  We present two realizations of an Otto cycle with a quantum planar rotor as
the working medium controlled by means of external fields. By comparing the
quantum and the classical description of the working medium, we single out
genuine quantum effects with regards to the performance and the engine and
refrigerator modes of the Otto cycle. The first example is a rotating electric
dipole subjected to a controlled electric field, equivalent to a quantum
pendulum. Here we find a systematic disadvantage of the quantum rotor compared
to its classical counterpart. In contrast, a genuine quantum advantage can be
observed with a charged rotor generating a magnetic moment that is subjected to
a controlled magnetic field. Here, we prove that the classical rotor is
inoperable as a working medium for any choice of parameters, whereas the
quantum rotor supports an engine and a refrigerator mode, exploiting the
quantum statistics during the cold strokes of the cycle.
