What would a quantum computer architecture look like?
Really my question comes down to where in the standard computer processing pipeline does the quantumness come in, in order to produce the intended theoretical behaviour of quantum computer?


The critical part to get the intended theoretical (scaling) behaviour of quantum computer is probably to achieve and maintain the coherence between the different qubits in the computer.

Let me illustrate this with a hypothetical architecture. Assume that you have ready made modules with a fixed number of qubits (say 49=7x7). Further assume that each module for itself is able to maintain coherence between its qubits (by using quantum error correction), and reliably operate on its qubits controlled by a classical computer (my mental model is similar to this simple model of a finite quantum coprocessor I asked about before, but it doesn't really matter for this illustration1).

Now assume that you scale your quantum computer by putting many of those modules together, using some suitable topology. The modules cannot talk directly to each other, but we may assume that the classical computer controlling those modules is able to freeze any quantum operation of the modules and then permute qubits between those modules according to the topology.

Now assume that the quantum operation of each module randomly changes its global phase in a way uncontrollable and unmeasurable by the controlling classical computer. This is not even true decoherence (as would occur in practice - the density matrix formalism could be used for a better model), but I guess it will be sufficient to destroy the intended theoretical (scaling) behaviour of quantum computer. But if the qubits of the different modules stay perfectly coherent with each other, and the global phase differences between the modules are known to the classical computer, then I guess it will be possible to achieve the intended theoretical (scaling) behaviour of quantum computer.




Edit Actually, the part of the model about initialization and measurement does matter a bit. Even if we assume that we know the global phase differences between the modules after initialization, we certainly have to assume that measurement is restricted to each module alone, and that coherence with other modules is lost after measurement. I still don't think that this will destroy the quantum speedup, since the global state can be (quantum/unitarily) transformed suitably before the measurement, hence one should still be able to perform any desired single measurement at the end of the quantum computation.