  The understanding of how the sub-nanoscale exchange interaction evolves in
macroscale correlations and ordered phases of matter, such as magnetism and
superconductivity, requires to bridge the quantum and classical worlds. This
monumental challenge has so far only been achieved for systems close to their
thermodynamical equilibrium. Here we follow in real time the ultrafast dynamics
of the macroscale magnetic order parameter triggered by the impulsive optical
generation of spin excitations with the shortest possible nanometer-wavelength
and femtosecond-period. Our experiments also disclose a possibility for the
coherent control of these femtosecond nanomagnons, which are defined by the
exchange energy. These findings open up novel opportunities for fundamental
research on the role of short-wavelength spin excitations in magnetism and
high-temperature superconductivity, since they provide a macroscopic probe of
the femtosecond dynamics of sub-nanometer spin-spin correlations and,
ultimately, of the exchange energy. With this approach it becomes possible to
trace the dynamics of such short-range magnetic correlations for instance
during phase transitions. Moreover, our work suggests that nanospintronics and
nanomagnonics can employ phase-controllable spin waves with frequencies in the
20 THz domain.
