Talk:No-communication theorem

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Added conflict of interest label, clear dispute by the author(s)on the subject, weasel words and general disagreement. Certainly not an expert in the subject; suggest weaving opposing viewpoints into single article that presents both ideas before removing label. —Preceding unsigned comment added by 130.215.232.181 (talk) 03:24, 5 December 2009 (UTC)

It's going to be hard to find anyone competent to write on such a specialised topic who is not close to it. And because it is technical, what can usefully be said without the mathematics? Mike --Netteville (talk) 11:08, 30 December 2009 (UTC)

Not sure if it's relevant but there was a semi-recent experiment that pretty much destroys this theorem. http://news.softpedia.com/news/Entangled-Light-Stored-in-Cooled-Atoms-80659.shtml --Mike reddog418

This proof only proves that it can't be done instantly since it assumes no time passes. This doesn't really seem to say anything about whether we can communicate faster than the speed of light, just that we can't do it in zero time flat. I keep hearing that the no communication theorem disproves superluminal communication completely- does this mean that there is a more complex proof somewhere that handles extremely brief time evolution? Particularly, the time frame that would be needed to surpass the speed of light? Even if it's too elaborate for the article, it should be described if the result exists. AllUltima (talk) 19:13, 27 March 2010 (UTC)

Not true. The no-communication and superluminal theories imply that the speed of light is a cosmological constant, and that with absolute certainty, cannot be violated. However, this assumes that particles have well-defined individual properties [hidden variables]; they do not. This was also the assumed hypothesis of the EPR experiment performed by John Bell-- if there was no communication going on that violates the speed of light, then John Bell's experiment would have resulted in locality, solidifying the cosmological constant of light, and proving that Einstein's objections of assumed realism were correct all along. It did not; information transmission between particles is instantaneous. Both this and the superluminal article approach quantum physics from the world-view of EPR, and not quantum physics in itself. Here's an article by Alain Aspect from a 1999 issue of Nature magazine that explains everything and cites further repeated experiments. — Preceding unsigned comment added by 209.105.184.93 (talk)

According to Special Relativity, for any two events which are connected by a super-luminal signal (which therefore are the endpoints of some space-like vector) there is a frame in which an observer would see the events as exactly simultaneous. In other words, for any space-like vector, there is a Lorentz transformation which makes the extremes to have the same time coordinate t (and vice-versa). Therefore, proving the theorem (in a Special Relativistic setup) for all simultaneous events means having proved it for any super-luminal speed (unless I'm missing something which goes beyond Special Relativity) 89.202.228.211 (talk) 14:06, 26 July 2010 (UTC)

Regarding erasers experiments. Do I understand correctly that the only reason they do not show FTL communication is because of the filtering? Is the filtering really needed? What if the source of the entangled particles already provides a well defined state - say, with a given polarization? If that is the situation, it would be better to return to the version were no final conclusion is made. Srjmas (talk) 10:28, 14 October 2010 (UTC)

The coincidence counters are what prevents FTL communication. Those counters are needed because a quantum eraser will actually produce two sets of interference patterns phase shifted from each other. So if you superimpose those interference patterns, you get essentially the same result as you would without an interference pattern. 184.100.111.110 (talk) 17:59, 15 November 2010 (UTC)

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means that Alice's measurement apparatus does not interact with Bob's subsystem

But this is what communication with Bell-pair is about: That Alice’s measuring apparatus interact spontaneous with Bob’s receiver system. The theorem only holds for the situation, where no successful connection is created between Alice’s system and Bob’s system. UChr (talk) 20:36, 6 January 2011 (UTC)

I think it is a no – proof – theorem. The proof is circular. It postulates: no connection / independency between Alice and Bob and uses this for later to show: no connection / independency between Alice and Bob. UChr (talk) 11:11, 15 January 2011 (UTC)

Opposing Viewpoint

The following is misleading in suggesting super luminal communication because it does not state all the conditions of the experiment.

"it is possible to cause or prohibit an ensemble of photons into making an interference pattern on a screen, by remotely manipulating their entangled twins"

There is no interference pattern without also the use of "Coincidence Logic" as shown in fig 3 of of the Zeilinger reference (Dopfer 1998 experiment). This coincidence logic requires a sub luminal communication from the non slit half before the interference pattern can be shown. Therefore there is no suggestion of super luminal communication and this section of the Opposing Viewpoint should be rewritten or removed. —Preceding unsigned comment added by 83.59.122.251 (talk) 00:23, 16 February 2011 (UTC)

--- The best way to argue against a theorem is to prove the opposite – here as a gedanken- experiment. Idea: To take advantage of quantum entanglement to create superluminal communication.

