I'm Scott Aaronson, quantum computing/computational complexity researcher. AMA

Yes, you can make plots of the number of qubits, coherence times, etc. as a function of year -- and if you listen to talks by John Martinis, Chris Monroe, or the other leading experimentalists, you'll often see such plots. But at the very least, you need to look at both dimensions (qubits and coherence time) -- not just at "number of qubits," which will be severely misleading! And even if you do, there are very few data points to use for extrapolation, since it's really only within the last ~6-7 years that people have even gotten qubits to work well in isolation, let alone scaling them up. So it's really hard to extrapolate.

Like, I'm hopeful that within the next decade, we'll have systems with a few hundred qubits that will be good enough to do some useful tasks that are classically intractable (such as quantum simulation), though they certainly won't be threatening public-key crypto yet. But I'm not sure even about that. And I'd prefer to see what happens with this before speculating about the timescale for the next step, of building a full universal QC (the kind that would break our existing public-key cryptosystems)!

You can't have quantum von Neumann computer or can you? Are realistic quantum computers always going to be quantum circuits?

Or better yet, we could bifurcate into:

(1) Why is our universe apparently unable to solve the halting problem for Turing machines?

The answer, presumably, is a combination of the physical Church-Turing Thesis (specifically, the apparent impossibility of Turing-uncomputable processes in our world), with Church and Turing and Post's theorem on the unsolvability of the halting problem.

Of course one could then push back and ask why the Church-Turing Thesis should be true of our world: i.e., why shouldn't there be physical "hypercomputers"? That's an enormous question, but my old survey "NP-Complete Problems and Physical Reality" contains some thoughts about it: https://arxiv.org/abs/quant-ph/0502072

And then there's:

(2) Why should we live in a universe that's unable to solve "its own" halting problem?

I.e., even if super-Turing computers were physically possible, we could presumably formulate a halting problem for those computers, which would then require a still more powerful computer to solve (a super-duper-Turing computer?), and so on forever. This is because we could simply repeat Turing's diagonalization argument at a higher-level up -- given very minimal properties of computation, such as the ability to feed one program to another program as code, the ability of one program to emulate another one, and the ability to compute the NOT function. Once you have those properties, the inability of any given computational model (even a super-Turing model) to solve its own halting problem is just a matter of logic, as schoen said.