  A quantum two-level system with periodically modulated energy splitting could
provide a minimal universal quantum heat machine. We present the experimental
realization and the theoretical description of such a two-level system as an
impurity electron spin in a silicon tunnel field-effect transistor. In the
incoherent regime, the system can behave analogously to either an Otto heat
engine or a refrigerator. The coherent regime could be described as a
superposition of those two regimes, producing specific interference fringes in
the observed source-drain current.
