  Following the rising interest in quantum information science, the extension
of a heat engine to the quantum regime by exploring microscopic quantum systems
has seen a boom of interest in the last decade. Although quantum coherence in
the quantum system of the working medium has been investigated to play a
nontrivial role, a complete understanding of the intrinsic quantum advantage of
quantum heat engines remains elusive. We experimentally demonstrate that the
quantum correlation between the working medium and the thermal bath is critical
for the quantum advantage of a quantum Szilard engine, where quantum coherence
in the working medium is naturally excluded. By quantifying the non-classical
correlation through quantum steering, we reveal that the heat engine is quantum
when the demon can truly steer the working medium. The average work obtained by
taking different ways of work extraction on the working medium can be used to
verify the real quantum Szilard engine.
