Microcanonical and resource-theoretic derivations of the thermal state of a quantum system with noncommuting charges

• Published in: Nature communications 2016-07, Vol.7 (1), p.12051-12051, Article 12051
• Main Authors: Yunger Halpern, Nicole, Faist, Philippe, Oppenheim, Jonathan, Winter, Andreas
• Format: Article
• Language: English
• Subjects: 639/766/483/640; 639/766/530/951; Entropy; Equilibrium; Humanities and Social Sciences; multidisciplinary; Physics; Science; Science (multidisciplinary); Statistical mechanics; Thermodynamics
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms12051

The grand canonical ensemble lies at the core of quantum and classical statistical mechanics. A small system thermalizes to this ensemble while exchanging heat and particles with a bath. A quantum system may exchange quantities represented by operators that fail to commute. Whether such a system thermalizes and what form the thermal state has are questions about truly quantum thermodynamics. Here we investigate this thermal state from three perspectives. First, we introduce an approximate microcanonical ensemble. If this ensemble characterizes the system-and-bath composite, tracing out the bath yields the system’s thermal state. This state is expected to be the equilibrium point, we argue, of typical dynamics. Finally, we define a resource-theory model for thermodynamic exchanges of noncommuting observables. Complete passivity—the inability to extract work from equilibrium states—implies the thermal state’s form, too. Our work opens new avenues into equilibrium in the presence of quantum noncommutation. A central concept in thermodynamics is the thermal state, which is the one towards which the system relaxes. Here, the authors derive the same state, through three different approaches, in the case of a quantum system whose conserved quantities correspond to operators that do not commute with one another.