A previous commenter has shamed me into posting more about QC. So here is a post about QC.
In the TAQC computational model, a quantum computation provides the user with an answer that is exact if the computer is in its lowest energy state at the end of the calculation. If the computer is not in the lowest energy state, the result we get is an approximate answer. The approximation gets worse the higher the energy is.
For this model of quantum computation, errors tend to cause the resulting answer we get to drift away from the exact answer. These errors, which drive the computer into higher energy states, are naturally counteracted by the computer system’s tendency to reduce its energy.
This tendency to reduce energy includes dissipative processes–ones that release energy. These types of process, helpful for TAQC in that they improve the approximation to exact that the computer provides, are anathema to the gate model of quantum computation.
In the gate model, there does not exist the same idea of approximate answers ranked by the energies of the states encoded by a processor. The exact solution to a problem could be any state, even the highest energy state. Errors in this model have to be removed because of this lack of a concept of approximate solution. Any change to the desired application of gates brings the computer into a state which could be very far from the desired state.
This unfortunate situation, that the gate model requires error correction in order to work at all, has led to a commonly repeated and very misleading QC dogma. Here it is in a recent preprint from Laforest et.al. quant-ph:0610038:
QEC [quantum error correction] is a requirement for scalable quantum information processing (QIP)
While technically this is true, what the authors are implying is that active quantum error correction, which is a complex and ridiculous process that will probably never be used outside a research lab ever, is required for quantum computation. It isn’t, which is a good thing. In the TAQC model, quantum error correction occurs passively; nature itself shepherds the system towards better and better approximate answers.
I would even go as far as to bet that no QC that requires active error correction will EVER perform a computation beyond the scope of conventional systems; having architectures and systems that are passively protected against errors is Rule #1 for anyone serious about building large-scale quantum computers.