Talk:Bell state

message: Hi, I think that to make the discussion more clear would be important to specify, within this article or within another, how is it actually possible to perform a Bell measurement as this is "It is the crucial part of quantum teleportation". Without a better description of what is a Bell measurement, from my point of view, both the article on quantum teleportation and this one remains too abstract and not accessible. 
Dave.bradi  —Preceding undated comment was added at 18:12, 22 August 2008 (UTC)
replies: text: One could replace bell measurement with measurement in the bell basis. Skippydo (talk) 00:10, 23 August 2008 (UTC)

text: I think the diagram related to this part of the article is not correct. The input-qubits need not be entangled c.q. be in a EPR-state like the diagram suggests. 2A02:A463:2848:1:2D3E:B130:FF82:193F (talk) 13:01, 8 August 2023 (UTC)
thread_title:  Bell state measurement  

message: I read:
A Bell state is defined as a maximally entangled quantum state of two qubits.
This seems wrong: |0>|+> + |1>|-> is also maximally entangled and is not one of the four Bell states.
Yves hochedez (talk) 08:36, 6 July 2013 (UTC)
replies: text: your state is not normalized correctly (factor 1/sqrt(2) is missing). When you normalize correctly indeed the von neumann entropy is also ln(2) like for the bell states. But the article states (at least now) that the bell states are specific maximally entangled states.--Biggerj1 (talk) 21:11, 10 April 2014 (UTC)
thread_title:  Definition of Bell states 

message: Unlike classical phenomena such as the nuclear, electromagnetic, and gravitational fields, entanglement is invariant under distance of separation...
This seems like a category mistake, or just sloppy writing. 
If "invariance" refers to the Lorentz group that enforces separability, then indeed, all of the phenomena listed, including all features of quantum mechanics, are invariant under this group.
Indeed, "nuclear, electromagnetic, and gravitational fields" are not "classical phenomena". The first two are quantum phenomena, and electromagnetic fields are the main place where entanglement phenomena can be physically demonstrated. 
On the other hand, if the quantum phenomena are modelled by Hilbert spaces and their operators controlled by a dagger category in a kind of "timeless" universe, then they can't be compared very well with such things as nuclear forces with respect to geometric questions.
178.38.97.233 (talk) 12:41, 2 May 2015 (UTC)
replies: text: I updated the introduction to be more precise, just now. 2001:2002:51E3:8007:B66D:83FF:FE0E:C298 (talk) 19:33, 23 August 2017 (UTC)
thread_title:  Doubtful statement 

message: Bell pair links here, but is not mentioned in this article.  Presumably, this is the same as an EPR pair, as mentioned in the article?  — Preceding unsigned comment added by 70.247.162.192 (talk) 04:05, 6 May 2016 (UTC)
replies: text: "EPR pair" in this article links to the article on entanglement, which however does not contain the term "EPR pair" anywhere.  What's the best fix?  Vaughan Pratt (talk) 17:22, 30 December 2017 (UTC)
replies: text: I am fairly certain that there once was an article named "EPR pair", that was merged into this article (Bell state). A lot of articles reference EPR pair and is redirected to this article, but then the phrase EPR pair does not appear in this article.. I think the two are exactly equivalent. That is, an EPR pair is a Bell state. It seems throughout the literature and random searches on the web, that EPR pair and Bell state is exactly identical. I have not found any definition or anything though!  · · · Omnissiahs hierophant (talk) 12:08, 19 April 2021 (UTC)
thread_title:  Bell pair 

message: When first defining what it means for a state to be a Bell state, the page defines what it means for the states to be entangled as follows.
Their entanglement means the following:
The qubit held by Alice (subscript "A") can be in a superposition of 0 and 1. If Alice measured her qubit in the standard basis, the outcome would be either 0 or 1, each with probability 1/2; if Bob (subscript "B") also measured his qubit, the outcome would be the same as for Alice. Thus, Alice and Bob would each seemingly have random outcome. Through communication they would discover that, although their outcomes separately seemed random, these were perfectly correlated.
This seems to specifically exclude the two Ψ Bell states (in which Bob's outcome would always be the exact opposite of Alice's). Perhaps this section could be clarified to indicate that this is only an illustration of what it would mean for two states to be entangled, rather than a definition covering all possible cases.  J.Gowers  11:29, 17 October 2023 (UTC)
thread_title:  Confusing example