Talk:Quantum superposition

living thing?
There is a temptation to put in 'gee-whizz' items. In particular, I refer to the claim implicit in one example here that viruses are living things. And now we have as another example a proposal to put a bacterium into a state that has a wave function. At best, such a state would be one of suspended animation. I have not bothered to struggle with the editor who passionately claims that viruses are alive, apparently because they are complex, while avoiding the fact that they don't metabolize. The new proposal about bacteria, in its cited text source, says they would be alive. The key physical fact is that for superposition, one needs a wave function, as stated clearly by Willis Lamb (see just above). This is so obvious that not many writers would bother to say it explicitly, and yet it does seem to escape the attention of too many. The present use of two such examples, without note that they rely on at least suspended animation, puts us in near 'gee-whizz' territory. I would prefer to see both removed because of their dodgy form, but I don't intend to struggle over it.Chjoaygame (talk) 00:49, 22 September 2015 (UTC)

"Concept"
Most of the material in the "Concept" section needs either a complete rewrite, or should simply be removed. For example the term "pure state" has a very specific meaning in quantum mechanics, and has little to do with anything described there. For a specific example, "pure with respect to the filter" is gibberish - a quantum state is either pure or mixed, there's nothing relative about it. The reference given (Messiah) says nothing that supports what is written. Another example: "For perfect superposition it is essential that the intermediate beams are mutually coherent." Again, this is gibberish - superposition is an exact principle, it's never "imperfect", nor is it necessary for beams to be "coherent" for their corresponding quantum states to be superpose-able. Given the major problems with these sections and the lack of sources that support what is written, the content there now detracts significantly from the article and from the understanding of anyone seeking to learn from the article. Therefore I am deleting the entire section. Hopefully it can be replaced later with a correct version.  Waleswatcher  ( talk ) 23:08, 5 November 2015 (UTC)

Something to add - there is really no reason to discuss "beams" at all, especially not in a "concept" section that is supposed to gently introduce the idea of quantum superposition. Superposition is a totally general concept in QM that applies to everything from individual isolated particles to macroscopic systems (assuming QM applies to them). It's far more general than "beams". Also, a minor note on English usage. "Quantal" is really not a common word, either among professionals or lay-people. Best to avoid it.  Waleswatcher  ( talk ) 23:13, 5 November 2015 (UTC)


 * It is always a pleasure to be corrected by you.Chjoaygame (talk) 01:00, 6 November 2015 (UTC)


 * I didn't check who wrote those sections - but if it was you, you're welcome. Please refrain from edits to wiki articles about quantum mechanics in the future, because it is apparent that you do not understand the basics of the topic (that's not meant to be insulting, very few people do). Thanks in advance.   Waleswatcher  ( talk ) 01:20, 6 November 2015 (UTC)


 * Silly old Feynman, he also could do with some correction by you.: "The answer is this: If the atoms are in a definite state with respect to $S$, they are not in the same state with respect to $T$—a $+S$ state is not also a $+T$ state. There is, however, a certain amplitude to find the atom in a $+T$ state—or a $0T$ state or a $−T$ state."[Feynman's format of —]Chjoaygame (talk) 01:14, 6 November 2015 (UTC)


 * What Feynman says is correct (assuming S, T are non-commuting operators). It has nothing to do with the material that I deleted, or with whether the state is pure or mixed.   Waleswatcher  ( talk ) 01:20, 6 November 2015 (UTC)


 * Quantal response equilibrium, Quantal theory of speech, Quantal neurotransmitter release.Chjoaygame (talk) 01:25, 6 November 2015 (UTC)


 * None of those have anything to do with quantum mechanics, which makes my point for me. To repeat myself, I'm not saying it's not a word, I'm saying it's not the right word in this context.  And in any case that's the least important of the points I made above.  Waleswatcher  ( talk ) 03:43, 6 November 2015 (UTC)


 * It's always a pleasure to be corrected by you. Hopefully, the section "can be replaced later with a correct version" by you.Chjoaygame (talk) 03:58, 6 November 2015 (UTC)

"much like waves in classical physics"?
Dirac 4th edition (1958), p. 14: "It is important to remember, however, that the superposition that occurs in quantum mechanics is of an essentially different nature from any occurring in the classical theory, as is shown by the fact that the quantum superposition principle demands indeterminacy in the results of observations in order to be capable of a sensible physical interpretation."

Dirac 4th edition (1958), p. 17: "The assumption just made shows up very clearly the fundamental difference between the superposition of the quantum theory and any kind of classical superposition."

The second sentence of the lead of the present article currently reads "... much like waves in classical physics, any two (or more) quantum states can be added together ..." How much like?Chjoaygame (talk) 15:31, 21 November 2015 (UTC)


 * Very much like, in precisely the sense stated. The biggest difference is that  ANY two states can be superposed, not just linear waves.   Waleswatcher  ( talk ) 12:18, 30 November 2015 (UTC)


 * Silly old Dirac, how wrong he was!Chjoaygame (talk) 12:46, 30 November 2015 (UTC)


 * He's not wrong, nor does what is written in the article now contradict what he says in either of those places. On p.17 he is pointing out that QM states must be normalized, so adding a state to itself doesn't change the state, where as superposing a classical wave with itself changes the amplitude.  That doesn't conflict with the statement that waves can be added together, and so can QM states.  Furthermore, the mathematics (solutions to linear diff. eqs. can be added together, and the result is still a solution) is identical in the two cases.   Waleswatcher  ( talk ) 15:45, 30 November 2015 (UTC)


 * It remains incongruous that Dirac repeatedly points out the profound differences between classical and quantum superposition, whereas the present lead of the present article seems to minimize or hide it. Classical waves do not exhibit wave–particle duality, a prime fact exhibited by quantum mechanics.Chjoaygame (talk) 18:13, 30 November 2015 (UTC)
 * This article is about quantum superposition. There is a very close connection between quantum superposition and the superposition of classical linear waves (mathematically, they are exactly the same thing).  Yes, there are differences between quantum wave functions and classical waves, and I certainly don't object to pointing them out.  However, bear in mind that the differences aren't really connected to superposition itself so much as they are to the interpretation of the wave.   Waleswatcher  ( talk ) 20:11, 30 November 2015 (UTC)
 * The Dirac passage quoted above is specifically about the addition of a state to itself with two different coefficients. In classical waves the result is a wave with different magnitude; Dirac gives the example of a drum head. In quantum waves, the magnitude is a probability amplitude; the scalar multipliers in QM are the eigenvalues. Johnjbarton (talk) 21:51, 5 November 2023 (UTC)

Experiments
Maybe we could eliminate the whole section Experiments and applications. Of course these are some examples, but isn't any QM experiment a demonstration of quantum superposition? All QM experiments are under the assumption they follow somehow Schrodinger equation or some version of Dirac-von Neumann axioms. I agree we could leave some examples but not as experimental evidence and only if it is argued why is a good example of Q superposition, like the slit and the qubit in the lead. --MaoGo (talk) 16:38, 14 May 2018 (UTC)


 * I partially agree. Some of the experiments on that list are interesting because they involve relatively large systems (like buckyballs or flu viruses), and it's interesting to check that such large systems can be put in coherent superpositions (of position eigenstates, for instance) and can demonstrate interference.  I think it's a good idea to keep some discussion of that somewhere in the article.  But much of what's there in that section is fluff.   Waleswatcher  ( talk ) 17:19, 14 May 2018 (UTC)
 * We might just move the list to pages like mesoscopic physics or quantum interference, they are maybe not suited here. --MaoGo (talk) 14:36, 15 May 2018 (UTC)