Atomic scale quantum sensing from ultrafast coherence measurement of single H₂ molecules in the STM junction

Quantum sensing takes advantage of the quantum properties of a qubit system to realize measurements with high sensitivity surpassing its classical counterpart. Scanning tunneling microscopy combined with femtosecond THz spectroscopy has shown the capability of coherence measurement of single molecules. We report here, by performing time domain THz rectification spectroscopy, the discovery of a molecular hydrogen quantum sensor in the STM junction with unprecedented atomic-scale spatial and femtosecond temporal resolution and sensitivity. The hydrogen molecule in the STM junction behaves as a two-level system with its coherent oscillation period and decoherence time highly sensitive to the underlying surface potential of Cu₂N grown on Cu(100) surface. We explore the sensitivity of single hydrogen molecules to the surrounding environment in a controllable way in all three spatial dimensions. Time domain THz rectification imaging was then acquired with various pump-probe delays to reveal the atomic scale surface potential distribution of Cu₂N. Our results demonstrate a new class of quantum sensor in the STM junction with ultrahigh resolution and sensitivity outperforming existing sensing protocols.