  We propose a new no-go theorem by proving the impossibility of constructing a
deterministic quantum circuit that iterates a unitary oracle by calling it only
once. Different schemes are provided to bypass this result and to approximately
realize the iteration. The optimal scheme is also studied. An interesting
observation is that for large number of iterations, a trivial strategy like
using the identity channel has the optimal performance, and preprocessing,
postprocessing, or using resources like entanglement does not help at all.
Intriguingly, the number of iterations, when being large enough, does not
affect the performance of the proposed schemes.
