  We study non-perturbatively the time evolution of cascade decay for generic
fields $\pi \rightarrow \phi_1\phi_2\rightarrow \phi_2\chi_1\chi_2$ and obtain
the time dependence of amplitudes and populations for the resonant and final
states. We analyze in detail the different time scales and the manifestation of
unitary time evolution in the dynamics of production and decay of resonant
intermediate and final states. The probability of occupation (population)
"flows" as a function of time from the initial to the final states. When the
decay width of the parent particle $\Gamma_\pi$ is much larger than that of the
intermediate resonant state $\Gamma_{\phi_1}$ there is a "bottleneck" in the
flow, the population of resonant states builds up to a maximum at $t^* =
\ln[\Gamma_\pi/\Gamma_{\phi_1}]/(\Gamma_\pi-\Gamma_{\phi_1})$ nearly saturating
unitarity and decays to the final state on the longer time scale
$1/\Gamma_{\phi_1}$. As a consequence of the wide separation of time scales in
this case the cascade decay can be interpreted as evolving sequentially $\pi
\rightarrow \phi_1\phi_2; ~ \phi_1\phi_2\rightarrow \phi_2\chi_1\chi_2$. In the
opposite limit the population of resonances ($\phi_1$) does not build up
substantially and the cascade decay proceeds almost directly from the initial
parent to the final state without resulting in a large amplitude of the
resonant state. An alternative but equivalent non-perturbative method useful in
cosmology is presented. Possible phenomenological implications for heavy
sterile neutrinos as resonant states and consequences of quantum entanglement
and correlations in the final state are discussed.
