Quadruply Bonded Mo 2 Molecules: An Innate Emitter‐Resonator Quantum System in Free Space

ABSTRACT In recent decades, significant progress has been made in constructing and studying individual quantum systems based on two‐level atoms (molecules) and photons. Here we demonstrate that the quadruply bonded Mo 2 unit, with a Mo─Mo bond distance as short as 2.1 Å, functions as an innate emitter–resonator molecular quantum system capable of trapping visible‐light photons between the two molybdenum atoms under ambient conditions, thereby generating an intense quantized local electromagnetic field with an extremely small mode volume. The resonance fluorescence spectra of three Mo 2 complexes indicate that the intermetallic Mo→Mo charge‐transfer transition is coherently coupled to the local scattering field, exhibiting vacuum Rabi splitting and Mollow triplets. Resonant coupling of single molecules and N ‐molecule ensembles to the scattered light through sideband excitation produces a sequence of discrete optical modes spanning a broad wavelength range, with polaritonic transitions identical to those observed in Ni 2 ‐based systems. These results establish the Mo 2 molecule as an independent emitter–resonator integrated quantum system that enables quantum‐optical experiments in free space using conventional spectroscopic instrumentation. This work extends quantum electrodynamics into molecular science, providing new insights into metal–metal bonding, molecular physics, and light–matter interactions.