Faster than light communication through entanglement

[bascule]

This was partially proposed in this thread:

 

http://www.scienceforums.net/forums/showthread.php?t=10803

 

And it really got me thinking. So here's a quick proposal.

 

We have locations A, B, and C, with AB and BC being equidistant (let's say they're one light day each) Our goal will be to quasi-instantaneously transmit a message between A and C.

 

B is a setup containing the following:

 

A laser emitting single photons at short and regular intervals, fired through a beam splitter which in turn passes either path through down converters.

 

The photons streaming out of this device travel a day until they reach their destinations. C directs the two possible beam paths across each other, generating an interference pattern so long as A doesn't attempt to garner 'which path' information from its two beams.

 

Now, the process of interpreting the results is somewhat messy as we have to wait and see if an interference pattern forms at C or not, but it looks like given this you have a simple way of instantaneously transmitting a yes/no 1/0 message across space; A can selectively and insantaneously destroy the interference pattern by observing which path information from the entangled photons, or choose not to observe them and allow the interference pattern to form.

 

Doesn't this work? I've always been told entanglement can't be used to send information faster than the speed of light, but it would seem to me that using this approach you can instantaneously tell what someone else did on the other side of the universe. You still have to wait for the photons to cross space at the speed of light, but once they do they would appear to be able to serve as a non-local medium for communication. Thoroughly impractical, perhaps, but not impossible?

[rajama]

Your set-up seems - to me - a little vague. I've only been here a short while, but I would guess you need more initial detail to engage the members here who can actually comment on it with any authority.

 

Maybe the same again, with more on the mechanism at B and the logic regarding how you can tell the interference pattern exists, etc.

 

Just a thought. Have fun.

[swansont]

How are the photons entangled, and how do you turn on/off the "which path" information?

[jutntog1]

I thought that acording to newtons law of gravity whenever you do anthing you instantly effect the rest of the universe's gravity, with that in mind it would be posible to do faster then light communication becouse you could somehow move things in a way that would allow you to comunicate.

 

this all sorta came from a universe in a nutshell (just stating my source)

[swansont]

I thought that acording to newtons law of gravity whenever you do anthing you instantly effect the rest of the universe's gravity' date=' with that in mind it would be posible to do faster then light communication becouse you could somehow move things in a way that would allow you to comunicate.

 

this all sorta came from a universe in a nutshell (just stating my source)[/quote']

 

Yes, but that's according to Newton's law. According to general relativity, gravity propagates at the speed of light.

[Vladimir]

Are'nt they testing communicatin with entaglemetn terought he great wall of china?

[bascule]

How are the photons entangled

 

The down converter generates two entangled photons which are half as energetic from a single source photon.

 

and how do you turn on/off the "which path" information?

 

As far as I can tell, it should be as simple as stopping the photons headed to A before their entangled partners generate the interference pattern at C. Anything which causes them to exhibit particle-like instead of wave-like behavior should do it.

[swansont]

As far as I can tell, it should be as simple as stopping the photons headed to A before their entangled partners generate the interference pattern at C. Anything which causes them to exhibit particle-like instead of wave-like behavior should do it.

 

Detecting the particle at A doesn't tell you which slit they are going through at C. "Which path" refers to which slit the particle goes through in a Young's double-slit apparatus. If you know which slit, then the interference pattern goes away.

 

What you want is known as a "quantum eraser" experiment. You put polarizers in place to give you the which-path information. I don't recall what the specific explanation is for why superluminal communication isn't possible, though.

[rajama]

Can I ask a dim question?

 

Wouldn't this setup - as described so far - still be the same if you moved the down converters out from B to A and C..?

[swansont]

Can I ask a dim question?

 

Wouldn't this setup - as described so far - still be the same if you moved the down converters out from B to A and C..?

 

I think so. But for the effect under investigation, I think you'll want to maximize the path length of the entangled photons.

[rajama]

Okay, thanks - er - I think.

 

When I first read #1, it looks as if I didn't pick-up on all of the (implicit) detail of the idea (also, re-reading my post #2 I think I was more than a little 'off' in trying to give advice - huh, foot-in-mouth again).

 

So - after returning to old text books and thrashing Google for useful technical details - a further (still dim, but longer) question:

 

The laser light driving the down converters goes through a beam splitter, so we can't do any kind of coincidence counting... we're looking at a single measurement of a non-local photon 'somewhere' at A or C, and some kind of averaging of the results over time to detect interference (or the lack of): is that correct, or have I missed something else?

[swansont]

Okay' date=' thanks - er - I think.

 

When I first read #1, it looks as if I didn't pick-up on all of the (implicit) detail of the idea (also, re-reading my post #2 I think I was more than a little [i']'off'[/i] in trying to give advice - huh, foot-in-mouth again).

 

So - after returning to old text books and thrashing Google for useful technical details - a further (still dim, but longer) question:

 

The laser light driving the down converters goes through a beam splitter, so we can't do any kind of coincidence counting... we're looking at a single measurement of a non-local photon 'somewhere' at A or C, and some kind of averaging of the results over time to detect interference (or the lack of): is that correct, or have I missed something else?

