  We investigate the behaviour of quantum heat engines, in which a qubit is put
through the quantum equivalent of the Otto cycle and the heat reservoirs are
due to the Unruh effect. The qubit is described by an Unruh--DeWitt detector
model coupled quadratically to a scalar field and alternately to a fermion
field. In the cycle, the qubit undergoes two stages of differing constant
acceleration corresponding to thermal contact with a hot and cold reservoir.
Explicit conditions are derived on the accelerations required for this cycle to
have positive work output. By analytically calculating the detector response
functions, we show that the dimensionality of the quadratic and fermionic
coupling constants introduces qualitatively different behaviour of the work
output from this cycle as compared to the case in which the qubit linearly
couples to a scalar field.
