Quantum Entangled FTL Communications?

[garys_2k]

Here's the scenario, please tell me where I'm messing up.

I have a set of entangled particles, say twenty-six pairs, designated A-1, B-2, C-3, etc. I put all the particles into individual containers where they are unobserved and send all the numbered particles, 1, 2, 3 etc, on my spaceship a light year away. The corresponding lettered particles, A, B, C, etc. stay home with me.

I have prearranged a code with my astronaut partner to send him FTL communications via the particles thusly:

Data will be sent in five bit words at specific times and intervals. We have clocks that will stay well synchronized with each other, compensating for special relativity effects, so that our communications will start at noon on May 1, according to our protocol. At noon on the "common time" clock I will place particle A in a + environment, driving his to -. He will NOT observe particle 1 until 12:05 pm to ensure that I've taken care of my end and will confirm that he sees a -. Our protocol is for me to "set" four more particles at ten minute intervals, in order of B, C, D and E. When I want to send a "1" I will do so by changing the next-up particle from whatever state the previous one was. Conversely, if I want to send a "0" I will do so by repeating the same state as the previous entangled particle.

So, my first message of 1, 1, 1, 0 would be sent as -, +, -, -, completing my five-bit (including the initial "synchronizing" bit) message. We agree to repeat this protocol every hour.

Note that the astronaut would only test his corresponding particles 2, 3, 4 and 5 at ten minute intervals, but five minutes AFTER I set my end's half of the entangled pairs. This should ensure his states are already established. He should see the following sequence: +, -, +, +. Combined with his sync bit of -, he should read the data sent as 1, 1, 1 ,0 -- exactly what it was meant to be.

What am I missing? I suspect that the idea of having really synchronized clocks may be the issue, but couldn't we build a clock smart enough to calculate the special relativity effects the astronaut encountered during his trip and provide a readout identical (well, within a few minutes of) the time display on the other end?

 

[neutrino_cannon]

I DO NOT CARE ABOUT QUANTUM INSANE PHYSICS OR RELATIVITY, THEY DO NOT MATTER. YOU CAN HAVE NO PROOF...

But seriously now.

Let me see if I've got this straight (sorry, I can be a bit S-L-O), but you intend to communicate using quantum entangled particles correct? These being particles that always have the smae quantum state?

I would suggest this doesn't work because you might destroy the entanglement messing around with them in the way you have proposed. But then I'm pretty far from an expert in this area.

 

[Beausoleil]

I confess I've wondered the same sort thing ever since I first came across Bell's theorem. The direction you choose to measure spin along for one set of particles affects the results of experiments on the other one. This happens even when no light speed signal could pass, we're told. So when you do the second set of experiments can't you deduce what happened somewhere inaccessible at light speed?

I'm hoping someone can save me from having to think about it at length!

 

[DrChinese]

Per the experimental setup contemplated by Bell's Inequality and tested by Aspect et al:

You don't set the state of the photon as being a particular one. You merely force it to take on a discrete value when you observe it. When you check the polarity of a photon, you get one of two answers randomly selected: absorbed by the polarizer or allowed to pass thru the polarizer. In a perfect test, 50% always make it through on each side.

Neither person has the information to determine what was going on at the other end at that time - since the result is perfectly random. That is why FTL communication does not occur in this case.

Does that help?

 

[metacristi]

DrChinese is right.The problem is that you cannot control the polarity of the photon [it can be vertical,horizontal and any other angle] but only the difference in polarization between the photons.
If you polarize them at let's say 180 degrees from each other you still do not know the state of your photon before measuring it.When you measure it you know it's polarization and consequently know the polarization of the entangled photon but the receiver cannot draw any meaningful conclusion from the measurements she makes...they would seem perfectly random.FTL transmission using nonlocality is not possible,at least in the light of what we know now.
However there is still possible to make such a data transmission if the sender measures the polarity of all his photons at the beginning,forcing the other photons in the complementary state,and sends part of this info to the receiver using a classical [subluminal] way of transmission.This info must contain the 'direction' of the polarity in the form 'up'-'down' not the exact polarity and moreover the photon pairs which are useful to encode the message and must be checked at the destination.
At the reception of this information your type of transmission [not identical to your proposal however] is possible,of course if the clock synchronization problem is solved.Unfortunately no one can claim that we send information with supraluminal speeds...


