Why does quantum entanglement not allow ftl communication

Hurkyl

Classical logic capable of treating other logics -- one never has to adopt a different logic as anything but a syntactic description of a traditional mathematical object.

Furthermore, every major 'interesting' logic of which I'm aware is completely subsumed by an ordinary, classical subject. e.g.

Intuitionistic logic is subsumed by topos theory
Constructivism is subsumed by computability theory (at least, some forms are)
Quantum logic is subsumed by C*-algebra

Ken G

Forget my remarks on Kuhn, I was probably a bit unnecessarily harsh and it makes no real difference in this thread because I'm going to argue that we are simply not seeing any paradigm-shift-driving issues here. The issue is what should count as an "anomaly" in a theory, versus the other possible classifications of something left unspecified by a theory, to wit: a limitation of a theory that is of no value to be concerned with until some specific observation points to a problem (as happened to Newton's laws), or a fundamental limitation of science, moreso than the theory (as is likely the case with quantum mechanics seen in the Copehagen interpretation). So we have (at least) three classifications for sticky philosophically unappealing elements of any theory and the resolutions they suggest:
1) anomaly-- get busy fixing it by considering existing observations
2) unconstrained limitation-- it will probably be fixed in the future, but current observations offer no guide, so there is simply no current "action item"
3) fundamental limitation-- don't bother trying to "fix" this, there's nothing to fix.

As an example of each, (1) is like a car with a nasty noise from its engine, (2) is like a car that you wish got 100 miles per gallon, and (3) is like a car that can't fly to the Moon.

So in light of those possibilities, let's look at the interesting issues you raise, issues that indeed come up often in this context:
I hear this a lot but to me this exposes a common misconception about measurement in quantum mechanics. In my view, there is very little question about what causes a measurement-- it is the decohering of the projections of a wave function onto a particular set of eigenstates. I know that has a lot of jargon in it, but it's really pretty straightforward-- you can always project a wavefunction onto a complete set of basis states, but the amplitudes that describe that projection retain coherences, which means you cannot simply pretend that one of the basis functions is "correct" while the others simply express your lack of knowing that. However, the first step in a measurement is the intentional destruction of those coherences, done expressly so that we can imagine that one of the basis functions is "correct" even if we don't yet know which one (or never look).

You might then ask, but how does the measurement "know" which set of basis states to perform this decoherence with respect to? The answer to that is, the question is being asked backward-- all we know about the measurement is what basis states it decoheres, indeed we chose that measurement expressly because of that property. How it accomplishes the decoherence is what we don't know, but that's not at all unusual in science-- at least we do know why we don't know: we don't know because we have chosen not to track that information (usually it would involve the coupling to macroscopic noise modes that are quite untrackable anyway, but the principle applies any time we simply choose not to track the information, as can occur for one part of an entangled system). So I really don't see any "measurement problem" at all-- it is category (3) above.

This is another very common story, but to me what it does is confuse the first step of measurement, described above (and which is a real connection with physical noise modes of an actual apparatus), with the second step, which is the recording of the result in a conscious mind. The second step is indeed a formal step in "measurement" as the term is used in science, but is in no way central to the quantum mechanics of the problem. The quantum mechanics was over in step 1, the destruction of the coherences. Step 2 is no different at all from classical situations like a person playing a shell game and revealing which shell the pea is under. It's under one of them already, by virtue of the decohering of the amplitudes or the lack of need for amplitudes in the first place, but the player just doesn't know which. Why people think quantum mechanics, once the coherences are destroyed by the classical apparatus doing the measurement, is any different from classical physics, is beyond me-- I don't see any problem there other than we have no idea what a conscious mind is doing.

Thus my answer to von Neumann's chain (if it was indeed him) is that the measurement in the quantum mechanical sense (step 1) occurs as soon as the coherences are destroyed, i.e., the first stage of that chain, but the classical meaning of measurement (step 2) is not resolved until some later and less well determined stage-- but that much was already true for the shell game, and quantum mechanics adds nothing to it. I would call this category (2) from above-- when we have a working model of what consciousness is, we can better address this issue, but until we have a greater body of experimental data on that topic, we are shooting blanks and really shouldn't bother ourselves with it at this juncture.

