Avoided Level Crossing and Entangled States of Interacting Hydrogen Molecules Detected by the Quantum Superposition...

Pump-probe measurements by ultrashort THz pulses can be used to excite and follow the coherence dynamics in the time domain of single hydrogen molecules (H₂) in the junction of a scanning tunneling microscope (STM). By tailoring the resonance frequency through the sample bias, we identified two spectral signatures of the interactions among multiple H₂ molecules. First, the avoided level crossing featured by energy gaps ranging from 20 to 80 GHz was observed because 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 interactions. A dipolar-type interaction was integrated into the tunneling two-level system model of H₂, enabling accurate reproduction of the observed behaviors. Our results obtained by the quantum superposition microscope reveal the intricate quantum mechanical interplay among H₂ molecules and additionally provide a 2D platform to investigate unresolved questions of amorphous materials.