Quantum fluctuations in dipolar gases: Quantum droplets, vortices and impurities

• Applicant: Professor Dr. Tilman Pfau
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 392530138

Correlations and fluctuations in a many-body system are the origin of physics beyond mean-field description of the system. This appears naturally if interactions are strong compared to the kinetic energy. In quantum systems, sometimes strong - and microscopically even intractable - quantum correlations can appear and lead to new physics that emerges on the macroscopic scale. However recently it was shown that weak perturbative correlations can also dominate the physics if two weak interactions almost compensate each other on the mean-field level while the fluctuations they induce add up. One first example of this kind of correlation-dominated physics was the surprising discovery of self-bound states of extremely dilute quantum liquids - so called quantum droplets. In this system long and short range interactions compensate each other while their fluctuations stabilize a many-body state that would be unstable on the mean-field level.Here we propose to investigate the fundamental properties of such droplets as well as the applications of these novel isolated quantum objects. We will do so by investigating many-body systems of strongly dipolar dysprosium atoms with a focus on quantum fluctuation effects in dipolar superfluidity, in low dimensions and on fermionic impurities. First, we will dramatically modify the behavior of dipolar quantum fluctuations via the dimensionality of the system. Interestingly, in two dimensions the theoretical description of quantum fluctuations in the presence of dipole-dipole interactions is not known to date calling for experiments to clarify their effect. In one dimension there is first theoretical work on dipolar quantum fluctuations, which however needs to be tested as there are controversial views on the validity of that theory. Again there is clear need for experimental clarification.Second we will investigate the effect of the fluctuations on the superfluid properties of the system. In the crossover from three to quasi-two dimensions we will study the anisotropy of dipolar fluctuations as well as the resulting anisotropy in the Landau critical velocity. Also the transition from Abrikosov vortex lattices to stripe arrangements of vortices will be studied. If those phenomena are observed close to mean-field instabilities they become most susceptible to the effects of fluctuations. Third, an important tool to study thermal and quantum fluctuations is to study dissolved impurities. In the case of a quantum droplet we plan to use the measured number of fermionic impurities in a droplet to do thermometry and determine thermodynamic properties like specific heat. These studies of impurities will try to answer questions on the evaporation process of the droplets. Finally we will investigate the nature of the transition from a gaseous to a liquid state using this tool.

• DFG Programme: Research Grants
• Major Instrumentation: Single frequency seed laser + fiber amplifier 100W
• Instrumentation Group: 5700 Festkörper-Laser