To reiterate my point in the other question, I think the scope should be quantum information, mostly including the subfields listed in the wiki page on quantum information science (though I do not completely agree with the classification used there). In other words the following should be on-topic:
- Quantum computing and related topics (which includes things such as quantum error correction, quantum complexity theory, etc.). "Quantum computing" here also includes anything related to ways to program a quantum device.
- Quantum communication, quantum cryptography, and related topics (such as quantum teleportation and quantum dense coding).
- Questions about quantum information that are not included in the above (e.g. conceptual questions about quantum entanglement or quantum entropy that not necessarily directly associated with a given protocol).
- Questions about the math used for results that belong to the above categories. Now this is a tricky one, because it would essentially make the whole of linear algebra in the scope of the site. I see two ways for these kinds of questions to be fit for this site:
- They are asked in direct relation with a topic belonging to the first three categories listed here (e.g. how do you compute the eigenvalues of this quantum circuit?).
- It might give a chance to prove the given result using tools common in quantum information theory (mostly bra-ket notation). An example of this sort of question is this one.
This list is not, nor could possibly be, exhaustive (though please suggest major topics that should be explicitly in it that I might be forgetting at the moment).
The main reason for having "quantum information" as scope is that I do not think it is feasible to draw any kind of meaningful line between what is "quantum computing" and what is, say, "only quantum communication".
But so then, one could argue, why is the site called "quantum computing" and not "quantum information"? Well, I think it's a good question, but it is what it is at this point.
About welcoming any question related to quantum mechanics, I quote myself in this related answer:
I strongly disagree with accepting all questions about quantum mechanics, as these are very much on-topic on physics.SE (I mean,
quantum-mechanics is literally the top tag on physics.SE). To accept all questions about QM indiscriminately would just lead to an unnecessary amount of duplication. Strongly related questions spread among multiple sites are a pain, for one, because you lose all the advantages provided by the
Related lists, and it is often harder to track them down.
Even limiting ourselves to quantum information topics, there will still be a significant amount of overlap with physics.SE, as quantum information questions are just as on-topic there as they are here.
There will also be overlap with other sites such as cstheory (for questions about complexity), maths (for questions about the math behind results), or stackoverflow (for questions about programming on quantum devices).
I don't think this is avoidable, but is also not that bad, as this sort of thing is nowadays pretty common among SE sites.
This said, it is also true that one cannot really define an hard distinction between what is "quantum information science" and what is "only quantum mechanics".
Again, this is unavoidable, but hopefully most cases will be clearly on- or off-topic, and the edge cases will be handled case by case.