Way back when, I wrote an answer about whether or not asking questions about not-yet built aspects of quantum computing were okay. To quote my answer:
I would argue that these sorts of questions should be on-topic, but that there is a line.
I think a good rule of thumb is: if it involves non-mainstream physics, no. If it involves mainstream physics, and even more so if the idea has been discussed, yes, it's probably on-topic.
Recent events have reminded me of this, and made me think that this should be made semi-official, to wit, questions about non-mainstream physics will be off topic. The new close reason (kudos to Physics.SE for the great wording) is
We deal with mainstream quantum computing here. Questions about the general correctness of unpublished personal theories are off topic, although specific questions evaluating new theories in the context of established science are usually allowed. For more information, see this meta post.
I will again quote Physics.SE:
We are not a substitute for peer-review, and cannot evaluate new theories. While some questions can lead to legitimate new theories, the question will need to be specific in order to fit this format.
If you are asking a question along the lines of
can we use string theory out of a spray can to solve world hunger and quantum computing gimme teh codez
(even stuff less blatantly bad) it will be closed once the close reason goes into effect.
This close reason has been put into effect as of 7.15.18. Please feel free to use this close reason on any question. Thank you for your comments and responses!
Note: Questions or answers that are extremely non-mainstream, or of very low quality, may be deleted at the discretion of the community and moderators.
To be clear, I still stand by what I wrote in that answer - questions about not-yet built stuff is okay, I just believe that we need to make it very clear that non-mainstream physics isn't allowed here.