  In some instances of study of quantum evolution of classical backgrounds it
is considered inevitable to resort to non-perturbative methods at the price of
treating the system semiclassically. We show that a fully quantum perturbative
treatment, in which the background is resolved as a multi-particle state,
recovers the semiclassical non-perturbative results and allows going beyond. We
reproduce particle-creation by a classical field in a theory of two scalars as
well as in scalar QED in terms of scattering processes of high multiplicity.
The multi-particle treatment also gives a transparent picture of why a
single-process transition from a classical to a quantum state, which we call
quantumization, is exponentially suppressed, whereas the opposite process,
classicalization, can take place swiftly if the microstate degeneracy of the
classical state is high. An example is provided by the $N$-graviton portrait of
a black hole: a black hole can form efficiently via a $2\to N$ classicalization
process in the collision of high-energy particles but its quantumization via a
decay $N \to 2$ is exponentially suppressed.
