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Cyclical heat engines are a paradigm of classical thermodynamics, but areimpractical for miniaturization because they rely on moving parts. A morerecent concept is particle-exchange (PE) heat engines, which uses energyfiltering to control a thermally driven particle flow between two heatreservoirs. As they do not require moving parts and can be realized insolid-state materials, they are suitable for low-power applications andminiaturization. It was predicted that PE engines could reach the samethermodynamically ideal efficiency limits as those accessible to cyclicalengines, but this prediction has not been verified experimentally. Here, wedemonstrate a PE heat engine based on a quantum dot (QD) embedded into asemiconductor nanowire. We directly measure the engine's steady-state electricpower output and combine it with the calculated electronic heat flow todetermine the electronic efficiency $\eta$. We find that at the maximum powerconditions, $\eta$ is in agreement with the Curzon-Ahlborn efficiency and thatthe overall maximum $\eta$ is in excess of 70$\%$ of the Carnot efficiencywhile maintaining a finite power output. Our results demonstrate thatthermoelectric power conversion can, in principle, be achieved close to thethermodynamic limits, with direct relevance for future hot-carrierphotovoltaics, on-chip coolers or energy harvesters for quantum technologies.

A quantum-dot heat engine operating close to the thermodynamic efficiency limits