Entanglement and interference

[bonepile]

I have recently finished reading John Gribbin's book <i>In Search of Shroedinger's Cat</i>, and it left me with a big question concerning quantum entanglement and FTL communication. It seems to me that there is a simple experiment that could demonstrate instantaneous communication if my understanding is correct.<br /><br />Before I get into this hypothetical experiment, let me say that I have heard a lot that relativity forbids FTL communication. Does anyone know why this is? I understand that light and matter cannot travel faster than light, but information is a theoretical concept with no direct dependence on any underlying physical reality.<br /><br />Anyways, on to my little experiment:<br /><br />1) Create a stream of entangled photons moving in opposite directions. The receiver will receive photon stream <i>R</i>, and the transmitter will receive stream <i>T</i>. The receiver and the transmitter are nearly equidistant from the source of the photons, with the transmitter being slightly closer.<br /><br />2) On the receiver side (stream <i>R</i>), set up a version of the double-slit experiment. However, instead of having two parallel slits, use orthogonal slits.<br /><br />Here is the big IF in this experiment. When the photons reach this double slit plate, the polarization is unknown. Therefore, the light should be able to pass through both the vertical and horizontal slits. This should create an interference pattern on a second plate behind the slits.<br /><br />Is that correct? If it isn't, then my experiment will not work, but if it is...<br /><br />3) At the transmitter side (stream <i>T</i>), simply transmit a "1" by inserting a polarizer in front of the photon stream. It does not matter what the polarization of each photon actually is - once you insert the filter, the polarization of photons in stream <i>R</i> should become "known", and the interference pattern should go away.<br /><br />4) At the receiver side, interference = 0, no interference = 1.<br />

 

[qso1]

BonePile:<br />I understand that light and matter cannot travel faster than light, but information is a theoretical concept with no direct dependence on any underlying physical reality.<br /><br />Me:<br />I'm no expert on this stuff either but I noticed one comment you made which I would address this way. Info may be a theoretical concept, but how we transmit info is based on currently understood scientific where radio waves and carrier signals are concerned. The limitation is the signal itself and the signal is what carries the information. I hope I got that right. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[alkalin]

Parallel slits work well because there is enough diffracted light from each to cause interference that is clearly visible. I would seriously doubt that the arrangement you describe would produce sufficient photons in interference to be seen readily but I could be wrong.<br /><br />Why not just look at the possibility that all you might need are polarizers since you can polarize light in certain ways, and you can detect that polarization with a polarization system at the receiver. You would look for intensity of the beam or its polarization, not interference.<br /><br />I would guess that the problem is finding a method of creating enough ‘entangled’ photons to do this, in fact I think photons are not likely candidates for this experiment, but I’m not sure of that. I have a few years of work in optical engineering, but I do not know some of the physics involved. I think you are right that this very likely has been thought of before. <br /><br />Another thing to consider is that if the experiment worked and entanglement is a real phenomenon with photons, then how to proceed to the next step. The full use of this, it seems to me, is to have electrons at a source and electrons at a receiver, where you observe their flip states resulting from changed states at the transmitter. Using electrons in this manner eliminates signal delay your experiment has with photons. But hasn’t this been done already?<br />

 

[jaxtraw]

Couple of quick observations. Firstly, IMPO "the speed of light" isn't an ideal term. C is IMV better described as "the maximum speed of information transference" (a bit of a clunky term <img src="/images/icons/smile.gif" /> ) Light is better seen as information in this context. C is just the maximum speed anything can go at, and light goes as fast as it can. A bit like a 40mph speed limit being described as "the speed of cars" <img src="/images/icons/smile.gif" /><br /><br />I think your experiment falls afoul of the Bell Inequalities. For some unknown reason the Universe strives very hard to stop anybody getting around its maximum speed c, and is clearly conspiring to prevent clever experimenters "cheating". So your polarisers will destroy the information sent by "T" and cause a fresh measurement which will be unpredictable (within the parameters calculated by a wave function). That's what the Inequalities show happens.<br /><br />I think you'll just get randomly polarised photons out, in line with some equation involving cos theta or something, I'll be bound. Much like the trick with three layers of polarising film- set 1 and 3 at 90 degrees, no light gets through. Insert film "2" between them at some arbitrary angle and light will get through now because two measurements are occurring- between films 1 and 2, and 2 and 3; the proportion of photons passed by each pair of films is um 1/cos theta squared or somethign, can't remember right now, luckily I'm not a physicist so my ignorance is allowable <img src="/images/icons/smile.gif" /><br /><br />Anyway, I think your R polarisers just destroy the information "sent" by T. If they don't, expect a call from the Nobel Prize committee...

 

[h2ouniverse]

Bonepile,<br /><br />Actually this kind of experiment has been run. Alain Aspect in 1981 was one of the first to get experimental evidence of the Einstein-PodolskyRosen paradox. As you said, this is on a statistical basis (violation of Bell's law).<br /><br />More recently in Geneva, Antoine Suarez even succeeded in breaking causality, with the interaction with the first photon occurring "before" the second measurement in the timeframe of the second member of the pair (by playing on relativistic effect, the other part of the experiment moving wrt the first part.<br /><br />Conclusion:<br />distance, time, and therofore speed are illusory notions<br /><br />Regards.<br /><br />

 

[jaxtraw]

...which demonstrates that our understanding and interpretation of QM is hopelessly inadequate <img src="/images/icons/wink.gif" />

 

[h2ouniverse]

I disagree. I think it simply demonstrates our so-called "commonsensical" understanding of the universe is really really inadequate. (I mean, more than usual) <img src="/images/icons/tongue.gif" /><br /><br />I no longer believe in locality. I am seriously pondering a vision of universal "co-location". <br />

 

[bonepile]

<blockquote><font class="small">In reply to:</font><hr /><p>I'm no expert on this stuff either but I noticed one comment you made which I would address this way. Info may be a theoretical concept, but how we transmit info is based on currently understood scientific where radio waves and carrier signals are concerned. The limitation is the signal itself and the signal is what carries the information. I hope I got that right.<p><hr /></p></p></blockquote><br />That is how I have understood it. In that case, relativity has nothing to say on whether or not some bizarre quantum phenomenon can be leveraged for FTL communication.

