Thing is fusion is known to be possible - the sun and hydrogen bombs. Quantum Computing lacks equivalent existence proofs. There's a lot of abstract theory but no real indication it is possible to scale up in the real physical world to useful problem sizes and reasons to doubt that it is possible.
Neither of those are contained fusion. I think gp is implying that it might be impossible to have net energy gain for contained fusion (which currently seems to be the case even with most advanced designs)
