Stefan Kehrein's Research

My main research interest lies in the many fascinating properties of quantum many-body systems. The models are mostly motivated from condensed matter physics, but there are also remarkable and useful connections to other fields of theoretical physics like black holes and quantum information theory. I am especially interested in non-equilibrium situations, for example photoexcited materials, because non-equilibrium offers a new route to realizing quantum matter with novel properties. While there has been a lot of progress in this direction in the past decade both experimentally and theoretically, the non-equilibrium map of correlated materials still contains a lot of terra incognita. This combination of practical questions with fundamental questions is rewarding, challenging and a lot of fun. My group and I pursue our research along those lines using mainly analytical tools supplemented by numerical methods.

The study of entanglement properties has become an important tool across many fields of physics from condensed matter physics to quantum information theory and black hole physics. Generically, the entanglement entropy of ergodic systems grows linearly in time until it saturates at a value given by the volume law for excited states. While the general validity of this behavior has been confirmed by many studies, a long standing debate in black hole physics centers around the very different entanglement dynamics described by the Page curve where the entanglement entropy has to decrease again after the so called Page time. My recent paper Phys. Rev. B 109, 224308 (2024) [ arXiv:2311.18045 ] describes an analytically solvable quantum many-body that shows exactly this kind of behavior. Interestingly, the increasing and decreasing regime of the entanglement entropy are separated by a quantum phase transition of the entanglement Hamiltonian and can therefore be thought of as different phases of quantum matter.

Thermalization of isolated quantum systems

The topic of thermalization of isolated quantum many-body systems continues to be of central interest in non-equilibrium quantum many-body physics. The generic non-integrable case poses considerable challenges both analytically and numerically, especially since one is usually interested in the thermodynamic limit. In a recent paper we investigated the quench dynamics of a generalised Sachdev-Ye-Kitaev model by mapping it to a simple numerical problem even in the thermodynamic limit. We find rich non-equilibrium dynamics leading to a final stationary thermal equilibrium state.