  We consider the role of quantum mechanics in a specific coherent control
scenario, designing a "coherent control interferometer" as the essential tool
that links coherent control to quantum fundamentals. Building upon this allows
us to rigorously display the genuinely quantum nature of a generalized
weak-field coherent control scenario (utilizing 1 vs. 2 photon excitation) via
a Bell-CHSH test. Specifically, we propose an implementation of "quantum
delayed-choice" in a bichromatic alkali atom photoionization experiment. The
experimenter can choose between two complementary situations, which are
characterized by a random photoelectron spin polarization with particle-like
behaviour on the one hand, and by spin controllability and wave-like nature on
the other. Because these two choices are conditioned coherently on states of
the driving fields, it becomes physically unknowable, prior to measurement,
whether there is control over the spin or not.
