Personal quantum computers would be truely useless. They break specific kinds of encryption, and simulate other quantum systems. Other than that, nobody’s been able to devise a way to make them do much practical work.

Really, it’s unfortunate they were named that, because they’re only like computers if you have a solid background in computing to understand the analogy. “Quantum emulator” or “programmable quantum system” might be a better word that wouldn’t make people think it’s the next semiconductor node. Alas, I have no time machine to fix it.