There are already Computer Science and Theoretical Computer Science sites that should cover Quantum Computing topics. There's no reason to needlessly multiply stack exchange sites.
Proposal: Quantum Computing
Looking on the Stack Overflow blog, Merging Season (it may be a number of years old, but it's linked on the Area 51 FAQ, so is still relevant)
So: the right size might be somewhere around the size of a university department
As time would have it, there is now at least one centre with a similar number of members to a small department (and other institutes with over 100 members). These will only increase in size as time goes on. This puts the area of Quantum Computing 'somewhere around the size of a university department', making it potentially 'the right size', although a better indicator of 'right size' would perhaps be given by having a large number of followers, which this proposal does have (currently the third highest of the proposals in the definition stage). This puts it in the place of being big enough to be a valid area for SE.
Now, the next point in the blog is the one being addressed in this question:
Here’s the best we could come up with for deciding whether site X should be subsumed by site Y:
- Almost all X questions are on-topic for site Y
While there are quantum computing tags on the sites you mentioned, there are also quantum computing tags on other sites (There are also a few quantum computing questions on Worldbuilding, but I feel they're substantially different enough to be not on topic here):
Or, to put it another way, while Computer Science and Theoretical Computer Science SEs "cover Quantum Computing topics", they don't cover all Quantum Computing topics. This can also be demonstrated by looking at the list in the What questions should be on topic for Quantum Computing? question, where there are example questions about the definition as well as from both the computer science and physics sides. Similarly, looking at the list of questions asked so far in this proposal, there are questions related to computer science, physics, maths, security and engineering.
The above is extremely strong evidence that Quantum Computing cannot just covered by one or two SE sites, but would require at least 7 or potentially 8 (including a feasible tag on engineering). This shows that the first rule is not valid. That is, Most Quantum Computing questions are not on topic for any other single SE site and so, Quantum Computing should not be subsumed by one.
Furthermore, as shown above, Quantum Computing/technologies/engineering is now starting to branch into being another subject and so no longer deserves to be split into smaller areas that are individually part of different subjects (maths, computer science, physics, engineering, security) but is now a valid area by itself and so, should be considered as a whole. Considering it as a whole area by itself then means that, as there is no single site that covers all of this, the best option is to have another SE site for Quantum Computing.
Edit: Question areas on Quantum Computing not on Topic on any other SE site:
One of the comments on this answer said that I needed to make the argument "all of Stack Exchange together does not sufficiently cover QC". I do disagree with this, as whether or not an area should be considered for an SE is based more on whether it is big before looking at potential issues such as 'can it be subsumed in Y' (i.e. The area as a whole should be looked at before deciding whether or not it should be subsumed into Y, never mind X, Y and Z). In addition, the question is about specifically subsuming it into Computer Science and Theoretical Computer Science which clearly would not work, although votes and said comment suggest that this isn't actually the problem, which instead appears to be subsuming it into many different parts of SE. Anyway, some questions areas that are not on topic on any SE site (that I'm aware of) include:
First of all, most questions about quantum computing wouldn't be on-topic at all on Computer Science or cstheory. If any, Physics is the site where most questions asked here would be more suitable for.
However, I don't think that this is really the question one ought to ask (there are so many examples of stackexchange sites that shouldn't exist if this was the only argument to consider). Rather, I believe the question is whether a site is able to attract a dedicated community that doesn't fit so much, and maybe wouldn't exist at all, on other sites.
I see this in a similar (though not identical) fashion to the current situation with maths vs mathoverflow or cs vs cstheory. In both these cases there are two sites dedicated to the same topic, differing in that one site is for undergraduate level questions and the other for postgraduate and research level questions. In the case of the QC proposal the situation is somewhat different in that QC is also dedicated to a specific topic. This topic is however one that would naturally attract higher level questions, which would make this site more focused on postgraduate-level questions (*). Moreover, a dedicated stackexchange for QC and related topics would attract many researchers that wouldn't otherwise have had any interest in frequenting physics or related sites.
Finally, QC is multidisciplinary enough that many questions wouldn't be so good a fit on physics/maths/cs/cstheory. This is the same kind of thing that happens all the time with SE sites dedicated to specific technologies, most questions of which would have also been on topic on stackoverflow/superuser or other existing sites.
(*) As was pointed out in the comments, both maths and cs were created after mathoverflow and cstheory, and not the other way around as I mistakenly thought (see this question for a full timeline). This invalidates using these sites as successful examples of higher-level sites being born from the lower-level counterparts, but I think the point still stand that these are examples of successful coexistence of two sites dealing with mostly the same topic.
Quantum computing is a broad and expansive topic, but it is also specialized. Therefore, the fact of the matter is that the overwhelming majority of people checking these other sites don’t know much about it, though I’d venture a guess that people in physics are more likely to know about quantum information. If quantum information questions are to gain a good deal of visibility among people who have the ability to answer, a focused site is needed. It would be most helpful to researchers in the field to have a place where we know that many experts are frequently checking it and contributing content. You won’t get nearly that same effect of frequently checking and contributing content on any other site. I’ve done work in quantum computing for about 3.5 years and I only go on Stackexchange sites when I have a specific question about a specific topic, whether it be math, physics, or computer science. I don’t frequently check any of these because I’ve been focused on quantum information and related topics. If however there was a high-quality Q&A site devoted to quantum information and if I knew that it attracted experts and others who are knowledgeable and somewhat deeply involved in the field, I might check it just because, that is just to see what is going on even when I don’t have a specific question. I expect others who are seriously involved in the subject to be more likely to do the same under those circumstances (given a dedicated site). That ensures a substantially greater proportion of high quality content, and actually a higher volume of content on the topic. Frankly, I’d be in favor of it being more broadly on quantum information and not just quantum computing: so I’d favor it including quantum communication, quantum cryptography, etc.
Quantum Computing did not exist with all other disciplines. It is quite new, and it will keep growing up. As everyone knows in general that the distance between transistors inside chips is decreasing and will only result in a quantum property trouble, we will need an entirely different computing; the Quantum Computing. I think the proposal for a new site for this is appropriate and very timely.