  Carnot established in 1824 that the efficiency of cyclic engines operating
between a hot bath at absolute temperature $T_{hot}$ and a bath at a lower
temperature $T_{cold}$ cannot exceed $1-T_{cold}/T_{hot}$. We show that linear
oscillators alternately in contact with hot and cold baths obey this principle
in the quantum as well as in the classical regime. The expression of the work
performed is derived from a simple prescription. Reversible and non-reversible
cycles are illustrated. The paper begins with historical considerations and is
essentially self-contained.
