One deals with the notion of superposition when studying Shor's algorithm, but how about entanglement? Where exactly does it appear in this particular circuit?
I assume it is not yet present in the initial state $\left|0\right>\left|0\right>$, but how about in further process, after applying Hadamard gates, the controlled-U gates and the inverse Fourier transform?
I understand that the first and second registers have to be entangled, otherwise, the final measurement on one of them wouldn't collapse the other one, which gives us the period (well, kind of, we need to use continuous fractions to infer it).