a beam splitter which goes into two fiber optic cables. Have the resulting beams cross. What you will see is an interference pattern.

 

Run the beams coming out of the beam splitter through a pair of down converters which generate entangled photos. Now you end up with two sets of interference patterns.

 

Now, right next to each of the screens, place a "which path detector" on one of the fiber optic cables. Now, whenever either side turns on this device, it destroys the interference pattern on the other side by collapsing the waveform of the entangled particle and thus destroying the interference pattern. The result is two way communication via quantum entanglement. Like morse code you can signal by collapsing the waveform.

 

I've been puzzling myself as to why this can't result in FTL communication.

 

My best guess is: it will take at least distance/c seconds to transmit enough photos to generate a distribution statistically relevant enough to deduce a collapsing vs. uncollapsed waveform from the photons received. Otherwise, it will be ambiguous.

 

Is this the case?

 

Any math to back it up?

[Klaynos]

If I understand your setup correctly then, the person viewing the second interference pattern will not be aware of the calapsed waveform so the interferance pattern will still exist. Untill he does some measurement to destroy it.

[Sayonara]

Could you not have two identical setups, but omit the detector in one, so that the recipient can compare the collapsed waveform pattern with the non-collapsed pattern?

[5614]

Run a laser through a beam splitter which goes into two fiber optic cables. Have the resulting beams cross. What you will see is an interference pattern.

 

Run the beams coming out of the beam splitter through a pair of down converters which generate entangled photos. Now you end up with two sets of interference patterns.

Are these two seperate setups? Or are the photons from one fed into the other?

 

Does the 2nd setup just entangle photons? Or are you using them to make an interference pattern aswell?

 

Now' date=' right next to each of the screens, place a "which path detector" on one of the fiber optic cables.[/quote']Next to the screen AND on the fiber optic cable? I think you're just saying attach a "which path detector" into the setup.

 

Now, whenever either side turns on this device, it destroys the interference pattern on the other side by collapsing the waveform of the entangled particle and thus destroying the interference pattern. The result is two way communication via quantum entanglement. Like morse code you can signal by collapsing the waveform.

I'm confused about this "other side", what is it? I think the confusion originates from not knowing how these two setups are linked (eg. photons from one fed into another, or totaly seperate setups?)

 

I think the only useful thing I can say is this:

 

Say you have a "which path detector" half way down your fiber optic cable. If you turn it on at t=1 then any photons which have already passed the detector (half way down the cable) before t=1 will display interference.

 

 

So from the time the detector is turned on the first photon to show no interference would take a/c seconds to emerge. Where a is the distance between the detector and the end of the cable and c is speed of photons travelling along the cable (it's a model, lets assume they travel at exactly c).

 

So for the first a/c seconds interference will still be shown.

 

However then you started talking about entanglement, which confused me, so maybe this is not what you mean.

[bascule]

Are these two seperate setups? Or are the photons from one fed into the other?

 

Yes, the first setup is just the double split experiment, only using a beam splitter rather than two slits to split the probability wave.

 

Does the 2nd setup just entangle photons? Or are you using them to make an interference pattern aswell?

 

It's doing both. The point of the first setup was to show that the second one was based on it, just with down converters on either side of the beam splitter generating entangled photons (or rather, entangled probability waves)

 

Next to the screen AND on the fiber optic cable? I think you're just saying attach a "which path detector" into the setup.

 

Yes

 

I'm confused about this "other side", what is it? I think the confusion originates from not knowing how these two setups are linked (eg. photons from one fed into another, or totaly seperate setups?)

 

There's a laser, going into a beam splitter, and then into down converters:

 

 

I think the only useful thing I can say is this:

 

Say you have a "which path detector" half way down your fiber optic cable. If you turn it on at t=1 then any photons which have already passed the detector (half way down the cable) before t=1 will display interference.

 

 

So from the time the detector is turned on the first photon to show no interference would take a/c seconds to emerge. Where a is the distance between the detector and the end of the cable and c is speed of photons travelling along the cable (it's a model, lets assume they travel at exactly c).

 

So for the first a/c seconds interference will still be shown.

 

However then you started talking about entanglement, which confused me, so maybe this is not what you mean.

 

Well, hopefully I've improved the description now...

[5614]

I think I understand it better now. But I can't see how entangling the photons changes the scenario.

 

As far as I can see when you enable the WPD it would still take a/c seconds for the first non-intefering photons to be observed, where 'a' is the distance between the WPD and the detection screen and c is the speed of light.

 

Remember a photon which has passed the WPD but has not yet left the cable will still show interference, as it has not passed through the WPD.

[swansont]

If the photon takes the upward path, you know it did by the WPD, because the photons are entangled. But you know if it took the downward path, too, because you see a flash but without a coincident signal from the WPD. I don't think that layer of complexity adds anything but confusion, and entanglement is confusing enough.

[rajama]

There may be something useful in this related thread: http://www.scienceforums.net/forums/showthread.php?t=10911

 

Have fun.