I mean, quantum computing does exist right now, it’s just not terribly stable or useful yet. With time and bucketloads of engineering it could be a huge change in how we solve problems.
My understanding is that it is incredibly useful for very obscure and specific things, and not very good at all for general purpose computing (and it may never be).
Not yet, I think. They're still too small from what I've heard to be useful for really anything at all, but they are improving and will likely start to be useful in the next 10 years or so