Way to transmit information faster than the speed of light?

maninasia

Is there a way to transmit information faster than the speed of light by using chains of entangled particles?

Separate the particles over a vast difference but maintain their entanglement.

While I know that it is impossible to determine the spin without observing it, perhaps there is another way to transmit the information, by using chains of entangled particles. We don't care about the spin, we just care about the closing of the quantum wave.

Checking the twin of each particle would set-off the other entangled particle (would that be able to generate a signal in the entangled particle?) and would varying the speed the observation along a chain enable you to transmit a message in binary code or any code you wished?

Morbert

Entangled particles allow for "superdense coding", a technique which can be used to potentially send large amounts of data efficiently.

It won't, however, allow us to send information faster than the speed of light. The key to utilising quantum entanglement is determining what transformations someone has applied to a quantum wavefunction, and this always requires a comparison of bits on both sides of the communication. This comparison is where the speed of light restriction is apparent.

Professor_Fink

maninasia wrote: »

Is there a way to transmit information faster than the speed of light by using chains of entangled particles?

No. Quantum mechanics is a no-signaling theory, meaning that while correlations may be stronger than classically possible, they cannot be used to signal.

To see this, imagine the maximally entangled state |00>+|11>. We consider the effect of measurements on each particle (in either, say, the X (|0>+|1>, |0>-|1>), Y (|0>+i|1>, |0>-i|1>), or Z (|0>,|1>) basis).

The state above can be described by the expectation of each possible measurement: <XX>=1, <YY> =-1, <ZZ> = 1, <XY>=<YX>=<XZ>=<ZX>=<YZ>=<ZY>=<IX>=<IY>=<IZ>=<XI>=<YI>=<ZI>=0. So forget about the state and think only about these correlations.

Since <XX>=1 but <XI>=<IX>=0, this means that measuring a single qubit yields only a random outcome (you can verify this for any other combination), but there is some correlation between the two random outcomes of the two measurements. What this means is that you cannot influence the outcome of a measurement of a distant particle, no matter what you do, even though you do have strong correlations in your measurement results.

This leads to a no-signaling theorem for quantum mechanics. It is simply impossible to transmit information via entanglement within any linear theory (QM is linear). Adding non-linear terms does make this possible, but that is simply making up new physics to suit your purpose.

Professor_Fink

Morbert wrote: »

Entangled particles allow for "superdense coding", a technique which can be used to potentially send large amounts of data efficiently.

Actually superdense coding only allows you to double the the number of bits you can transmit. There is something called the Holevo bound which says the best you can ever do is 1 bit per qubit. This is still true in superdense coding, it is simply that the receiver holds half the qubits. As it turns out, this is provably the best you can do, as you can probably see by symmetry.

maninasia

I'm not very up to speed with high level mathematics so difficult for me to understand.
So there is no way to setup a stream of entangled particles and tell if they have been 'read' previously on the other side, correct?


Because the readout is essentially random...

http://en.wikipedia.org/wiki/Photon_entanglement
'Instantaneous communication by means of quantum entanglement is actually impossible because neither side can manipulate the state of the entangled particles, they can only measure it (see No-communication theorem). This fact means that if you measure one particle you cannot infer anything meaningful about the observers measuring the other particle, except you know what state they will measure, or have already measured. Thus causality is preserved.'

So in this paragraph it states that we can tell they have already measured the entangled particle.

Why would this scheme be useful, it could be used as a device to detect alien civilisations over vast distances....i.e. a measurement had occurred. Thus allowing us to know alien life exists in that sector and passing information to us instantaneously, not of the state of the particle but of the presence of an observer.

maninasia

In this link there is a good debate on the subject.
http://www.technologyreview.com/blog/arxiv/24759/

You can see in this debate (and above) that the physicists tend to get side-tracked rather than answering the question of whether it would be possible to setup a protocol to follow that a series of entangled particles located on both Earth and Mars could be read out in some way.

