Electron spin resonance in scanning probe microscopy: From single-spin spectroscopy to atomically engineered quantum operations.

Electron spin resonance (ESR) is one of the most powerful spectroscopic methods for probing systems with unpaired electronic spins. Conventional ESR has usually relied on the detection of large spin ensembles, thereby averaging over local environments and suppressing microscopic information that is central to surface science, molecular magnetism, and quantum nanoscience. Scanning probe microscopy (SPM) changes this situation. By combining magnetic resonance with the spatial resolution and manipulation capabilities of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM), ESR can now be performed on individual atoms and molecules on surfaces. This short perspective discusses three connected developments. First, ESR-STM has enabled all-electrical detection and coherent control of individual spins with atomic precision. Second, ESR-AFM has recently provided a complementary route to single-molecule spin resonance based on force-detected pump-probe protocols. Third, these advances are turning surface spins into atomically engineered quantum systems in which spin Hamiltonians, interactions, sensing architectures, and elementary quantum operations can be designed locally. The resulting field does not merely miniaturise conventional ESR; it transforms magnetic resonance into a local quantum tool for spectroscopy, sensing, and coherent manipulation at the atomic scale.