Even if thinking does somehow depend on quantum effects, it seems hugely unlikely that it would depend on the specific quantum state of individual atoms.
If it does, you don't need a spec of that state, since we know that it can emerge from something simpler (humans start out as a single cell, after all, and so in fact did all of humanity). You don't need the whole system, just the right initial conditions.
At that point you're growing a brain rather than engineering one, and maybe it takes you no closer to understanding the mechanics. But the point stands that it must be possible to construct a brain in principle, because it's already happened so many times before.
If it does, you don't need a spec of that state, since we know that it can emerge from something simpler (humans start out as a single cell, after all, and so in fact did all of humanity). You don't need the whole system, just the right initial conditions.
At that point you're growing a brain rather than engineering one, and maybe it takes you no closer to understanding the mechanics. But the point stands that it must be possible to construct a brain in principle, because it's already happened so many times before.