  We show theoretically that a thermoelectric heat engine, operating
exclusively due to quantum-mechanical interference, can reach optimal
linear-response performance. A chiral edge state implementation of a
close-to-optimal heat engine is proposed in an electronic Mach-Zehnder
interferometer with a mesoscopic capacitor coupled to one arm. We demonstrate
that the maximum power and corresponding efficiency can reach 90\% and 83\%,
respectively, of the theoretical maximum. The proposed heat engine can be
realized with existing experimental techniques and has a performance robust
against moderate dephasing.
