Boosting the efficiency of quantum heat engines

A team of researchers from IFISC (UIB-CSIC) has demonstrated that a quantum chiral conductor driven by an alternating current voltage can operate with efficiencies much higher than the hitherto insurmountable Carnot limit. In the paper, published in Nature Communications, the authors propose a general class of quantum devices driven periodically by alternating current (AC), which can convert heat into electricity with an increased performance.

One of the implications of the second law of thermodynamics is that the power generated by an ideal thermal machine cannot exceed the Carnot efficiency limit in the classical regime. However, this upper limit can in principle be exceeded if we assume that quantum coherence is also a resource for entropy production. Therefore, understanding how the entropy resource can be controlled in different scenarios is key to achieving higher efficiency in quantum engines and refrigerators.

To do so, the team considered a pumped quantum engine consisting of an arbitrary energy-dependent transmission tunneling scatterer coupled to hot and cold electronic reservoirs in the presence of an external AC bias voltage. A crucial point to understand is that most AC voltage sources inject net energy into the motor, thus reducing the power developed. The main finding of the study is to show how to cancel this detrimental effect using chiral conductors. These conductors are characterized by a spatial separation of the electronic motion, such as those created with topological matter. Then, the proposed device selectively applies an AC field to the electrons depending on their direction. This boosts the quantum engine efficiency beyond the Carnot bound.

The paper discusses two necessary ingredients to observe this effect: on the one hand, an irreversible production of entropy by the photo-assisted excitation processes induced by the AC field and, on the other hand, the absence of net energy injection thanks to chirality. The device also extracts work from baths held at the same temperature, thus seemingly violating the Kelvin-Planck law. However, this does not mean that the second law of thermodynamics is not satisfied, since with a proper definition entropy production is always positive.

These results are relevant within modern developments of quantum thermodynamics that aim at understanding the role of nonequilibrium effects in quantum machines.

Ryu, S., López, R., Serra, L. et al. Beating Carnot efficiency with periodically driven chiral conductors. Nat Commun 13, 2512 (2022). https://doi.org/10.1038/s41467-022-30039-7

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