 

I think that the down-converter contributes one photon to each path in the scenarios I've read about, so there's no need for a beam splitter.

[rajama]

Posted by bascule:

 

A laser emitting single photons at short and regular intervals, fired through a beam splitter which in turn passes either path through down converters.

 

Okay, I assumed two down converters here..?

 

(should it be 'either' or 'each'? This is what confused me in the first place, I guess...)

[bascule]

That was poorly worded. There are two down converters, one connected to either of the two possible paths coming from the beam splitter.

 

So you have the two beams coming from the beam splitter being duplicated by the down converters and sent to both A and C.

 

And yes, otherwise you have the idea correct.

[rajama]

Thanks for that (been sick, just getting back now) - really useful.

 

Most of the experiments I've read to do with entanglement don't go beyond the demonstration of the underlying QM - that makes this interesting...

 

So, you have a single photon distributed over the whole experiment... until it's detected. It will only ever arrive on one side or the other - that is correct?

 

Also, returning to my post #9, this implies you could move the down converters out to A and C and get the same result - whatever that is?

[bascule]

So, you have a single photon distributed over the whole experiment... until it's detected. It will only ever arrive on one side or[/b'] the other - that is correct?

 

The laser would generate a constant stream of photons which the beam splitter seperates and the down converters split. In cases where A has a detector on for its photon streams, the photons will exhibit particle-like behavior and the interference pattern on the other side will be destroyed. In cases where A has no particle detector, the photons should effectively take "both paths" through the beam splitter simultaneously (exhibiting wave-like behavior) and the interference pattern would appear.

 

Also, returning to my post #9, this implies you could move the down converters out to A and C and get the same result - whatever that is?

 

The down converters are located at B, the midpoint between A and C. You can't move the down converters because otherwise the photons wouldn't arrive at A and C simultaneously.

 

The whole setup is too confusing, so perhaps I should phrase the question more generically...

 

Is it possible for A to use entanglement to "toggle" the interference pattern on and off at C's side if entangled particles emitted at B arrive at A and C simultaneously?

[rajama]

...maybe you use an equivilent set-up derived from two double slit experiments?

[bascule]

The problem with that diagram is the position of the down converters. Each down converter should fire one beam at A and one beam at C, such that the two beams fired in opposite directions are comprised of entangled photons.

 

If we have DC #1 and DC #2 attached to the two output paths of the beam splitter, DC #1 fires beam W at A and beam X at C, DC #2 fires beam Y at A and beam Z at C.

 

C redirects beams X and Z so that they cross and interfere. A can selectively flip on and off a "which path" detector for either beams W or Y, which will destroy the interference pattern observed at C when active or allow it to occur when disabled.

[rajama]

I did wonder, but - okay, suddenly everything is very clear... and bright. That's why you can't move the down-converters!

 

I'll be back...

[CanadaAotS]

Yes, but that's according to Newton's law. According to general relativity, gravity propagates at the speed of light.

 

has anyone tried to measure the speed of gravity? I always thought that gravity was instantaneous... like if say the sun disappeared it would take 8 minutes to see that it disappeared but the gravity would leave instantaneously...

[bascule]

has anyone tried to measure the speed of gravity? I always thought that gravity was instantaneous... like if say the sun disappeared it would take 8 minutes to see that it disappeared but the gravity would leave instantaneously...

 

Wrong thread perhaps?

[rajama]

The problem with that diagram is the position of the down converters. Each down converter should fire one beam at A and one beam at C' date=' such that the two beams fired in opposite directions are comprised of entangled photons.

 

If we have DC #1 and DC #2 attached to the two output paths of the beam splitter, DC #1 fires beam W at A and beam X at C, DC #2 fires beam Y at A and beam Z at C.

 

C redirects beams X and Z so that they cross and interfere. A can selectively flip on and off a "which path" detector for either beams W or Y, which will destroy the interference pattern observed at C when active or allow it to occur when disabled.[/quote']

 

I seem to be obsessing about diagrams - what about:

[bascule]

I seem to be obsessing about diagrams - what about:

 

Well, you convinced me to bust out OmniGraffle and diagram the thing. Here's my version. BS is the beam splitter, DC is the down converter, and WPD is the "which path detector", which can be selectively toggled by a person at A.

 

In this diagram, A consists of the two beams leaving the down converters plus the "which path detector." C would consist of the blue screen and the two white mirrors/prisms/whatever directing the beams onto the screen. B consists of the lasers, the beam splitter, and the down converters.

 

Hope this helps in understanding the idea.

[bascule]

Perhaps this will make it clearer still:

[rajama]

Wow - drop shadows and everything.

 

I know it may be a property of the diagram rather than the experiment, the top half looks a little like the usual EPR set-up, while the bottom half is a reference path to compare against.

 

And..?