Strong light-matter coupling and field quantization in a dinickel molecular resonator

Field quantization is fundamental to quantum optics and polaritonic chemistry and is usually explored in microcavities through light-matter interactions. Here, we report strong resonant coupling of the two-level excitation of a dinickel (Ni₂) complex molecule with its intrinsically recycled scattering field in free space under ambient conditions using weak, off-resonant excitation. The Ni₂ unit acts simultaneously as a Jaynes-Cummings (JC) molecular emitter and an angstrom-scale optical resonator. The system exhibits well-resolved resonance-fluorescence signatures, including vacuum Rabi doublets and Mollow triplets. Simulations of the second-order correlation function g ⁽²⁾(τ) confirm photon antibunching and underdamped Rabi oscillations, consistent with JC dynamics. Collective coupling scales with molecular number N as ${\Omega }_N=N\sqrt{N}{\Omega }_1$ , distinct from the $\sqrt{N}{\Omega }_1$ Tavis-Cummings scaling, enabling ultrastrong coupling with few molecules. These results demonstrate that a Ni₂ complex can be a chemically defined molecular quantum emitter for nonclassical light generation and used for studying polaritonic control of chemical reactivity.