Doing computations with single atoms also doesn't sound very practical. Measuring distances, I suppose there could be numerous ways, like measuring the gravity

Doing computations with single atoms also doesn't sound very practical. Measuring distances, I suppose there could be numerous ways, like measuring the gravity or electric pull (not sure what it is called in English). I think only Quantum theory says we can not measure to arbitrary precision, or at least if we do, there are other issue. Still, that would be another argument than simply pointing at the number of atoms.