Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Avenida dos Estados 5001, 09210-580 Santo André, São Paulo, Brazil.
Faculty of Physics, University of Vienna, Boltzmangasse 5, Vienna A-1090, Austria.
Phys Rev Lett. 2015 Nov 6;115(19):190601. doi: 10.1103/PhysRevLett.115.190601. Epub 2015 Nov 2.
Irreversibility is one of the most intriguing concepts in physics. While microscopic physical laws are perfectly reversible, macroscopic average behavior has a preferred direction of time. According to the second law of thermodynamics, this arrow of time is associated with a positive mean entropy production. Using a nuclear magnetic resonance setup, we measure the nonequilibrium entropy produced in an isolated spin-1/2 system following fast quenches of an external magnetic field. We experimentally demonstrate that it is equal to the entropic distance, expressed by the Kullback-Leibler divergence, between a microscopic process and its time reversal. Our result addresses the concept of irreversibility from a microscopic quantum standpoint.
不可逆性是物理学中最引人入胜的概念之一。虽然微观物理定律是完全可逆的，但宏观平均行为具有时间的优先方向。根据热力学第二定律，这个时间箭头与正的平均熵产生有关。使用核磁共振装置，我们测量了在磁场快速淬火后，一个孤立的自旋-1/2 系统中产生的非平衡熵。我们的实验结果表明，它等于微观过程与其时间反转之间的熵距离，用柯尔莫哥洛夫-莱布勒散度表示。我们的结果从微观量子的角度解决了不可逆性的概念。
