It is long known that the dimensionality of a system has great influence on the laws of physics. In one dimension, the Hohenberg-Mermin-Wagner theorem predicts that, under some constraints,  long-range order cannot be established in one dimension, as thermal fluctuations drive the system to an unordered situation. But what happens when going from two to one dimension? We have studied this crossover together with colleagues from the RPTU Kaiserslautern-Landau in a quantum gas of light.

Photons confined in a potential with tunnel-coupled minima in a ring shape can populate the hybridized states of that ring. The ground state in such a ring is the symmetric superposition of the eigenstates of the individual wells, a smeared-out ringlike state. Using cooling enabled by thermal contact to a dye solution, we were able to coll directly into this ground state, and verify the phase coherence of the superposition state. The results have been published in Physical Review Letters. 

The quantum Rabi model describes the coupling of a two-level system to a bosonic mode, one prominent example is an atom coupled to a light field. When the coupling gets stronger than the relevant energy scales of the atom and the light field new effects are expected, for example a collapse ad revival of the initial state. In optical systems, however, this regime is not achievable, but cold atoms can simulate this regime. This proposal, conceived together with our theory colleagues from Bilbao, has been tested successfully in our lab.

Dr. Julian Schmitt from the Institute of Applied Physics received the Wissenschaftspreis 2024 of the Industrie-Club Düsseldorf for his groundbreaking research on quantum gases of photons, which on the one hand expand the understanding of quantum states in fundamental science and on the other hand can produce new technological components for the sensor technology or control of laser beams.

A fundamental relation connecting the temporal fluctuations and the response behaviour has been experimentally observed in a quantum gas made of photons, the particles of light. The validity of this so-called quantum regression theorem could so far not be directly revealed for quantum gases.

Dr. Julian Schmitt of the Institute for Applied Physics of the University of Bonn is awarded the Rudolf-Kaiser-Prize 2023 - one of the most renowned German prizes for young researchers in physics. 

An important step towards the realization of a VUV photon condensate has been achieved.

Our paper “Quantum Rabi dynamics of trapped atoms far in the deep strong coupling regime“ has been published in the journal Nature Communications. We report on a study of ultrastrong coupling between two mechanical modes of trapped cold atoms in the quantum Rabi regime. Using a novel experimental scheme, we achieve a Rabi coupling of 6.5 times the field mode frequency, so that the coupling term clearly dominates over all other energy scales for the first time.

The grand-canonical coupling of photons to a bath of dye molecules leads to fluctuations of the particle number. According to the fluctuation-dissipation theorem this is connected to a viscosity of the photon gas. In our experiment, we were able to prove the relation for a Bose-Einstein condesed gas for the first time.

Andreas Redmann presented his PhD project at the Excellence Slam of the university.

Our group members Steffi Moll and Niels Wolf presented experiments on the foundations of quantum physics in Regensburg.

[ Press release ]
[ Science Perspective  article by R. Fletcher and M. Zwierlein ]
[ New Scientist article by A. Wilkins ]
[ Media Inaf article by V. Guglielmo ]
[ Physics World article by S. Jarman ]