  We propose a nanoscale heat engine that utilizes the physics of resonant
tunneling in quantum dots in order to transfer electrons only at specific
energies. The nanoengine converts heat into electrical current in a
multiterminal geometry which permits one to separate current and heat flows. By
putting two quantum dots in series with a hot cavity, electrons that enter one
lead are forced to gain a prescribed energy in order to exit the opposite lead,
transporting a single electron charge. This condition yields an ideally
efficient heat engine. The energy gain is a property of the composite system
rather than of the individual dots. It is therefore tunable to optimize the
power while keeping a much larger level spacing for the individual quantum
dots. Despite the simplicity of the physical model, the optimized rectified
current and power is larger than any other candidate nano-engine. The ability
to scale the power by putting many such engines into a two-dimensional layered
structure gives a paradigmatic system for harvesting thermal energy at the
nanoscale. We demonstrate that the high power and efficiency of the layered
structure persists even if the quantum dots exhibit some randomness.
