(1) Background in the Research Field and Issues in Previous Studies
In practical situations concerning thermodynamic and quantum devices, a primary goal is achieving high-speed operation and low energetic costs. Recent advances in the thermodynamics of small-scale systems have revealed a universal trade-off between operation speed and energetic costs. Active research is exploring how quantum effects contribute to overcoming this trade-off relation. Notably, quantum heat engines that provide nonzero power output while asymptotically reaching the ideal Carnot efficiency—which is not possible for classical heat engines—have been discovered. Despite these findings, the fundamental limit of enhancement of the performance of thermodynamic devices via quantum effects has not been clarified.
(2) Key Findings
By constructing a unified theoretical framework based on symmetry, this research analytically derived the fundamental limit on how much the trade-off relation can be improved and determined the specific symmetry conditions required to achieve this optimal improvement. As a result, increasing symmetry leads to higher-speed and lower-energy-cost operations. Application of these principles to quantum heat engines allows formulating a model that drastically surpasses the power of conventional heat engines. In particular, this study demonstrated the possibility of constructing a quantum engine that asymptotically achieves Carnot efficiency while delivering power that scales ranging from quadratic to exponential with respect to the number of particles (Fig. 1).
(3) Significance for Society
This research establishes a theoretical framework for analysing how symmetry affects quantum thermodynamics. The findings are expected to lead to design principles for thermodynamic devices and quantum information processing devices that combine high-speed operation with energy efficiency. Even with current quantum information processing technologies, the proposed quantum heat engine can be experimentally demonstrated and validated using a small number of qubits. In the long term, scaling up the number of qubits could enable the realization of heat engines with significantly higher efficiency and power than traditional engines, contributing to advancements in sustainable and high-performance energy technologies.
Fig1: Application of symmetry principles to quantum heat engines
(a): Plot of the heat-to-work conversion efficiency in units of Carnot efficiency. When the number of qubits increases, the efficiency asymptotically approaches the Carnot efficiency. (b) Plot of the output power. The scaling of the power in terms of the number of qubits becomes quadratic (blue) to exponential (red) by considering the symmetry principle obtained in this research. This scaling drastically exceeds the power of conventional heat engines (green).
Papers
Journal: Physical Review Letters
Title: Symmetry induced enhancement in finite-time thermodynamic trade-off relations
Authors: Ken Funo* and Hiroyasu Tajima