I agree that quantum tic tac toe is an interesting game (congratulations), with some parallels with quantum mechanics that needn't be taken too literally. But given my answer above, I think you are trying to solve a "problem" of category (3). It is already clear to me that measurement in quantum mechanics (step 1 above) is an objective process, very akin to your quantum tic tac toe, and the Copenhagen interpretation already includes that just fine. I really don't know what all the buzz is about (and I know about non-unitariness and so forth, note that I already addressed that when I mentioned all the information that we have chosen not to track when a step-1 measurement occurs). The coupling to a device we can trust to behave classically, and therefore we know we are not going to track the full information of the reality, is an integral part of objective science, there's no other way to do science and therefore there is nothing to fix. I believe that is true to Bohr's way of looking at things.

ThomasT

This is not eliminated by Bell's theorem.

Common timelike cause and common spacelike cause are how quantum entanglements are experimentally produced in the first place. There just isn't a generally accepted expression with a visualizable (classical) analog to explain the correlations. What Bell showed is that orthodox quantum mechanics is incompatible with such an explanation.

Common spacelike causality is already an experimental fact. This has been done to entangle even somewhat large groups of atoms if I'm not mistaken.

The other sort of spacelike causality -- ie. instantaneous action at a distance -- is physically meaningless.

Of course, something is happening instantaneously in EPR-Bell experiments. When the setting at one end or the other is changed, then the global setting (and the probability of joint detection) instantaneously changes. Of course, this angular difference isn't a local object. It's simply an observational perspective.

There isn't any evidence to suggest that ftl or instantaneous actions or connections have anything to do with quantum entanglement. Thus, the appropriate path to take in considering all the stuff related to EPR, Bell, quantum entanglement, etc. is to assume that nature is local -- at least until something a bit more compellingly suggestive of ftl or instantaneous actions or connections is discovered or invented.

Ken G

I agree, and that's why I object whenever I hear someone claim that Bell-type experiments exhibit a nonlocal influence when a measurement is made. I see it as entirely local influences, being used to intentionally "unpack" nonlocal information. You only run into trouble when you ask "where is the information stored", and combine local thinking with realism. But these are problems for philosophy, not physics, and really just say that we need to tailor successful philosophies more carefully for them to be informed by physics. Philosophies should not, on the other hand, be used to inform physics-- the history of trying that is pretty clear on that point. (Even the principle of relativity, which is often pointed to as a kind of philosophy-informing-physics, is actually just philosophy-informing-form, i.e., informing pedagogy, not physics itself. In my view, anyway-- there's a relativity thread on this which draws much fire for that position and I'd have to say it's still unresolved).

Degeneration

I dont understand. Isnt it that there is no SIGNIFICANT information sent? Suppose I wish to receive a signal to turn on a lamp and I have one of two entangled particles. When the particle has an up spin, I am to turn on the lamp. My partner, a couple lightyears away decides to do something to his particle to change its spin to down. My particle instantly reacts with an up spin meaning that I am to turn on my lamp. Isnt information sent here, as primitive of a method it might be?

Ken G

No, because your partner cannot "decide" to make his particle be down, and expect that will make your particle be up. If the partner makes a decision and gets a certain spin by design, that would break the entanglement. The entanglement is only unbroken if the partner makes no such decision and simply measures the spin-- but then he has no way to influence whether or not you turn on the lamp. Nothing is transmitted, nonlocal information is simply being "unpacked" by the experiment.

It only seems like a nonlocal "influence" if you imagine that the information being unpacked is somehow stored in the two particles, such that changing that information represents a physical change in both particles, but I would argue that such is a purely philosophical picture that is clearly problematic and retains no value in quantum mechanics, any more than imagining that any wave function is "stored" in the same region of space as it takes on its values. I would say that the place a wave function "resides" is in the mind of the physicist using it, not in the region of space where it takes on its values, and many people may not even realize they are implicitly assuming the latter instead of the former when they agonize over entanglement and delayed choice.

Dar Kthulu

hello everyone, I am very new to this discussion and i have just a few questions regarding this topic.

1.) how are these particles affected by speed. do they gain mass? can that be measured?
2.) Is the communication of these paricles affected by gravity such as the gravity well around massive objects.
3.) what do you suppose of this experiment on an entangled pair? One is left here on earth and the other is placed aboard the International Space Station. both are observed.

