  The decoherence of a central electron spin due to the dynamics of a coupled
electron-spin bath is a core problem in solid-state spin physics. Ensemble
experiments have studied the central spin coherence in detail, but such
experiments average out the underlying quantum dynamics of the bath. Here, we
show the coherent back-action of an individual NV center on an electron-spin
bath and use it to detect, prepare and control the dynamics of a pair of bath
spins. We image the NV-pair system with sub-nanometer resolution and reveal a
long dephasing time ($T_2^* = 44(9)$ ms) for a qubit encoded in the
electron-spin pair. Our experiment reveals the microscopic quantum dynamics
that underlie the central spin decoherence and provides new opportunities for
controlling and sensing interacting spin systems.
