  Performance of nano- and micro-scale heat engines can be improved with a help
from quantum mechanical phenomena. Recently, heat reservoirs with quantum
coherence have been proposed to enhance engine performance beyond the Carnot
limit even with a single reservoir. However, no physical realizations have been
achieved so far. Here, we report the first proof-of-principle experimental
demonstration of a photonic quantum engine driven by superradiance employing a
single heat reservoir composed of atoms and photonic vacuum. Reservoir atoms
prepared in a quantum coherent superposition state underwent superradiance
while traversing the cavity. This led to about 40-fold increase of the
effective engine temperature, resulting in a near-unity engine efficiency.
Moreover, the observed engine output power grew quadratically with respect to
the atomic injection rate. Our work can be utilized in quantum mechanical heat
transfer as well as in boosting engine powers, opening a pathway to development
of photomechanical devices that run on quantum coherence embedded in heat
baths.
