Department of Physics and Astronomy, University College London, London, WC1E 6BT, UK.
Department of Physics, University of Oxford, Oxford, OX1 3PU, UK.
Nat Commun. 2017 Dec 1;8(1):1895. doi: 10.1038/s41467-017-01505-4.
Passivity is a fundamental concept in thermodynamics that demands a quantum system's energy cannot be lowered by any reversible, unitary process acting on the system. In the limit of many such systems, passivity leads in turn to the concept of complete passivity, thermal states and the emergence of a thermodynamic temperature. Here we only consider a single system and show that every passive state except the thermal state is unstable under a weaker form of reversibility. Indeed, we show that given a single copy of any athermal quantum state, an optimal amount of energy can be extracted from it when we utilise a machine that operates in a reversible cycle. This means that for individual systems, the only form of passivity that is stable under general reversible processes is complete passivity, and thus provides a physically motivated identification of thermal states when we are not operating in the thermodynamic limit.
被动性是热力学中的一个基本概念，它要求量子系统的能量不能通过任何作用于系统的可逆、幺正过程降低。在许多这样的系统的极限下，被动性依次导致完全被动性、热态和热力学温度的出现。在这里，我们只考虑单个系统，并表明在较弱的可逆性形式下，除了热态之外的每个被动态都是不稳定的。事实上，我们表明，对于任何非热量子态的单个副本，当我们利用在可逆循环中运行的机器时，可以从中提取最佳量的能量。这意味着对于单个系统，在一般可逆过程下稳定的唯一被动形式是完全被动性，因此在我们不在热力学极限下操作时，为热态提供了一种物理上合理的识别。
