  We consider a harmonic oscillator under periodic driving and coupled to two
harmonic-oscillator heat baths at different temperatures. We use the
thermofield transformation with chain mapping for this setup, which allows us
to study the unitary evolution of the system and the baths up to a time when
the periodic steady state emerges in the system. We characterize this periodic
steady state, and we show that, by tuning the system and the bath parameters,
one can turn this system from an engine to an accelerator or even to a heater.
The possibility to study the unitary evolution of the system and baths also
allows us to evaluate the steady correlations that build between the system and
the baths, and correlations that grow between the baths.