As far as I know the statement is usually that classical information needs to be sent to make quantum teleportation and therefore communication possible. The observer on one side needs to know the protocol the other observer followed in order to read the spin of the particle on his side.

Why not set a very accurate clock to do measurements at a predetermined rate and using a predetermined detection system on both sides. You have sent the classical information ahead of the game, so to speak.

Also, by varying the amount of quantum energy when you wanted to send information (instead of to the predetermined pattern), surely the observer on the other side could detect this and get information in this manner.. a change in energy level from the expected protocol has occurred e.g.. No. 151 at time 5 days, 23minds, 15 secs in the huge number of entangled particles we have already prepared showed a deviation from the programmed energy level.

So come on physicists, can you answer the question at hand?

Morbert

maninasia wrote: »

So come on physicists, can you answer the question at hand?

I'm not very up to speed with high level mathematics so difficult for me to understand.

The question already has been answered. You seem to simply be unwilling to absorb it.

Why would this scheme be useful, it could be used as a device to detect alien civilisations over vast distances....i.e. a measurement had occurred. Thus allowing us to know alien life exists in that sector and passing information to us instantaneously, not of the state of the particle but of the presence of an observer.

How would we know alien life has made a measurement? Professor Fink just explained that, although you get a special kind of correlation between entangled particles, it is impossible for a measurement on one side to actually influence the state of the particle on the other side.

Professor_Fink

maninasia wrote: »

So come on physicists, can you answer the question at hand?

I believe I already have. Operations on different subsystems commute, and so the time ordering of any operations makes no difference to the result.

I realise that might be a bit mathsy for you, but unfortunately quantum mechanics is a relatively advanced topic, and so higher level maths is par for the course. What this means is that the outcome of measurements of seperate systems (be they entangled or not) cannot depend on the order of the measurements.

maninasia

Morbert wrote: »

The question already has been answered. You seem to simply be unwilling to absorb it.



How would we know alien life has made a measurement? Professor Fink just explained that, although you get a special kind of correlation between entangled particles, it is impossible for a measurement on one side to actually influence the state of the particle on the other side.

No, when measurement is made on one side the spin of the entangled particle is instantaneously altered, is that not an alteration of state?

maninasia

Professor_Fink wrote: »

I believe I already have. Operations on different subsystems commute, and so the time ordering of any operations makes no difference to the result.

I realise that might be a bit mathsy for you, but unfortunately quantum mechanics is a relatively advanced topic, and so higher level maths is par for the course. What this means is that the outcome of measurements of seperate systems (be they entangled or not) cannot depend on the order of the measurements.

What about latest theories of quantum energy transferrance? Could energy levels be altered on a series of entangled particles to read out information?

http://www.technologyreview.com/blog/arxiv/24759/

maninasia

Morbert wrote: »

Entangled particles allow for "superdense coding", a technique which can be used to potentially send large amounts of data efficiently.

It won't, however, allow us to send information faster than the speed of light. The key to utilising quantum entanglement is determining what transformations someone has applied to a quantum wavefunction, and this always requires a comparison of bits on both sides of the communication. This comparison is where the speed of light restriction is apparent.

What about sending the information ahead of time?

Morbert

maninasia wrote: »

What about sending the information ahead of time?

Then you're just sending information earlier. Say I have a particle and you have a particle, and both these particles are entangled. I can preform a transformation on my particle and send it to you. If I want to send the bits 00, for example, I can perform an identity transformation. If I wanter to send the bits 11, I could perform the transfromation ZX (= iY). You would then receive my particle and decode the information. You would not be able to decode the information until after you receive my particle.

What Professor Fink showed was that, if you don't have my particle, you can't infer anything from a measurement on your particle alone as the statistics won't tell you anything. It won't even tell you if I have made a measurement at all. This is independent of the distance between us, or the nature of the entanglement.

Professor_Fink

maninasia wrote: »

No, when measurement is made on one side the spin of the entangled particle is instantaneously altered, is that not an alteration of state?

