Super-Poissonian Squeezed Light in the Ground State of Strongly Coupled Light-matter Systems

Strong light-matter coupling enables hybrid states in which photonic and electronic degrees of freedom become correlated even in the ground state. While many-body effects in long-range dispersion interactions are known to reshape electronic properties under such conditions, their impact on quantum-optical observables remains largely unexplored. Here, we address this problem using quantum electrodynamical density-functional theory (QEDFT) combined with the recently developed photon-many-body dispersion (pMBD) functional, which can capture higher-order electron-photon correlations and multi-photon processes. We compute ground-state photonic observables including photon number fluctuations, second-order correlations, and quadrature variances, and find squeezing and super-Poissonian photon statistics emerging from light-matter interactions in the strong coupling regime. Our results demonstrate that capturing the full hierarchy of many-body, electron-photon and multi-photon correlations is essential for a consistent description of quantum-optical properties in strongly coupled molecular systems, establishing QEDFT as a first-principles framework for predicting nonclassical photonic features in the ground state of complex systems.

💡 Research Summary

In this work the authors address a fundamental gap in the theoretical description of strongly coupled light‑matter systems: the lack of a first‑principles framework that can simultaneously treat electronic structure and quantum‑optical observables beyond simple one‑photon exchange approximations. While quantum electrodynamical density‑functional theory (QED‑DFT) provides a formally exact route to coupled electron‑photon problems, existing exchange‑correlation (xc) functionals have been limited to low‑order photon‑mediated corrections. Consequently, predictions of ground‑state photon statistics, squeezing, and entanglement have been either absent or unreliable.

To overcome this limitation the authors employ the recently developed photon‑many‑body dispersion (pMBD) functional, an extension of the many‑body dispersion (MBD) method that incorporates long‑range electron‑photon correlations and multi‑photon processes. They reformulate the pMBD Hamiltonian in the velocity gauge, which yields a quadratic Hamiltonian involving both atomic momentum operators and cavity coordinate operators. By diagonalizing the resulting 2(3N_a+N_p) matrix and applying a Bogoliubov transformation, they obtain collective normal‑mode operators ( \hat b, \hat b^\dagger ) together with transformation matrices (X) and (Y). These matrices encode the hybridisation between electronic and photonic degrees of freedom and provide direct access to all Gaussian moments of the ground‑state field.

Key analytical results include closed‑form expressions for the quadrature variances \