[knzhou]

The answer is a combination of 1 and 2. Not every computing device is actually useful. For example, you can find plenty of brain parts that look vaguely like GPUs or FPGAs. None that look anything like a standard CPU. This would be basically impossible to build out of cells, and not useful anyway.

The same thing applies to quantum computers. They’re much much harder to build because they’re more delicate. We’re talking about effects that usually are completely destroyed by a single unwanted atom coming in and hitting something. And there are a lot of atoms flying around in cells. Propagating any quantum signal from even one cell to an adjacent one is impossible. Finally they’re less useful. I can’t think of problems a biological brain needs to solve that require even a moderately fast CPU, let alone a quantum computer, which provides speedups over the CPU for only certain specific problems.

But none of this really matters, because your comment is one long isolated demand for rigor. You wave away my long list of examples because you think something very distantly related exists (in which case, with those low standards, quantum computers already exist), or because the examples are clearly impossible or not useful (without equally seriously considering the same for quantum computers). This is what I mean by skepticism of QC being driven mostly by intellectual fashion.