The Quench Action

TL;DR

The paper introduces the Quench Action methodology for calculating time-dependent expectation values after a quantum quench, combining exact overlap data with variational reasoning, applicable to various observables and times.

Contribution

It provides a comprehensive pedagogical overview of the Quench Action method, highlighting its general validity and potential for future research in out-of-equilibrium quantum systems.

Findings

Valid for arbitrary times post-quench

Applicable to a wide class of observables

Combines exact overlaps with variational principles

Abstract

We give a pedagogical introduction to the methodology of the Quench Action, which is an effective representation for the calculation of time-dependent expectation values of physical operators following a generic out-of-equilibrium state preparation protocol (for example a quantum quench). The representation, originally introduced in [Caux and Essler, Phys. Rev. Lett. 110, 257203 (2013)], is founded on a mixture of exact data for overlaps together with variational reasonings. It is argued to be quite generally valid and thermodynamically exact for arbitrary times after the quench (from short times all the way up to the steady state), and applicable to a wide class of physically relevant observables. Here, we introduce the method and its language, give an overview of some recent results, suggest a roadmap and offer some perspectives on possible future research directions.