Talk:No-communication theorem

Sorry, but...

I know, I Know, this is not a forum. But I honestly think this theorem is very, very broken. He shoves dozens of mathematical obscure formulas to try to furiously deny that it can happen something that is logically simple to analyze: We have two points, each with a photon "tied" to another by quantum entanglement. We know that if the spin of the photon "A" change, this change is reflected immediately in the photon "B".

Knowing this, the observer "B" does _not_ need dozens of mathematical expressions to know that if the photon "B" spin change, was because the observer "A" changed the spin of the photon "A". How can this be so difficult? More valid questions would be:
a) The change in spin is really instantaneous? This can be tested, although it is difficult;
b) The change in spin can be caused by external interference? If yes, so which ones?
c) The act of trying to measure the spin breaks, or can break, the entanglement?


Less "Oh No!! It will break my preferred theory! heretic!", more "Let's see what happens" science please. 200.189.118.162 (talk) 17:11, 18 March 2013 (UTC)

Changing the spin of system A does change the spin of system B, of course. The point of the theorem is that a scientist at lab A cannot control whether the system decoheres into spin up or spin down (in the entangled photon example- whatever analogue is applicable for other systems) along the axis of measurement. The result is that, regardless of anything the scientist in lab A does, system B has (not knowing the result of the measurement of system A) a 50/50 chance of observing spin up or spin down. A scientist at lab A can, after measuring her system, determine what system B will be measured as, but can in no way affect that measurement. Somephysicist (talk) 01:15, 16 February 2014 (UTC)

The article provides a solid overview of the no-communication theorem but could clarify its implications for quantum entanglement and causality. Specifically, it should explain why "spooky action at a distance" doesn’t enable faster-than-light communication. Adding references to experimental confirmations and theoretical discussions would also help provide a more complete understanding of the theorem's significance. AaronLuo.tulane (talk) 15:37, 31 October 2024 (UTC)Aaron

@Somephysicist that does not seem to be a general statement. That is a fact for that particular example but it doesn't seem translate to other experiments where there are more than two possibilities. Additionally more and more quantum eraser experiments (Kim et.al, Dopfer) are indicating our accepted understanding of the wave function is wrong.

Take the case of entangled photons in a momentum state such as in the Kim et.al. QE experiment, one photon that is "path-observed" changes the interference pattern of its partner. INSTANTLY. If this is true then the only thing that is missing to form an FTL communication device is being able to produce TONS of entangled photons at a time and instantly imaging the interference pattern. That should be a function of technology.

So what I'm saying is that one could in essence send an intense pulse of entangled photons where the partners after the non-linear (e.g. BBO) crystal are sent via fiber to e.g. NY and LA from Austin, TX. Although not shown to be necessary let's say that path lengths to each detector in NY and LA from the Austin source are exact. Then LA could observe "which path" information and affect INSTANTANEOUSLY the interference pattern of the clump of photons at NY. NY would not need a classical channel enabled coincidence counter to extract the interference pattern over time. They could just see it. Instantly. A little like shown in this video https://www.youtube.com/watch?v=wGkx1MUw2TU.

1's and 0's are transmitted by the change of the pattern. The pulse repetition rate would determine the bandwidth. But like I said this requires a source that produces a clump of momentum state entangled photons. This does NOT require a bunch of cloned photons. This probably is not the only way either. Even in the video above to really see the pattern they have to take several averaged frames but that is a technicality not a limit of physics.

It almost seems like the wavefunction is an object that propagates and evolves instantly, or superluminaly at least, while the transfer of energy (not information) or matter must happen at or below c. Ldussan (talk) 22:44, 2 April 2014 (UTC)

Wiki Education assignment: Quantum Information Science and Engineering

This article was the subject of a Wiki Education Foundation-supported course assignment, between 19 August 2024 and 13 December 2024. Further details are available on the course page. Student editor(s): AaronLuo.tulane, LockeW (article contribs). Peer reviewers: Mahabib01, Ethan Scott Bravo Quinonez.

— Assignment last updated by Phyhuan (talk) 04:11, 13 December 2024 (UTC)

Large additions to "Informal overview" and Key Concepts

@LockeW Thanks for your recent additions. I hope you have just started and are open to some feedback. The use of dates in the Informal overview makes it sound like a History section. We should have History or Overview but not mix it up. The section should be based on secondary references because just taking the word of the primary ref is not reliable. And Key Concepts seems odd: its just a list of things with nothing gluing it together or to the article and no sources. Johnjbarton (talk) 04:20, 6 December 2024 (UTC)

Thanks for your reply. Based on the comments in the talk section of this page, we are trying to find out in which paper no communication was first mentioned. Next, we will try to find the secondary references of the three papers we added. AaronLuo.tulane (talk) 08:46, 6 December 2024 (UTC)

Relation to no-signalling principle.

So far at least I have found only one secondary reference that discusses the "no-communication theorem". Very little shows up on Google Scholar for this term. The only reference cited in the article that specifically says "no-communication theorem" is the Peres/Terno review. That review matches the math (AFAICT) in the "Formulation" section and seems to match the GHIRARDI paper.

Here are some things I did find:

• This impossibility of faster-than-light communication assisted by quantum entanglement is known as the no-signaling principle. Azuma, Koji, et al. "Quantum repeaters: From quantum networks to the quantum internet." Reviews of Modern Physics 95.4 (2023): 045006.
• More generally, we refer to the impossibility of sending information arbitrarily fast as the no-signaling principle. Masanes, Ll, Antonio Acín, and Nicolas Gisin. "General properties of nonsignaling theories." Physical Review A—Atomic, Molecular, and Optical Physics 73.1 (2006): 012112.

So it seems to me that the no-signalling principle is an axiom rather than a theorem equivalent to the no-communication theorem. The proofs of the no-communication theorem seem to be QM, whereas Eberhard's work cited here is QFT. The article claims without a source that the Eberhard proof is equivalent. They have similar consequences but I doubt the non-relativistic proof is really equivalent to the QFT result. Johnjbarton (talk) 00:15, 15 December 2024 (UTC)

@ReyHahn Any thoughts on this article? As far as I can sort out, the "no-communication theorem" is a strictly QM thing, no real space or time involved. Its conclusions match the axiom "no-signalling" but that is the only overlap. And the Eberhard work is completely separate. Johnjbarton (talk) 00:13, 20 December 2024 (UTC)

I have to take a look but I think that the no communication theorem is a more obscure theorem that has popularized due to Wikipedia, the most commonly cited statement in papers is the no-signalling theorem but I have to understand what is the difference too. Note that some people call it the no-signaling theorem.

After a careful reading, I think I agree with you.--ReyHahn (talk) 08:43, 20 December 2024 (UTC)