The condensed matter theory group works on a wide range of modern topics centered around correlated electrons, quantum magnetism, nanostructures and non-equilibrium dynamics. We employ various numerical methods like QMC, DMRG and ED, and analytical approaches like field theoretic methods and flow equations.
Research group Prof. Stefan Kehrein
The main focus of research in the group of Stefan Kehrein is the non-equilibrium dynamics of quantum many-body systems. This has become a very active field of research in the past decade due to both experimental advances and new theoretical methods for dealing with non-equilibrium problems. Kehrein's group addresses a wide range of problems like thermalization, buildup of correlations, irreversibility and method development.
Irreversible dynamics in quantum many-body systems
Irreversibility, despite being a necessary condition for thermalization, still lacks a sound understanding in the context of quantum many-body systems. In this work we approach this question by studying the behavior of generic many-body systems under imperfect effective time reversal, where the imperfection is introduced as a perturbation of the many-body state at the point of time reversal. Based on numerical simulations of the full quantum dynamics we demonstrate that observable echos occurring in this setting decay exponentially with a rate that is intrinsic to the system meaning that the dynamics is effectively irreversible. [more...]
Flow Equation Holography
The Ryu-Takayanagi conjecture establishes a remarkable connection between quantum systems and geometry. Specifically, it relates the entanglement entropy to minimal surfaces within the setting of AdS/CFT correspondence. This Letter shows how this idea can be generalised to generic quantum many-body systems within a perturbative expansion where the region whose entanglement properties one is interested in is weakly coupled to the rest of the system. A simple expression is derived that relates a unitary disentangling flow in an emergent RG-like direction to the min-entropy of the region under consideration. Explicit calculations for critical free fermions in one and two dimensions illustrate this relation. [more...]