Talk:Quantum state

Spin

"which" tag

There is a {{which}} template in the Spin section, but I think that it can be safely removed. "A quantum system" means "any quantum system", therefore in my eyes it does not make sense to ask which system. Petr Matas 09:09, 8 September 2016 (UTC)

Pinging 70.247.163.128. Petr Matas 09:13, 8 September 2016 (UTC)

Other problems

This section has additional problems. Relativity is not needed for spin. Not even QM is needed for spin (the classical EM field has spin one). (Also The Pauli equation, usually tossed off as "phenomenological", can be deduced with about the same rigor as the Dirac equation, at least this is the claim in Quantum mechanics, by Walter Greiner.) It is all about under which (projective) representation of SO(3) (non-relativistic theory) or under which (projective) representation of SO(3, 1) (non-relativistic theory) the solutions of the theory transforms. YohanN7 (talk) 10:38, 10 September 2016 (UTC)

Error in article?

I think there is a severe problem with the part of this article between the phrases "A more complicated case is given" and "the particles are arbitrarily far apart."

The problem is with the premise "superposition of joint spin states for two particles with spin 1/2".

In particular, the problem is that a requirement that the two particles be entangled, that is, that they share a single quantum state, is omitted.

The problem can be seen in the case of two particles with spin 1/2 that are not entangled. For any two such particles, this section is incorrect. For example, both particles can be measured with their spins in the down direction. David Spector (talk) 13:18, 14 May 2020 (UTC)

The article is perfectly correct. The state in question is the singlet, and it is defined in an equation. The properties that are mentioned are properties of the singlet, not of any quantum state. It wouldn't hurt to mention that this state is entangled, though. Tercer (talk) 15:35, 14 May 2020 (UTC)

I went to edit this myself, but then realised that the entire lead is a catastrophe and needs to be rewritten anyway. Tercer (talk) 15:39, 14 May 2020 (UTC)

Yes, that bit of the lead is technically correct, but the lead overall is a disorganized, overgrown mess. It's hard to tell how even to start fixing it; quite possibly, the best thing to do would be to ignore it and start an entirely new write-up in a sandbox. XOR'easter (talk) 18:18, 17 May 2020 (UTC)

OK, I tried my hand at shortening the introduction. I think it's better than it was before (which is of course not the same thing as being good outright). XOR'easter (talk) 19:38, 17 May 2020 (UTC)

Thanks for your work! It did improve, although I concur that it's still not good. It is still too long, and I'm skeptical of putting equations in a lead. Most of the material should me merged into the body of the article. I'll get to it eventually, but I can't make promises as I'm rather busy IRL. Tercer (talk) 21:06, 17 May 2020 (UTC)

Yes, I can imagine times when putting an equation in the lead would make sense, but this isn't really one of them — why that vector instead of any of the other equally famous examples? This is a busy time off-wiki for me, too, and not a week/month when I should be starting big new projects here, and planning out a new organization for this article (which might be what's necessary) sounds at least moderately big. XOR'easter (talk) 14:55, 18 May 2020 (UTC)

I waded right in here. Please review the new lead, but perhaps reply on the following topic to avoid getting too deep. Johnjbarton (talk) 03:23, 16 June 2023 (UTC)

"Quantum" but not "state".

The lede is pretty good here, but all of the rest of the article fall strong on "quantum". There is no beginner level discussion of "state" and no link to address this question either.

The disambiguation page has "State, a complete description of a system in classical mechanics," which as exactly zero content on "state". There is Phase space and it has pretty pictures, but again nothing at a beginner level for "state".

Since almost 100% of QM discussions are about "state", it's easy to overlook the fundamental strangeness of the concept for beginners. I wonder if anyone has any good resources to use as references to write more about "state"? Johnjbarton (talk) 16:28, 14 June 2023 (UTC)

I hijacked the lead to address this issue. Please review.

The hijacking did some damage to the pure/mixed content which was already a muddle. I'll take that up next. Johnjbarton (talk) 03:25, 16 June 2023 (UTC)

I ended the lead with a summary paragraph that serves as an outline and thus as a plan for revising the article. The concept that emerged was to use this article as an overview of concepts and as gateway, a summary for the many other pages discussing "quantum state". I will proceed along that plan until someone stops me ;-) Johnjbarton (talk) 14:49, 16 June 2023 (UTC)

I reorganized along the lines I outlined, with major sections "Wave function" and "Formalism" then pure and mixed underneath. Some content in the wave function part still needs to move to Formalism.

Broadly I hope the Wave function part will be more approachable matching how QM is mostly taught. I'm leaning pretty heavy on Messiah's classic which might not be everyone's favorite but reliable.

I feel like I have cleaned up the damage I did earlier; I believe the material that was in the article previous is in as good a shape as it was and now it sits in a better context. Suggestions and comments welcome. Johnjbarton (talk) 17:30, 16 June 2023 (UTC)

photon travel

The current revision has this (unreferenced) sentence in the section on superposition:

The photon state is a superposition of two different states, one corresponding to the photon travel through the left slit, and the other corresponding to travel through the right slit.

As is typical in many cases where "photon" appears, this description misleads readers to imagine tiny balls flying through slits. "travel" implies time and the interference effect in the double-slit experiment is not based on time. Or tiny balls.

I will correct this and try to re-point the content towards addressing "quantum state". Johnjbarton (talk) 23:55, 17 June 2023 (UTC)