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.