  We study the unitary time evolution of a simple quantum Hamiltonian
describing two harmonic oscillators coupled via a three-level system. The
latter acts as an engine transferring energy from one oscillator to the other
and is driven in a cyclic manner by time-dependent external fields. The
S-matrix of the cycle is obtained in analytic form. The total number of quanta
contained in the system is a conserved quantity. As a consequence the spectrum
of the S-matrix is purely discrete and the evolution of the system is
quasi-periodic.
