  Quantum interference is at the heart of what sets the quantum and classical
worlds apart. We demonstrate that quantum interference effects involving a
many-body working medium is responsible for genuinely non-classical features in
the performance of a quantum heat engine. The features with which quantum
interference manifests itself in the work output of the engine depends strongly
on the extensive nature of the working medium. While identifying the class of
work substances that optimize the performance of the engine, our results shed
light on the optimal size of such media of quantum workers to maximize the work
output and efficiency of quantum energy machines.
