One of our central objectives is the study of dynamical correlations in frustrated quantum spin systems in order to predict experimentally accessible fingerprints of emergent quantum phases. This entails for example the numerical calculation of spectral functions which can be compared to neutron scattering experiments. Here we made an important methodological breakthrough by developing an efficient algorithm that allows to obtain the dynamical response functions for two-dimensional lattice systems. Using this algorithm, we obtained the dynamic spin structure factor of the paradigmatic Heisenberg model on the square lattice and reproduced a puzzling feature which had been observed in experiment. We also considered the Heisenberg model on the triangular lattice. Our novel algorithm made the dynamic spin structure factor in this highly-quantum model accessible for the first time with an unbiased method. Remarkably, we found that the emergent quasiparticle excitations are long-lived —in contradiction with the common belief— and developed a simple microscopic theory to explain this phenomenon. We also investigated dynamical signatures of symmetry-broken and liquid phases in an S=1/2 Heisenberg antiferromagnet on the triangular lattice, providing reference data for experiments on triangular lattice materials.
We made substantial progress on the investigation of quantum phases that emerge when a many-body system is brought out of equilibrium. Here we discovered the concept of Stark many-body localization (MBL) and showed that several existing experimental probes, designed specifically to differentiate between these scenarios, work similarly in the Stark many-body localization (MBL) setting. Moreover, we found several new dynamical phases of matter in quantum many body systems and were able to prove their robustness in different settings. In this context, we investigated the stability of a many-body localized phase that can be realized in a central spin system. In a series of works, we discovered a new phenomenon dubbed “Hilbert space fragmentation”. In particular, we showed that the combination of charge and dipole conservation leads to an extensive fragmentation of the Hilbert space, which, in turn, can lead to a breakdown of thermalization. Excitingly, this new phase of matter can protect symmetry protected topological order in highly excited states in certain settings. This might be highly relevant to build robust quantum memories. We demonstrated this phenomenon to be robust in higher dimensions and also in open quantum systems.
In order to efficiently simulate complex quantum many-body systems, we developed novel tensor-network based tools and quantum algorithms for using noisy intermediate-scale quantum (NISQ) devices. Tensor network algorithms are efficient classical algorithms for simulating quantum many-body systems. To go beyond the one-dimensional limit, we introduced the class of isometric tensor network states (isoTNS), which enables novel algorithms for the simulation of two-dimensional quantum lattice models. We extended this algorithm to open systems and identified models that have an exact solution in terms of isoTNS. In addition to developing algorithm for the use on classical computers, we have also been investigating the possibility of using quantum computers for simulating condensed matter physics. Our first work considered Trotterized evolution of non-equilibrium quantum dynamics and was able to see signatures of non-trivial many-body physics in small scale systems. We demonstrated the first successful implementation of topological order on a quantum processor, inspiring many other group to extend our results to other topological orders.
Overall the project has resulted in 99 peer reviewed articles and 76 invited talks at international conferences. Moreover, the PI organized 8 scientific workshops and summer schools for PhD students related to the project.