Alice and Bob are measuring corresponding particles from pairs of quantum mechanically entangled photons - so-called Bell-couples – here called A and B.

The distance between Alice and the source are shorter than between Bob and the source, so with synchronized watches Alice will measure A before B reach Bob.

Alice can choose between inserting a mirror or not. She keeps her choice for an agreed period – for instance 1/100.000 sec. In that time perhaps 1000 pairs are sent to Alice and Bob, respectively.

By measuring his part of the 1000 pairs Bob should, with at least 99% probability, guess what Alice has chosen? Can he do so there is a practical basis for communication? As the exchange between Alice's particle and Bob's particles takes place immediately it will for a growing distance between Alice and Bob create superluminal communication.

Alice 0 = No mirror: A meets a polarizing beam splitter = PBS (0 / 90) who transmits horizontal polarized light and reflects vertical polarized. Photo detectors ensures that A has chosen one of the two options and not just remains in a superposition of both.

Alice 1 = with a mirror: A meets a PBS (+45 / -45). Photo detectors should again detect approximately 50% / 50%.

BOB's RECEIVER: Bob's receiver starts with a PBS (0 / 90).

The theory says that B will behave as polarized opposite to A - i.e. perpendicular to the direction A was measured.

Alice 0: Perpendicular to the vertical is horizontal and perpendicular to the horizontal is vertical. B will therefore behave as polarized either vertically or horizontally. When B reaches Bobs polarizing beam splitter (0 / 90 = horizontal / vertical), B will choose the road corresponding to its polarization. B will follow exactly one of the routes.

Alice (1): Perpendicular to ‘+45’ / -45’ are ‘-45’ / +45’. Both the ‘+45’- and the ‘-45’ - photons have a dilemma. With probability 0.5 to 0.5 they must split up in a horizontal part and a vertical part – a superposition.

I: With reasonable coherent light we could just use a ‘Mach–Zehnder’ interferometer (starting with the PBS (0 / 90) as Bob’s receiver.

II: But probably we must expect a difference on a half wave between the ‘+45’- and the ‘-45’ – photons, so the wave-difference repeals each other. If the Alice 0 light is reasonable coherent we could use the following construction: We let both the horizontal polarized light and the vertical light be followed of a ‘Mach–Zehnder’ interferometer, which is starting with a PBS (+45’ / -45). Alice 1: because of the half wave difference the two detectors at the interferometer will show approximately 50% / 50% howsoever we have tuned the interferometer. Alice 0: If the light is reasonable coherent it should give different output, when we have tuned the interferometer.

III: If all the light has the problem with a half wave-difference we start the Receiver as earlier with a ‘Mach–Zehnder’ interferometer (starting with the PBS (0 / 90). Alice 0: the horizontal photons will follow one route – meet a mirror and a balanced beam-splitter BS – and leave by gate 1. The vertical photons will follow the other route – meet a mirror and a half wave plate, which will change the vertical polarizations to horizontal – and leave through the BS by gate 2. Both the outcomes will be a mix with half a wave – difference, so when we let the photons from gate 1 meet a new ‘Mach–Zehnder’ interferometer (starting with a PBS (+45’ / -45), the detectors will show approximately fifty – fifty howsoever we have tuned the interferometers. (and same procedure from the photons from gate 2). Alice 1: If we tune the starting interferometer smartly we could split the mix so the outcomes from each gate are reasonable coherent, and then we could get interference by the next interferometers. UChr (talk) 01:48, 10 March 2011 (UTC)

Text deleted UChr (talk) 18:51, 5 June 2011 (UTC)

New Attack Plan

This so called theorem has many problems on this wiki because there is more than one version of the quantum eraser experiment, at least three. Each of them cannot be analyzed for FTL communication (or lack thereof) in the exact same way.

I suggest we analyze each QE experiment first before we give reasons as to why there is or there is not a no communication theorem. I suggest we just state the no comm theorem in this page and give no reasoning behind it. We could even demote it to the no comm hypothesis.

Let's discuss the viability of the no comm hypothesis as we analyze all three versions of the quantum eraser experiments. Then once a consesus is reached we can come back and give reasoning for why the hypothesis is or is not valid. --97.104.194.126 (talk) 03:29, 5 July 2011 (UTC) ldussan

I have now made a critical comment UChr (talk) 17:54, 21 September 2011 (UTC)