Not all properties of matter can be described purely by classical physics in terms of particles that follow Newton’s laws. For example, one can nowadays cool atoms to temperatures near absolute zero, where they lose their classical character and instead behave like electrons in a solid or form a quantum fluid, a so-called Bose-Einstein condensate. Our ever-advancing ability to control such ultracold atoms has enabled a broad range of fundamental science and provides a backbone for developing quantum technologies from quantum simulations and quantum computing to quantum-enhanced sensing. In our research we study the generation and control of atomic interactions by external fields or laser-light, and explore their fascinating and often surprising effects on the properties of such cold quantum matter.

A quantum fluid of particles with dipole-dipole interactions can form a so-called supersolid upon raising its temperature, i.e., it can crystallize by heating (Nature Comm. 14, 1868 (2023), opens an external URL in a new window).

Observing a quantum system inevitably leads to backaction effects on the state of the system that are central to our understanding of quantum mechanics. While such an interaction with an external observer or environment typically causes decoherence and the emergence of classical behavior, it can also be engineered to steer quantum dynamics and to even generate highly non-classical states. In our work, we apply and develop methodology to describe the evolution of open quantum many-body systems, under the influence of dissipation, measurements, strong particle interactions and coherent driving. Hereby, we investigate nonequilibrium many-body dynamics and seek to understand collective phenomena that may emerge from light-matter interactions in complex quantum systems.

An atomic gas that is continuously excited and probed by laser light exhibits spontaneously emerging oscillations of its optical transmission. The measured oscillations (left) can be explained by a theoretical model that predicts an oscillating phase in the non-equilibrium phase diagram as a function of the frequency detuning (\(\Delta\)) and amplitude (\(\Omega\)) of the applied laser. Such phases that spontaneously break time translational symmetry are called time crystals (arXiv:2305.20070, opens an external URL in a new window).

Photons - the elementary particles of light - are devoid of any type of mutual interaction in free space. While this fundamental property of light is crucial to the success of modern day optical communication, the ability to induce synthetic interactions between photons, like the ones between electrons in electronic circuits, may open up new possibilities for quantum science and technology. In our research we study the coupling of light to different optical interfaces and explore how effective interactions between photons may emerge under such conditions. To this end, we investigate effects of strong particle interactions between atoms and in solid-state systems or collective optical phenomena and employ numerical simulations or advanced analytical techniques.

Effective interactions between photons can emerge in a range of physical systems, including atomic gases (left), regular arrays of closely spaced atoms (middle), or semiconducting materials (right).