Cavity QED with two-dimensional quantum magnets

• Applicant: Dr. Lukas Weber
• Subject Area: Theoretical Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2022 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 503774589

Recent advances in the engineering of highly reflective electromagnetic cavities have made it possible to increase the effective interactions between quantum light and matter far beyond their strength in vacuum quantum electrodynamics. While the influence of such enhanced light-matter interactions on atomic systems has been studied in the field of quantum optics, nowadays, the potential for strong light-matter interactions in condensed matter applications is becoming increasingly apparent. Here, cavity photons may be used to alter material properties or stabilize novel quantum phases.The main objective of this project is to explore the effects of strong light-matter interactions in matter systems which by themselves are strongly interacting via their intrinsic electron-electron interactions. As a particular example of this, I will consider quantum magnets, where the electronic charges are immobile due to the Coulomb repulsion and the electron spins interact through exchange interactions. Quantum magnets thus have a reduced number of degrees of freedom, yet show a large spectrum of strongly interacting phenomena from spontaneous symmetry breaking to topological order. A distinct part of this spectrum are also quantum critical points and quantum critical phenomena in general, which are characterized by the divergence of the susceptibility of the system to certain external influences. Thus, for example, a small change in the external magnetic field or pressure can have a large effect on the quantum magnet as a whole.In this project, the external influence on the quantum critical magnet is given by the coupling to the cavity photon field, leading to strong correlations between magnetic spins and cavity light. In such a setup, using different microscopic models of quantum magnets, I will investigate the interplay with a cavity from three distinct angles. First, I will quantify the effect of the cavity on the magnetic phase diagram, including for instance, the influence on existing phase boundaries and critical temperatures or the appearance of new magnetic phases. Second, I will consider the the back action of the magnetic correlations on the cavity, which may lead to exotic photonic states. Third, fusing the two previous questions, I will search for strongly entangled light-matter states that arise as a result of the interactions in the system.An extension to these goals will be the identification of realistic models to find pathways towards observing the studied phenomena in experiments. Further, I will consider the addition of a weak few-photon drive to the cavity, generalizing the main results of this project, which will be obtained in thermal equilibrium, to a nonequilibrium setting.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Dr. Ángel Rubio