Edit to add:

It's clear from above that we need a much greater number of entangled particles even if only two complementary states were possible [as in the case of spin].
Anyway the fact that counts is that without the info passed at speeds equal or smaller than light's the data transmitted cannot be 'decoded' at the receiver.

 

[BillyJoe]

We have clocks that will stay well synchronized with each other, compensating for special relativity effects.....

What are you intending to do here?

You will have no problem calculating the time on the spaceman's clock relative to your clock provided you know his speed relative to you.
Similarly he will have no problem calculating the time on your clock relative to his clock provided he knows your speed relative to him.

The calculations are, of course, the same.

If at twelve o'clock on your clock you calculate the time on the spaceman's clock to be eleven o'clock, then at twelve o'clock on his clock he will calculate that it is eleven o'clock on your clock.

Is this what you mean by synchronizing clocks?

 

[phobos]

What am I missing? I suspect that the idea of having really synchronized clocks may be the issue, but couldn't we build a clock smart enough to calculate the special relativity effects the astronaut encountered during his trip and provide a readout identical (well, within a few minutes of) the time display on the other end?

Synchronising the clocks is not a problem; if relativity is a difficulty, have the starship at rest relative to the Earth, but still several light-years away. Your problem is here:

At noon on the "common time" clock I will place particle A in a + environment, driving his to -.

Doesn't work that way, I'm afraid. Suppose that our two states are encoded in the polarisation of a photon. Now, if I want to create photons polarised in what we'll call the + direction, I can pass a beam of photons through a + polariser, and all the photons that emerge will be + polarised. Great, right? But the problem is that not all the photons emerge.

So suppose I pass my entangled photon (which is in a superposition of + and - with each equally likely) through a + polariser. I now have a 50% chance that it will emerge the other side and be in a + state, and a 50% chance that it will not emerge the other side (and so it was in a - state). Either way, I know that my colleague's photon has the opposite polarisation.

From the perspective of the person at the other end of the entanglement, his photon will turn out to be + polarised half the time, and - polarised the other half - which is exactly what it would be if I hadn't done a thing to mine in the first place. No information gets sent.

 

[Tez]

The classical analog:

Imagine that you have a box containg four compartments, and in one compartment is a marble, although you have no idea which compartment the marble is in. I have a box, which the marble is guaranteed to be in the same compartment as the one in your box, although I also dont know which compartment its in.

We each assign a "state" of [1/4,1/4,1/4,1/4] to our individual boxes.

However, we really have an "entangled" state. If I label the compartments A,B,C,D then we have the "state"

1/4 AA + 1/4 BB + 1/4 CC+1/4 DD

in what I hope is an obvious notation.


Now, I want to communicate to you once we're far separated. Lets say I make one of two different measurements, according to whether I want to communicate a bit of zero or a bit of one.

If I want to communicate 0, then I do a measurement which tells me "Is the marble in my box in compartments A,B" or is it in "C,D"." Note, that I dont determine exactly which compartment its in, just the pair its in.

Similarly, to communicate a 1 I measure to determine whether the marble is in "A,C" or "B,D".

At the end of the measurement, I have "collapsed" the marble in your box. For example, lets say I find that my marble is in either A or B. Then I collapse the state of my marble to [1/2,1/2,0,0].

But notice that I also collapse your marble to [1/2,1/2,0,0].

Aha - i have changed the state of your marble from [1/4,1/4,1/4,1/4] to [1/2,1/2,0,0] - and therefore can communicate to you, even though youre far away!

Obviously ridiculous.

