  Proposals for systems embodying condensed matter spin qubits cover a very
wide range of length scales, from atomic defects in semiconductors all the way
to micron-sized lithographically-defined structures. Intermediate scale
molecular components exhibit advantages of both limits: like atomic defects,
large numbers of identical components can be fabricated; as for
lithographically defined structures, each component can be tailored to optimize
properties such as quantum coherence. Here, we demonstrate what is perhaps the
most potent advantage of molecular spin qubits, the scalability of quantum
information processing structures using bottom-up chemical self-assembly. Using
Cr7Ni spin qubit building blocks, we have constructed several families of
two-qubit molecular structures with a range of linking strategies. For each
family, long coherence times are preserved, and we demonstrate control over the
inter-qubit quantum interactions that can be used to mediate two-qubit quantum
gates.
