Coherent Interactions and Entanglement of Hydrogen Molecules in the STM Junction

Ultrashort THz pulses can be used to excite single hydrogen molecules (H₂) in the junction of Scanning Tunneling Microscope (STM), whose dynamics manifest itself as coherent oscillation through the pump-probe measurements. By tailoring the oscillation frequency through external bias, we report two spectral signatures of the interaction among multiple H₂ molecules. First, the avoided level crossing featured by energy gaps ranging from 20 to 80 GHz were observed, as a result of the level repulsion between two H₂ molecules. Second, the tip can sense the signal of H₂ outside the junction through the projective measurement on the H₂ inside the junction, owing to the entangled states created through the interaction. Dipolar-type interaction was incorporated in the two-level system (TLS) model of H₂, enabling us to accurately reproduce the aforementioned behavior. Our results reveal the peculiar interplay among H₂ molecules and allow us to envision further quantum operations of these polar molecules on surfaces.