Quantum states are more subtle than classical probability distributions in many ways, but ultimately they are epistemic "catalogs of information" much like the classical probability distribution. For understanding examples such as this the classical analog is perfectly fine...

 

[garys_2k]

Thank you to all, I understand now. What I misunderstood was a possibility to "write" a particular quantum state at the transmitting end instead of just being able to read its state. Since a particle can only tell me its existing state, and I cannot influence that state, I'd have no way to really encode data. I would have to send a classical transmission to my astronaut describing what my particle told me about its state, destroying any superluminal communications.

As far as the clock thing, I agree that keeping track of the "other end's" clock shouldn't be too difficult from either position. We could agree beforehand which to use as the common one.

 

[a_unique_person]

The developments in this area make me wonder if listening for advanced civilisations in space via radio waves wouldn't be a waste of time. If they were that advanced, wouldn't they be using a form of communication that travels faster than light if it was there?

 

[Tez]

The developments in this area make me wonder if listening for advanced civilisations in space via radio waves wouldn't be a waste of time. If they were that advanced, wouldn't they be using a form of communication that travels faster than light if it was there?

Its funny you say this AUP.

Rcently I (and two colleagues) discovered that communication without reference frames is possible, if we allow the communicating parties to exchange entangled systems. http://xxx.lanl.gov/abs/quant-ph/0302111

Why is this interesting? Well, we always presume (including our SETI communication) the existence of a reference frame - in SETI its basically assumed that the Hamiltonian of the Universe is such that nondegenerate states that the Aliens emit are received as such here. If the aliens live near a strong gravitaitonal field, this would not be true for example.

So our results amount to states that really advanced civilizations may communicate with. Of course they may even have critters that are smarter than me (hard to believe, I know) and so communicate in even better ways. I dont believe in FTL communication (and entanglement provably doesnt allow it), but your point is certainly well made...

 

[metacristi]

The developments in this area make me wonder if listening for advanced civilisations in space via radio waves wouldn't be a waste of time. If they were that advanced, wouldn't they be using a form of communication that travels faster than light if it was there?

I am not at all sure that FTL communications are impossible.Despite the fact that we cannot do that,at least for the moment,this possibility is still viable,in my opinion.We do not have yet sufficient reasons,beyond all reasonable doubt,to safely discard it.
As for the SETI's failure to intercept any alien communications the commonest opinion is:'the most reasonable is to simply doubt the existence of the aliens,the sad truth is that we are alone...'.
This is a hasty conclusion in fact,as some scientists have pointed out,the major part of interstellar communications belong to advanced races.Or we should expect that the number of these races is very small in the universe:the majority of the technical civilisations self destroy,due to internal crisis,before arriving at the point of using interstellar communications on great scale.
Also it is very possible that these advanced races have found other types of communications than electromagnetic waves:FTL transmissions,wormholes,the artificial 'bending' of spacetime and so on.Indeed it would be superfluous for type 3 civilizations [which 'harness' the energy of their entire galaxy] to use electromagnetic waves and wait millions of years for a response...This is valid even if electromagnetic waves were the only mean of interstellar communication,why send messages which,possible,no one will ever intercept?
One of the most interesting theories explaining the failure of SETI is that advanced civilisations do not send their messages using the same frequency.Instead they break them in small portions sending them using a wide range of frequencies,in order to compensate for the possible loss of signal.The receiver know these frequencies and could still understand the message even if some small portions were lost.
But all other 'listeners' will 'hear' only an unintelligible noise...

 

[phobos]

I am not at all sure that FTL communications are impossible.Despite the fact that we cannot do that,at least for the moment,this possibility is still viable,in my opinion.We do not have yet sufficient reasons,beyond all reasonable doubt,to safely discard it.

Though we can't absolutely rule it out, one thing we do know is that if there is any way of propagating an effect faster than a light signal, then signals can be sent back in time. Accepting FTL travel or communications requires that you also allow time machines.