Really, that tag should be gotten rid of.
At least, it should be renamed. Why would anyone not choose a tag named "Quantum Computer" in "Quantum Computing" forum?
Looking at the description of the quantum-computer tag, it's currently described as:
For questions about computers that run on a quantum architecture; the machines themselves, not just any computing that could take place on one.
Personally, I find this a reasonable start and think that questions tagged quantum-computer should be about either the physical or hypothetical machine that is called a quantum computer1. That is, just because a question is about qubits, noise or simulating a quantum computer2 does not mean it should also have the quantum-computer tag as well, unless relating to an actual, physical (or hypothetical) machine that we could call a quantum computer.
To give a couple of examples, the question Why must quantum computers be kept near absolute zero? is asking why a physical-machine-that-can-perform-arbitrary-quantum-computations must be kept near absolute zero, so is suited for the tag, while asking What is the difference between qubit and normal bits? doesn't appear to have anything to do with a physical or hypothetical machine in the same way that talking about a (classical) bit is not the same as talking about a (classical) computer.
However, as just shown, current questions on said tag don't always match this, so some re-tagging would need to be done...
1 Technically, this says nothing about whether the machine is universal or not, so we might want to think about tags for (non-)universal quantum computers
2 Simulating a QC might be a grey area, to be fair, although there is already a simulation tag