Liu Junjie, Jung Kenneth A
Department of Physics, International Center of Quantum and Molecular Structures, Shanghai University, Shanghai 200444, China.
Institute for Quantum Science and Technology, Shanghai University, Shanghai 200444, China.
Phys Rev E. 2024 Apr;109(4-1):044118. doi: 10.1103/PhysRevE.109.044118.
Whether the strong coupling to thermal baths can improve the performance of quantum thermal machines remains an open issue under active debate. Here we revisit quantum thermal machines operating with the quasistatic Carnot cycle and aim to unveil the role of strong coupling in maximum efficiency. Our analysis builds upon definitions of excess work and heat derived from an exact formulation of the first law of thermodynamics for the working substance, which captures the non-Gibbsian thermal equilibrium state that emerges at strong couplings during quasistatic isothermal processes. These excess definitions differ from conventional ones by an energetic cost for maintaining the non-Gibbsian characteristics. With this distinction, we point out that one can introduce two different yet thermodynamically allowed definitions for efficiency of both the heat engine and refrigerator modes. We dub them excess and hybrid definitions, which differ in the way of defining the gain for the thermal machines at strong couplings by either just analyzing the energetics of the working substance or instead evaluating the performance from an external system upon which the thermal machine acts, respectively. We analytically demonstrate that the excess definition predicts that the Carnot limit remains the upper bound for both operation modes at strong couplings, whereas the hybrid one reveals that strong coupling can suppress the maximum efficiency rendering the Carnot limit unattainable. These seemingly incompatible predictions thus indicate that it is imperative to first gauge the definition for efficiency before elucidating the exact role of strong coupling, thereby shedding light on the ongoing investigation on strong-coupling quantum thermal machines.
与热库的强耦合是否能提高量子热机的性能仍是一个正在积极讨论的开放性问题。在此，我们重新审视以准静态卡诺循环运行的量子热机，旨在揭示强耦合在最大效率方面的作用。我们的分析基于从工作物质的热力学第一定律的精确表述中导出的额外功和热量的定义，该表述捕捉了在准静态等温过程中强耦合时出现的非吉布斯热平衡态。这些额外定义与传统定义的不同之处在于维持非吉布斯特性的能量成本。基于这种区别，我们指出可以为热机和制冷机模式的效率引入两种不同但热力学上允许的定义。我们将它们称为额外定义和混合定义，它们在定义强耦合时热机增益的方式上有所不同，分别是仅通过分析工作物质的能量学，或者通过评估热机作用的外部系统的性能。我们通过分析证明，额外定义预测在强耦合时卡诺极限仍然是两种运行模式的上限，而混合定义则表明强耦合可以抑制最大效率，使卡诺极限无法达到。因此，这些看似相互矛盾的预测表明，在阐明强耦合的确切作用之前，必须首先确定效率的定义，从而为正在进行的强耦合量子热机研究提供启示。
