To 3: NP-complete Problems and Physical Reality by Scott Aaronson is a pretty good review. To 2: It is about scaling. The noise might be an order of magnitude

To 3: NP-complete Problems and Physical Reality by Scott Aaronson is a pretty good review. To 2: It is about scaling. The noise might be an order of magnitude smaller, but it does not matter, because this is a constant factor that is overpower by the fact that noise adds up quickly with number of elements/operations, no matter what hardware you use. It is the adding up that is the problem. To your last comment: you really will be surprised. It is our duty as scientists to balance excitement about the unknown (better analog computers would indeed be awesome) with the desire to speculate about what the solution would be. Lastly, learning why something is improbable (e.g. a perpetual motion machine) frequently brings amazing insight into the workings of the universe.