All computers are quantum computers… it’s just that we’ve engineered it down to a point where the quantum effects don’t matter to the answer. For example, the voltage on a wire depends on quantum effects, so we make the wires thick enough (12 atoms was considered the minimum when I took hardware engineering classes in 1995) such that the quantum effects are reduced to such a level as to not affect the answers. (Note that we can’t bring that to zero effect, so there are still some errors while transmitting info on a digital computer, but it’s quite rare.)