Open systems acquire time-dependent coupling constants through interaction
with an external field or environment. We generalize the Lewis-Riesenfeld
invariant theorem to open system of quantum fields after second quantization.
The generalized invariants and thereby the quantum evolution are found
explicitly for time-dependent quadratic fermionic systems. The pair production
of fermions is computed and other physical implications are discussed.Comment: LaTex, 10 pages, no figures; replaced by the version to be published
in Phys. Lett. A; the validity of the Liouville-von Neumann approach is
clarified for non-normalizable and degenerate states; references adde
