Explore quantum many-body physics with multimode cavity QED in this comprehensive lecture from the Open Quantum Systems program at ICTS Bangalore. Delve into topics like the Dicke model, superradiance, matter-light interactions in Coulomb gauge, and dipole approximation. Examine the two-level system concept, analyze graphs and diagrams, and investigate the Dicke model and Tavis-Cummings model. Study classical harmonic oscillators, magnetic fields, phase transitions, and mathematical proofs. Gain insights into cutting-edge research in mesoscopic quantum optics, cavity optomechanics, and quantum gases in cavities from expert Jonathan Keeling.