There is absolutely no evidence for that. There is a conserved quantity in the measurement results which leads to a stronger set of correlations than are possible classically. These correlations in no way rely on instantaneous changes.

Professor_Fink

maninasia wrote: »

What about sending the information ahead of time?

The bits that need to be communicated are the measurement results from the teleportation measurements, and so they can only be transmitted after the measurements are made.

maninasia

maninasia wrote: »

What about latest theories of quantum energy transferrance? Could energy levels be altered on a series of entangled particles to read out information?

http://www.technologyreview.com/blog/arxiv/24759/

What about this? I'm not being pedantic but this seems like a very new area to explore.

maninasia

Professor_Fink wrote: »

The bits that need to be communicated are the measurement results from the teleportation measurements, and so they can only be transmitted after the measurements are made.

Ok, thanks for both your answers.

Can we not create entangled particles to a set schema, like a set energy level or a set spin. Then split them and send them to opposite sides of the solar system. At the same time we sent ahead the expected read-out we should get day by day , say reading 10 particles a day from our stock of 5000 or so entangled particles.

We then measure our split particle on the other side 1 year later. If one of the pair deviate from our planned setting that means we have changed from our pre-set plan and therefore added information.

IF we cannot set the expected spin or energy level of the particle then I can concede how this scheme will not work.

Professor_Fink

maninasia wrote: »

Can we not create entangled particles to a set schema, like a set energy level or a set spin. Then split them and send them to opposite sides of the solar system. At the same time we sent ahead the expected read-out we should get day by day , say reading 10 particles a day from our stock of 5000 or so entangled particles.

We then measure our split particle on the other side 1 year later. If one of the pair deviate from our planned setting that means we have changed from our pre-set plan and therefore added information.

Changing the measurement or operation performed by one side does not alter the measurement result of the other. I know that seems hard to get your head around, but it is really no different from classical correlations. The only change is that stronger correlations are allowed.

maninasia wrote: »

IF we cannot set the expected spin or energy level of the particle then I can concede how this scheme will not work.

The point is that when you create an entangled state (let's assume spin entanglement for the moment) state the spins no longer have a definite spin: The state of the spin becomes delocalised over the whole system, so while the total spin may be conserved, the individual spins are random. Making a measurement in any basis yields a random outcome, independent of what the other party has done, and so there is no way to infer information from it.

maninasia

Ok, it looks like information cannot be obtained through the use of entangled spin. But is there an instantaneous change happening between entangled particles? Are they somehow linked through time and space?

Professor_Fink

maninasia wrote: »

Ok, it looks like information cannot be obtained through the use of entangled spin. But is there an instantaneous change happening between entangled particles? Are they somehow linked through time and space?

No on both counts. In fact there is a mathematical theorem which proves that superluminal signaling is not possible within quantum mechanics.

maninasia

So you don't accept quantum entanglement then?

Professor_Fink

maninasia wrote: »

So you don't accept quantum entanglement then?

I'm not sure how you can possibly have gotten that idea. I am simply saying that there is a mathematical theorem that says that you cannot use entanglement to transmit information faster than light.

maninasia

maninasia wrote: »

Ok, it looks like information cannot be obtained through the use of entangled spin. But is there an instantaneous change happening between entangled particles? Are they somehow linked through time and space?

Professor Fink, please read what I wrote here. Do you agree they are entangled, that there is a link and that this is independent of time and space?

We may not be able to get any information out of this entanglement, but there is a link. The reason that quantum entanglement fascinates me, aswell as heisenberg's uncertainty principle, is that they seem to suggest that space/time are artificial constructs. According to Heisenberg's uncertainty principle an electron could indeed inhabit the far reaches of the universe at any given time. It may be an extremely tiny probability but it is a real possibility. When I see this I know there are a lot of unanswered questions. Fundamental conflicts.

