Realizations of Quantum Heat Engines

Quantum heat engines (QHEs) utlize thermodynamics at the level of individual energy quanta to produce work from noise. However, the implementations to date rely on external control with energetic cost that overwhelms the work. We experimentally demonstrate [1] such an externally driven Otto-cycle QHE by coupling a flux-tunable transmon as the working medium and a quantum-circuit refrigerator as a two-way tunable bath. In good agreement with simulations, single-shot readout over repeated cycles reveal positive power and efficiencies of the engine itself but—consistent with the grand challenge for QHEs—the room-temperature generation of the control fields consume more energy than the engine produces at the device level. To break through this barrier, we propose [2] and experimentally realize an autonomous superconducting QHE that transforms thermal noise into coherent microwave work by the way of intrinsic non-Markovian circuit quantum electrodynamics, requiring only a thermal bias and no external time-dependent control fields. As the theory predicts, the QHE turns on given a sufficiently large initial impulse and keeps producing coherent microwaves indefinitely until the thermal gradient powering the engine is reduced below a relatively sharp threshold. Finally, we discuss some potential use cases and future evolutions of the engine.