  We experimentally demonstrate an approach to scale up quantum devices by
harnessing spin defects in the environment of a quantum probe. We follow this
approach to identify, locate, and control two electron-nuclear spin defects in
the environment of a single nitrogen-vacancy center in diamond. By performing
spectroscopy at various orientations of the magnetic field, we extract the
unknown parameters of the hyperfine and dipolar interaction tensors, which we
use to locate the two spin defects and design control sequences to initialize,
manipulate, and readout their quantum state. We finally create quantum
coherence among the three electron spins, paving the way for the creation of
genuine tripartite entanglement. This approach will be useful to assemble
multi-spin quantum registers for applications in quantum sensing and quantum
information processing.