Perhaps some other physicists would like to comment also, I notice some of the smart-arses dropped out of the debate a long time ago. Don't be afraid to admit what you don't know

Professor_Fink

maninasia wrote: »

Professor Fink, please read what I wrote here. Do you agree they are entangled, that there is a link and that this is independent of time and space?

I do not accept the premise of your question. The first part seems to ask "Do you accept particles entangled particles are entangled?". Certainly entanglement exists, so you are free to imagine entangled particles. It's not even particularly hard to make them.

The second part asks "Do you accept that there is a link and that this is independent of time and space?". That's a bit more tricky. There is a correlation, as I have mentioned before, and the correlation is independent of the spatial locations of the particles, but there is certainly no causal link, which is what you keep insisting I must believe in if I believe entanglement exists. That is simply wrong, entanglement is not a causal link.

maninasia wrote: »

We may not be able to get any information out of this entanglement, but there is a link.

A correlation certainly exists, but it is not a causal link.

maninasia wrote: »

The reason that quantum entanglement fascinates me, aswell as heisenberg's uncertainty principle, is that they seem to suggest that space/time are artificial constructs.

They really don't. But entanglement does get rid of the notion of local realism, which is why Einstein thought it must be wrong. Turns out, however, that local realism is wrong.

maninasia wrote: »

According to Heisenberg's uncertainty principle an electron could indeed inhabit the far reaches of the universe at any given time. It may be an extremely tiny probability but it is a real possibility. When I see this I know there are a lot of unanswered questions. Fundamental conflicts.

There is no conflict. You seem to be making the mistake of construing your understanding of quantum mechanics with the present state of knowledge. Just because something seems to be strange to you does not mean we (meaning people other than you, for example quantum physicists) do not understand it. I guess with the uncertainty principal you are thinking of signalling by for example measuring the exact momentum of a particle, so that it will become entirely delocalised. It turns out that this is impossible: momentum is not a local operator. When we make a local measurement of some quantum system we are not really measuring it's exact momentum, just an approximation to it.

maninasia wrote: »

Don't be afraid to admit what you don't know

I'm not. I don't know how to dance, I don't know how to speak Japanese, but I do know quite a lot about quantum mechanics, including everything covered in this thread.

maninasia

So local realism is wrong, there is a correlation between particles no matter where in the universe, perhaps instantaneous, and there is nothing wrong with our picture of space-time? Here is something that has no requirement for space or time. This just makes me question a lot of things, speed of light limit aswell. It makes me question the issue of time making me feel it is artificial (this is not the only thing that makes me think time is an artificial construct and many physicists have pointed out time doesn't come into their equations), and if time is artificial then the whole concept of speed of light limit could be artificial. Well this is all very rough half-assed conjecture I know...


Anyway...of course you know your subject much better than me. I think you misunderstood the last statement of mine, which is again pointed at the fact that there is so much more to be learned than what is currently known. You might be an expert of quantum mechanics as it currently stands but that could be only a small part of the bigger picture.

Professor_Fink

maninasia wrote: »

So local realism is wrong, there is a correlation between particles no matter where in the universe, perhaps instantaneous, and there is nothing wrong with our picture of space-time? Here is something that has no requirement for space or time.

This isn't something unique to quantum mechanics. I really don't see anything special in terms of space-time structure due to entanglement. Why would it change as particles travel through space and time?

maninasia wrote: »

This just makes me question a lot of things, speed of light limit aswell.

I understand why that may seem to be the case, but there is absolutely no evidence for superluminal signaling, and in fact both quantum mechanics and relativity have theorems proving the impossibility within those theories.

maninasia wrote: »

It makes me question the issue of time making me feel it is artificial (this is not the only thing that makes me think time is an artificial construct and many physicists have pointed out time doesn't come into their equations), and if time is artificial then the whole concept of speed of light limit could be artificial. Well this is all very rough half-assed conjecture I know...

Well, this just seems to be you rediscovering the idea of spacetime, where space and time are not really separate entities.