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

Xia, Yunpeng; Wang, Likun; Bai, Dan; Ho, Wilson · ACS Nano · 2023

basic_science · Level V

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Abstract

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 (H<sub>2</sub>) 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 H<sub>2</sub> 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 H<sub>2</sub> molecules. Second, the tip can sense the signal of H<sub>2</sub> outside the junction through the projective measurement on the H<sub>2</sub> 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 H<sub>2</sub>, enabling accurate reproduction of the observed behaviors. Our results obtained by the quantum superposition microscope reveal the intricate quantum mechanical interplay among H<sub>2</sub> molecules and additionally provide a 2D platform to investigate unresolved questions of amorphous materials.