Collective Many-Body and Multi-Photon Effects in Ground-State Polaritonic Systems

When light and matter interact strongly inside optical cavities, they form hybrid light--matter states known as polaritons, which fundamentally alter molecular and material properties. The study of these phenomena, known as polaritonic chemistry, explores how cavity confinement can modify molecular and chemical properties. Among the theoretical approaches developed to describe such systems, quantum-electrodynamical density-functional theory (QEDFT) is one of the most promising first-principles frameworks, extending traditional DFT to include quantized electromagnetic fields. However, most existing photon exchange--correlation (XC) functionals have been limited to single-photon processes, restricting the exploration of richer quantum-optical behavior such as photon correlations and field fluctuations. In this work, we employ the photon many-body dispersion (p-MBD) functional [1], which captures anisotropic and higher-order photon--electron interactions beyond previously developed XC functionals. This allows us to investigate how higher-order effects shape the exchange--correlation energy, photon number scaling, and quantum-optical observables such as photon statistics, field quadratures, and Mandel-Q parameters, highlighting the role of multi-photon processes in polaritonic systemss. These findings help establish a bridge between quantum optics and QEDFT.

1] C. Tasci, L. A. Cunha, and J. Flick, Photon Many-Body Dispersion: Exchange-Correlation Functional for Strongly Coupled Light–Matter Systems, Phys. Rev. Lett. 134, 073002 (2025).