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

Abstract

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.

Figures (3)

• Figure 1: Electron-photon exchange-correlation energy and photon number for chains of Ar atoms. (a) Electron-photon exchange-correlation energy predicted by pMBD (black), photon-GA (brown), and pMBD up to first (blue), second (red), and tenth order (green). The inset shows deviations between the second order and tenth/full pMBD results for a large number of atoms (90-100). In (b), we compare the ground-state photon number of pMBD (black) and photon-GA (brown).
• Figure 2: Uncertainties and squeezing parameters for chains of Ar atoms. (a) Cavity quadrature uncertainties $\Delta q$ (green squares) and $\Delta p$ (red triangles), and their uncertainty product $\Delta q \Delta p$ (blue circles), as a function of the number of atoms $N$. The gray dashed line marks the Heisenberg uncertainty limit ($0.5$). (b) The squeezing parameter $r$ predicted by pMBD (black circles), whereas the photon-GA functional (gray dashed line) predicts no squeezing ($r=0$). Insets display the Wigner quasiprobability distributions $W(\tilde{q}, \tilde{p})$ in dimensionless phase-space coordinates ($\tilde{q}=\omega_{\alpha}q, \tilde{p}=p/\omega_{\alpha}$) for $N=1, 50,$ and $100$, visualizing the phase-space deformation predicted by pMBD.
• Figure 3: Entanglement and correlations for chains of Ar atoms. (a) Mandel $Q$ parameter (blue squares) and photon number variance $\Delta n^2$ (red circles). The gray dotted line marks the Poissonian limit ($Q=0$). (b) Von Neumann entanglement entropy $S_{vN}$ vs photon number. The pMBD (black diamonds) predicts less entropy than the thermal limit (dashed black line), whereas the photon-GA (red circles) predicts a thermal-like behavior.