  Full insight into the dynamics of a coupled quantum system depends on the
ability to follow the effect of a local excitation in real-time. Here, we trace
the coherent evolution of a pair of coupled atomic spins by means of scanning
tunneling microscopy. We use a pump-probe scheme to detect the local
magnetization following a current-induced excitation performed on one of the
spins. Making use of magnetic interaction with the probe tip, we are able to
tune the relative precession of the spins. We show that only if their Larmor
frequencies match, the two spins can entangle, causing the excitation to be
swapped back and forth. These results provide insight into the locality of
electron-spin scattering, and set the stage for controlled migration of a
quantum state through an extended spin lattice.
