[n4r9]

Again, a totally fair point. I just mean that when you posed the question "what exactly are quantum computers useful for?", I kind of assumed you meant an ideal quantum computer that had this kind of control over its state.

For reference I am out of my depth! My doctorate was in quantum information theory but I've been out of academia for many years now.

[bowsamic]

Yeah, I did mean that, and I stand by my opinion that even the ideal cases are not particularly compelling. I'm just also pointing out that the ideal case is not actually possible, even theoretically. There will also be decoherence, for example, and even the tomography of the state (i.e. actually measuring the state) is limited by fundamental quantum noise (e.g. ultimately the Heisenberg limit).

[n4r9]

I guess it depends on what you find compelling. I'm not interested in finance. A lot of it just seems like moving money around to make wealthy people wealthier. But it does make sense that a lot of heads will perk up if there's a potential for speeding up market simulation models.