  We present experimental observations and a study of quantum dynamics of
strongly interacting electronic spins, at room temperature in the solid state.
In a diamond substrate, a single nitrogen vacancy (NV) center coherently
interacts with two adjacent S = 1/2 dark electron spins. We quantify
NV-electron and electron-electron couplings via detailed spectroscopy, with
good agreement to a model of strongly interacting spins. The electron-electron
coupling enables an observation of coherent flip-flop dynamics between
electronic spins in the solid state, which occur conditionally on the state of
the NV. Finally, as a demonstration of coherent control, we selectively couple
and transfer polarization between the NV and the pair of electron spins. These
results demonstrate a key step towards full quantum control of electronic spin
registers in room temperature solids.
