QUASICOMBS · Quantum simulator for strongly correlated many body systems

Final Activity Report Summary - QUASICOMBS (Quantum Simulator for Strongly Correlated Many Body Systems)

The project focussed on quantum simulations of strongly interacting quantum systems in artificial crystals of light. Understanding the complex interplay of a large number of particles according to the laws of quantum mechanics represents one of the largest challenges in the physical sciences. Our ability to advance material design and control our ambient world at the microscopic level is, however, crucially tied to our fundamental understanding of this collective behaviour. Already in the 1980s, the Nobel Prize winner in physics, R. P. Feynman, outlined how artificial quantum mechanical systems could help to analyse such problems, which tend to become quickly intractable on classical computers. As such analog quantum simulators, our research team has developed two distinct experimental setups for quantum simulations of bosonic, fermionic and bosonic-fermionic mixtures of ultracold atoms. In these setups, atomic quantum gases at temperatures as low as a few Nanokelvin are placed in artificial crystal structures formed by laser light, so called optical lattices, which are completely controlled in the experiment. The lattice depth, the dimensionality and even the symmetry of the lattice structures are designed in the experiment to fulfil a specific configuration to be analysed. In addition, the interaction strength between the particles can be tuned via so called Feshbach resonances. Most of the microscopic system parameters can also be controlled dynamically in the experiment. During the course of the project, our work focussed on the investigation of strongly correlated many-body quantum phases in such optical lattices. For the analysis of strongly interacting phases, the research team has been able to demonstrate a novel quantum optics based detection method, which allows one to reveal the real space ordering of the particles in the lattice. By employing a correlation analysis of the fundamental quantum noise present in these systems, the team could show for two examples of a Mott insulating bosonic quantum gas and a fermionic band insulator how these states leave detectable traces in their quantum noise. Furthermore, within these experiments, the first 'antibunching' of fermionic atoms has been demonstrated. This hallmark effect of quantum mechanics - originally demonstrated by Hanbury-Brown and Twiss for the case of photons - had previously only been observed for charged fermionic particles and has now been unambiguously demonstrated in the course of QUASICOMBS for the case of neutral particles. In the experiments, the paradigm of the transition from a weakly interacting gas to a strongly interacting many-body quantum system of a superfluid to a Mott insulating state of matter has been extensively analysed. The momentum distribution, the number statistics, as well as the change in the spatial density profiles in the transition have been revealed in the course of QUASICOMBS. This has led to fundamental insights into the properties and the dynamics of this exemplary transition in condensed matter physics. Next to static properties, dynamical properties of correlated atom systems were analysed in a final series of experiments. In this area, the team has shown how the dynamical evolution of atoms tunnelling through a barrier can dramatically change in the presence of interactions between the particles. Whereas for negligible interactions the particles can tunnel independently, in the presence of strong repulsive interactions, atoms can only tunnel together as a pair. In the experiments, we have been able for the first time to fully resolve the dynamics of correlated tunnelling over the full range of parameters from weak to strong interactions. Currently, the team is focussing on experiments with mixtures of bosonic and fermionic quantum gases in optical lattices. Such mixtures are ideal systems for the investigation of new quasi-particles that emerge in the case of strong interactions. First experiments have been carried out, in which such a 'dressing' of the particles could be observed.

Data: CORDIS, © European Union