Research group
Nonequilibrium Quantum Matter
lead by Priv.-Doz. Dr. Michael Buchhold

Institute for Theoretical Physics
Cluster of Excellence ML4Q 

University of Cologne 

Zülpicher Str. 77a , 50937 Cologne

Research interests 

The research in our group focusses on driven open quantum matter and nonthermal quantum states in 

This places our research at the interface of condensed matter, quantum optics, quantum information and statistical physics. If you are interested in our work, you may check our publications, watch a recent talk at Perimeter Institute about self-organized criticality in driven Ryberg systems or read a recent viewpoint about our work on entanglement transitions. 

Monitored Quantum Systems

We show that a large variety of entanglement transitions in monitored circuits can be understood from loop models of 2D statistical mechanics.

We apply this framework to augment quantum circuits with classical agents to engineer unconventionally entangled states.

We identify regimes with unconventional scaling behavior of the entanglement entropy of monitored fermions.

Field Theory for Monitored Quantum Systems
Physical Review X 11, 041004 (2021)
SciPost Phys. 12, 009 (2022)

A wave function subject to unitary time evolution and exposed to measurements undergoes pure state dynamics, with deterministic unitary and probabilistic measurement-induced state updates, defining a quantum trajectory. For many-particle systems, the competition of these different elements of dynamics can give rise to a scenario similar to quantum phase transitions. We construct an n-replica Keldysh field theory for the ensemble average of the n-th moment of the trajectory projector. 

Quantum Materials Out-of-Equilibrium

Strongly driven correlated insulators have the potential to exhibit exotic transient phenomena that are forbidden in thermal equilibrium. In such far-from-equilibrium regimes, nonthermal metastable states emerge, which go beyond the conventional framework of time-dependent Ginzburg-Landau descriptions. We approach such situations from the viewpoint of nonequilibrium universality and the Keldysh path integral framework. 

Effects of weak disorder in Weyl materials:
Phys. Rev. Lett. 121, 215301 (2018)
Phys. Rev. B 98, 205134 (2018)

Disorder-bound quantum states in topological semimetals are fragile: their contribution to the density of states effectively vanishes in many cases when averaged over a continuous disorder distribution. This protects the integrity of the Weyl nodes against weak disorder.

Driven Open Quantum Gases

Scale invariance without fine tuning:
Self-organized criticality and Griffiths phases in Rydberg gases
Nature Communications 12, 103 (2021)
Nature 577, 481 (2020)

Theory of emergent epidemics and self-organized criticality in driven Rydberg gases
Phys. Rev. Lett. 126, 123401 (2021)
Phys. Rev. A 99, 053616 (2019)
Phys. Rev. A 98, 062117 (2018)
Phys. Rev. Lett. 116, 245701 (2016) 

The competition between strong driving and strong interactions in a gas of ultracold Rydberg atoms gives rise to nonequilibrium scenarios that are close to epidemics: phase transitions into absorbing states and self-organized criticality.

Discontinuous and continuous non-equilibrium phase transitions in dissipative Dicke setups: Phys. Rev. A 97, 023807 (2018)
 Phys. Rev. A 95, 063824 (2017)

Thermalization and out-of-equilibrium dynamics of interacting Luttinger Liquids (arXiv1, arXiv2)