Just some random thoughts and questions from a non student. Thanks for your time and information. :)

Ken G

They don't change rest mass, and if you consider the change in what is known as "relativistic mass", that is just a frame-of-reference issue, not a physical difference that should affect entanglement.
The GR effects should just affect the background spacetime through which the system moves, but I don't see a direct impact on entanglement except perhaps in strong gravity environments where we would need a combined theory of quantum mechanics and gravity.I think the normal quantum mechanical expectations, referenced to the system proper times, should work fine there.

DrChinese

This is one of those areas where there are some interesting opportunities to consider QM and GR as a pair. If GR is correct, and there is no graviton, then you would certainly expect that entanglement is not affected by gravitational field. That might not be true, on the other hand, if the graviton exists. There have been a few papers that have speculated on this point. Of course without a specific QG candidate to work with, it is hard to say too much. But there might be some limits which could be derived to steer a potential candidate theory.

http://arxiv.org/abs/0910.2322

"We propose a thought experiment to detect low-energy Quantum Gravity phenomena using Quantum Optical Information Technologies. Gravitational field perturbations, such as gravitational waves and quantum gravity fluctuations, decohere the entangled photon pairs, revealing the presence of gravitational field fluctuations including those more speculative sources such as compact extra dimensions and the sub-millimetric hypothetical low-energy quantum gravity phenomena and then set a limit for the decoherence of photon bunches and entangled pairs in space detectable with the current astronomical space technology. "

Ken G

That's interesting, it would be somewhat ironic if gravity waves are first detected via their interaction with sublimely constructed entangled quantum states, rather than the more brutely classical application of watching them make masses jiggle!

foxy1968

By using a minimum of 2 sets of qubits in isolation and by freezing the spin of the entangled particles and specifying that set 1 is used to indicate the start of a message and set 2 is used to send the message. By influencing the spin of the particles at one site and monitoring the spin changes at the other site why is this not possible. In this manner would it not be possible to send data over an infinate distance with no delay and therefore ftl.

K^2

Because you can't check whether the spin was "influenced" by the sender or by your own attempt to check whether it was influenced. Both give you exactly the same result, and so no information is carried.

Demystifier

Sec. 3 of
http://xxx.lanl.gov/abs/1006.0338
gives a simple explanation why entanglement cannot be used for ftl signalization.
It also proposes how this inability (to use it for ftl signalization) could, in principle, be overcame.

Brokinarrow

So basically the reason FTL communication is not possible using quantum entanglement: Currently we cannot control the state of the entangled particles, we can only observe the changes that nature is making to the state of the particles. If we could figure out a way to control the state of these particles, FTL communications would be possible.

Albert V

I've read some papers that were aiming to use linearly and circularly polarized light as a protocol for communication - however, it seems difficult/impossible to distinguish these two when you have to rely on incident photons (eg. Physics Letters A
Volume 251, Issue 5, 1 February 1999, Pages 294-296). Anyone with an idea?

I recently saw another ideá from Arxiv.org. I am not able to discover the flaw in his argument, but I suspect that there will be no interference?

http://arxiv.org/abs/1106.2257

Albert V

This is a discussion of the Cornwall paper on superluminal communication from

http://arxiv.org/abs/1106.2257

quantum theory says that whatever you do on one side does not change what you observe on the other:

Total state |Phi> = (|H>|V> + |V>|H>)/sqrt2.

Not using the polarizing filter (no modulation)

rho = |Phi><Phi|
= (1/2) ( |H>|V><H|<V| + |H>|V><V|<H| + |V>|H><H|<V| + |V>|H><V|<H| ).

In order to see what we observe on the left side we have to "trace out" the right side

rho_right = Tr_left(rho) = (1/2) (|H><H|+|V><V|),

which is eihter a photon in the mode H or a photon in the mode V, which will give no interference.

Am I mistaken?

N468989

If I understand correctly, then quantum entanglement is explained by the simple fact that two particles behave the same way after being separated.

Take Machine A and B, each compute numbers from 1 to 10 and are synchonized. Separate the machines and get the output at a given moment in time. We know what the other machine reads, is this correct?

Another thing is to assume that something is propagating through space... (http://www.youtube.com/watch?v=QErwOK3S5IE)