  Electronic spin defects in the environment of an optically-active spin can be
used to increase the size and hence the performance of solid-state quantum
registers, especially for applications in quantum metrology and quantum
communication. Previous works on multi-qubit electronic-spin registers in the
environment of a Nitrogen-Vacancy (NV) center in diamond have only included
spins directly coupled to the NV. As this direct coupling is limited by the
central spin coherence time, it significantly restricts the register's maximum
attainable size. To address this problem, we present a scalable approach to
increase the size of electronic-spin registers. Our approach exploits a
weakly-coupled probe spin together with double-resonance control sequences to
mediate the transfer of spin polarization between the central NV spin and an
environmental spin that is not directly coupled to it. We experimentally
realize this approach to demonstrate the detection and coherent control of an
unknown electronic spin outside the coherence limit of a central NV. Our work
paves the way for engineering larger quantum spin registers with the potential
to advance nanoscale sensing, enable correlated noise spectroscopy for error
correction, and facilitate the realization of spin-chain quantum wires for
quantum communication.
