  Understanding and controlling decoherence in open quantum systems is of
fundamental interest in science, while achieving long coherence times is
critical for quantum information processing. Although great progress was made
for individual, isolated systems, and electron spin resonance (ESR) of single
spins with nanoscale resolution has been demonstrated, the construction of
complex solid-state quantum devices requires ultimately controlling the
environment down to atomic scales, as potentially enabled by scanning probe
microscopy with its atomic and molecular characterization and manipulation
capabilities. Consequently, the recent implementation of ESR in scanning
tunneling microscopy (STM) represents a milestone towards this goal and was
quickly followed by the demonstration of coherent oscillations, tunable dipolar
and exchange couplings and access to nuclear spins with real-space atomic
resolution. Atomic manipulation even fueled the ambition to realize first
artificial atomic-scale quantum devices. However, the current-based sensing
inherent to this method limits coherence times. Here, we demonstrate pump-probe
ESR atomic force microscopy detection of electron spin transitions between
non-equilibrium triplet states of individual pentacene molecules. Spectra of
these transitions exhibit sub-nanoelectronvolt energy resolution, allowing
local discrimination of molecules that only differ in their isotopic
configuration. Furthermore, the electron spins can be coherently manipulated
over tens of microseconds, likely not limited by the detection method but by
the molecular properties. Single-molecule ESR atomic force microscopy can be
combined with atomic manipulation and characterization, and thereby paves the
way for atomically well-defined quantum components with long coherence times
and local quantum-sensing experiments.