 

[bonepile]

Alkalin,<br /><br />Thanks for the input!<br /><br />If you use polarization or electron spin by themselves, I do not see how you could know if the polarization or spin measurement had occurred on the transmitter side. The problem is that you do not know what these properties were to begin with. Now if there were some way to control them... but any effort to do so would probably violate the Uncertainty Principle.

 

[bonepile]

<blockquote><font class="small">In reply to:</font><hr /><p>Anyway, I think your R polarisers just destroy the information "sent" by T.<p><hr /></p></p></blockquote>That wouldn't surprise me. However, in this case, how could you verify that the photons were entangled to begin with, if measuring the polarization of <i>T</i> results in the randomization of <i>R</i>? However, if only the other two axis of polarization (other than the one you measured) get re-randomized, then the interference pattern would still be destroyed. Of course, the <i>T</i> side polarizer must be oriented the same way as one of the orthogonal slits. (Actually, now that I think about it, that might pose a practical problem over long distances.)<br /><br />I suspect I'm overlooking something big here.

 

[h2ouniverse]

Bonepile,<br />I really think Aspect's experiment was very close to what you are considering. You can have statistics in such a context to demonstrate the entanglement.<br /><br />To me there is unicity of the particle as long as the wave-function has not collapsed.<br /><br />Regards.

 

[vandivx]

<font color="yellow"> I have heard a lot that relativity forbids FTL communication. Does anyone know why this is? I understand that light and matter cannot travel faster than light, but information is a theoretical concept with no direct dependence on any underlying physical reality.</font><br /><br />information may be abstract concept but getting it accross a distance is a damn right physical affair and relativity applies here<br /><br />entanglement and FTL communication has a lot similarities to perpetuum mobile thing, I mean the latter has been proved imposible in principle but people still think that if you do some involved shenigans that it may work<br /><br />entanglement in its bare form (in principle) has been shown incapable of FTL communication and you now think that if you do some sheningans that nobody has thought about yet (as you think) that you could make it work<br /><br />you have to understand that once something had been shown not to work in its basic bare form and in principle that no sophisticated build up of the experiment can't change that fact, all that does is it presents more obfuscation making it seem plausible that it might work<br /><br />but like with perpetuum mobile people will keep coming up with involved schemes that they think might work because they do not understand the reasoning why it won't work in the basic case, in principle<br /><br />what defeats us in the case of entanglement is the randomness of the phenomenon which prevents us from being able to use that for any sort of FTL signalling<br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

 

[nova_explored]

I don't think you would need a polarizer on the transmitter side. Considering entanglement involves 'a hidden variable' where we cannot know the state of the particle until we act on it, by doing so we would see just what that state is by how it affects its counterpart. The catch is, not having a predetermined state, while in keeping with the uncertainty principle, doesn't allow for specific transfer of information. I gather that is why you suggested using a polarizer on the transmitter to begin with. <div class="Discussion_UserSignature"> </div>

 

[nova_explored]

Oh, and the scientific reason why information cannot be transferred FTL is that for quantum transfer, or teleportation, requires a classical system to register that information, and all classical systems operate within the parameters of C.<br /><br />You can vie back and forth between the two states in different ways, converting source information back and forth much like we do in classical physics with electrons, however, unlike classical physics that allows dual interchangeability, you cannot take a quantum system and put it into a classical system and back again, simply because that quantum state breaks down once converted and can never be duplicated, or re arranged back. since a quantum state exists in multiple matrix (density matrix) it requires more than one measurement, and since it breaks down upon measuring, it is impossible to get an accurate measure for a 'pre-determined' state.<br /><br />This may be getting murky, so i'll stop for now.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[bonepile]

<blockquote><font class="small">In reply to:</font><hr /><p>The catch is, not having a predetermined state, while in keeping with the uncertainty principle, doesn't allow for specific transfer of information.<p><hr /></p></p></blockquote>In a classical system, this would be true. However, the double slit experiment invalidates this premise by the simple fact that you can detect whether or not a photon has a predetermined state. Sure, you have no idea what that state is, but that is not important. In my opinion, this is enormously significant. Nature seems to have left a gaping loophole. Once you can reduce a system to a "yes" or "no" (does this particle have a known state or not), and if you can control the result of this in any way, then the system holds usable information.<br /><br />In the double slit experiment, you <i>can</i> control the results, but there is no practical benefit because you do not know whether your photons have a known state or not until they travel to the plate (at C). So we know that we can detect if a photon has a known state or not, and we know we can control the results. Add in some entanglement, and there does not seem to be a fundamental reason why we could not communicate FTL.<br /><br />It is true that placing a polarizer in the path of a photon would eliminate polarization uncertainty on one axis but create new uncertainty on others. This is ok. We know that placing a polarizer in the path of a photon does not result in the complete randomization of that photon's polarity because light will not pass through two orthogonal polarizers. This means that if entangled photon A passes through a vertical polarizer, then entangled photon A' should not pass through a horizontal one. The axis of polarization that you measured should be known for both photons. I suppose you could say that after placing a polarizer in front of photon A, photons A and A' are only partially entangled.<br /><br />Anyways, there are obviously a lot of details here that I d