  The conventional continuous quantum heat engines rely on incoherent heat
transfer with the baths and, thus, have limited capability to outperform their
classical counterparts. In this work, we introduce distinct continuous quantum
heat engines that utilize coherent heat transfer with baths, yielding
significant quantum enhancement in performance. These continuous engines,
termed as coherent engines, consist of one qutrit system and two photonic baths
and enable coherent heat transfer via two-photon transitions involving
three-body interactions between the system and hot and cold baths. The closest
quantum incoherent analogs are those that only allow incoherent heat transfer
between the qutrit and the baths via one-photon transitions relying on two-body
interactions between the system and hot or cold baths. We demonstrate that
coherent engines deliver much higher power output and a much lower
signal-to-noise ratio in power, where the latter signifies the reliability of
an engine, compared to incoherent engines. Coherent engines manifest more
non-classical features than incoherent engines because they violate the
classical thermodynamic uncertainty relation by a greater amount and for a
wider range of parameters. Importantly, coherent engines can operate close to
or at the fundamental lower limit on reliability given by the quantum version
of the thermodynamic uncertainty relation, making them highly reliable. These
genuine enhancements in performance by hundreds of folds over incoherent
engines and the saturation of the quantum limit by coherent engines are
directly attributed to its capacity to harness higher energetic coherence which
is, again, a consequence of coherent heat transfer. The experimental
feasibility of coherent engines and the improved understanding of how quantum
properties can enhance performance may find important implications in emerging
quantum technologies.
