  We develop general quantitative criteria for dynamic interference, a
manifestation of double-slit interference in time which should be realizable
with brilliant state-of-the-art high-frequency laser sources. Our analysis
reveals that the observation of dynamic interference hinges upon maximizing the
difference between the dynamic polarization of the initial bound and the final
continuum state of the electron during the light pulse, while keeping depletion
of the initial state small. Confirmed by numerical results, we predict that
this is impossible for the hydrogen ground-state but feasible with excited
states explicitly exemplified with the hydrogen 2p-state.
