  The ongoing miniaturization in nanoscience and -technology challenges the
sensitivity and selectivity of experimental analysis methods to the ultimate
level of single atoms and molecules. A promising new approach, addressed here,
focuses on the combination of two well-established complementary techniques
that have proven to be very successful in their own fields: (i) low-temperature
scanning tunneling microscopy (STM), offering high spatial resolution for
imaging and spectroscopy together with the capability of manipulating single
atoms and molecules in a well-controlled manner; (ii) radio-frequency (rf)
magnetic resonance techniques, providing paramount analytical power based on a
high energy resolution combined with the versatility of being sensitive to a
great variety of different properties of matter. Here, we demonstrate the
successful resonant excitation and detection of nuclear and electronic magnetic
transitions of a single quantum spin in a single molecule by rf tunneling of
electrons applied through the tip of a modified STM instrument operated at 5 K.
The presented rf-STM approach allows the unrivalled spectroscopic investigation
of electronic hyperfine levels in single molecules with simultaneous
sub-molecular spatial resolution. The achieved single-spin sensitivity
represents a ten orders of magnitude improvement compared to existing methods
of magnetic resonance - offering, atom-by-atom, unprecedented analytical power
and spin control with impact to physics, chemistry, biology, medicine,
nanoscience and -technology.
