  Scalable quantum technologies will require an unprecedented combination of
precision and complexity for designing stable structures of well-controllable
quantum systems. It is a challenging task to find a suitable elementary
building block, of which a quantum network can be comprised in a scalable way.
Here we present the working principle of such a basic unit, engineered using
molecular chemistry, whose control and readout are executed using a nitrogen
vacancy (NV) center in diamond. The basic unit we investigate is a synthetic
polyproline with electron spins localized on attached molecular sidegroups
separated by a few nanometers. We demonstrate the readout and coherent
manipulation of very few ($\leq 6 $) of these $S=1/2$ electronic spin systems
and access their direct dipolar coupling tensor. Our results show, that it is
feasible to use spin-labeled peptides as a resource for a molecular-qubit based
network, while at the same time providing simple optical readout of single
quantum states through NV-magnetometry. This work lays the foundation for
building arbitrary quantum networks using well-established chemistry methods,
which has many applications ranging from mapping distances in single molecules
to quantum information processing.
