exp. quantum physics, quantum gases, low-dimensional quantum systems, ultracold molecules

Hanns-Christoph Nägerl is one of the worlds leading quantum physicists in the field of ultra-cold multi-particle quantum systems. He is particularly known for his work on atomic quantum wires and molecular quantum gases. In his research Hanns-Christoph Nägerl deals with atomic and molecular quantum gases near the absolute temperature zero point. It is his goal to make quantum simulators based on laser-cooled atoms and molecules in the laboratory, in order to elucidate complex multi-particle quantum processes directly in the experiment, e.g. behind the previously unexplained high-temperature superconductivity in solids that enable or prevent electronic transport in future electronic circuits, or which are relevant for the formation of novel superfluids. The complete control of multi-particle quantum systems and the targeted production of quantum matter are important projects in modern quantum physics. Thus quantum states are to be specifically engineered in order to make them available for simulation or for new measurement tasks. This approach is particularly relevant to dynamic multi-particle quantum processes, which, due to quantum correlations, cannot usually be controlled by classical simulation methods on conventional computers. To this end, the latest methods from quantum optics, laser physics and the physics of quantum gases are used. Quantum gases are particularly well suited for the study of multi-particle quantum dynamics, since their dimensionality and interaction properties can be controlled under excellent shielding from the classical outside world. One focus of his work is quantum control and the study of quantum dynamics in so-called quantum wires. In view of the continuing rapid miniaturisation of electronic components, it is expected that the ultimate limit in lateral expansion will soon be reached, namely the trace width of only one atom. Quantum gases in narrow light tubes can even now be used to simulate many of the expected effects. Quantum wires are characterised by a wealth of exciting multi-particle effects. These were discovered and investigated by Nägerl and his colleagues, such as the fermionisation in the case of boson particles and their effect on quantum transport, the over fermionisation in the case of strong attractive interaction and the formation of quantum crystals and the associated localisation by potential disturbances however weak they may be. Another focus of his work is the production and investigation of molecular quantum gases. These are characterised by rich interaction effects, which should lead to novel multi-particle quantum phases and a variety of previously unexplored phenomena in the field of quantum magnetism. This may be the key to understanding high-temperature superconductivity. In recent years, Nägerl and his team have demonstrated how quantum gases can be produced from molecules at high particle density and temperatures in the nanokelvin range. This was a quantum-physics surprise, given that the technology of laser cooling cannot easily be extended from atoms to molecules. This opened the door to a new field of research. Among other things, it should be possible to completely control quantum spin systems with hundreds, if not thousands of quantum spins. The support of the Wittgenstein Prize will enable Hanns-Christoph Nägerl to take his work on quantum control of multiple-particle systems to the next level. The aim is to detect the molecules individually and state-selectively from the molecular quantum gases and subsequently manipulate them individually. Among other things, the spatial correlation functions and their time- dependency become accessible directly, which makes it possible to quantify the quality of quantum simulation. Other possible applications include precision metrology and answering the question of whether fundamental natural constants are really constant.

The Wittgenstein Project has supported various quantum simulation activities in the Nägerl group. Three highlights stand out: the quantum simulation of one-dimensional anyons (published in Nature), the observation of many-body dynamical localization (MBDL, in press with Science), and the generation of Bethe-string states (submitted). Based on ultracold atoms initially collectively in a Bose-Einstein condensate (BEC) state, strongly interacting anyons in one-dimensional quantum gas quantum wires could be observed by measuring the momentum distribution of impurity atoms in the zero-temperature limit. The mean momentum acts as a statistical phase, which could be tuned from bosonic to anyonic to fermionic. For driven one-dimensional strongly interacting systems, the phenomenon of Anderson localization in momentum space (MBDL) was discovered, which remains stable even in the presence of strong interaction. For attractive interactions, so-called Bethe strings were realized for the first time in the context of ultracold quantum gases. These represent the elementary excitations of strongly attractively interacting one-dimensional quantum wires. Other highlights of the Wittgenstein Project include the observation of confinement resonances in a three-dimensional lattice under strong interactions, the measurement of temperature in strongly interacting one-dimensional (1D) and two-dimensional (2D) quantum gases, and the observation that the interplay of strong interactions and strong confinement can lead to cooling effects (published in "Science Advances"), the realization of a cesium BEC in an excited quantum state (published in "Nature Communications"), and the observation of the 1D-2D crossover regime for strongly interacting quantum gases (published in "Nature Physics"). The Wittgenstein Project has opened the door to a number of follow-up research projects, including anyons, Bethe strings, and localization effects. It is planned to carry out this and further work in the future within the framework of an ERC Advanced Grant recently awarded by the ERC.