(Non) equilibrium dynamics: a (broken) symmetry of the Keldysh generating functional

• Main Author: Camille Aron, Giulio Biroli, Leticia F. Cugliandolo
• Format: Article
• Language: English
• Published: SciPost 2018-01-01
• Series: SciPost Physics
• Online Access: https://scipost.org/SciPostPhys.4.1.008

We unveil the universal (model-independent) symmetry satisfied by Schwinger-Keldysh quantum field theories whenever they describe equilibrium dynamics. This is made possible by a generalization of the Schwinger-Keldysh path-integral formalism in which the physical time can be re-parametrized to arbitrary contours in the complex plane. Strong relations between correlation functions, such as the fluctuation-dissipation theorems, are derived as immediate consequences of this symmetry of equilibrium. In this view, quantum non-equilibrium dynamics -- e.g. when driving with a time-dependent potential -- are seen as symmetry-breaking processes. The symmetry-breaking terms of the action are identified as a measure of irreversibility, or entropy creation, defined at the level of a single quantum trajectory. Moreover, they are shown to obey quantum fluctuation theorems. These results extend stochastic thermodynamics to the quantum realm.