  Spin-bearing molecules are promising building blocks for quantum technologies
as they can be chemically tuned, assembled into scalable arrays, and readily
incorporated into diverse device architectures. In molecular systems, optically
addressing ground-state spins would enable a wide range of applications in
quantum information science, as has been demonstrated for solid-state defects.
However, this important functionality has remained elusive for molecules. Here,
we demonstrate such optical addressability in a series of synthesized
organometallic, chromium(IV) molecules. These compounds display a ground-state
spin that can be initialized and read out using light, and coherently
manipulated with microwaves. In addition, through atomistic modification of the
molecular structure, we tune the spin and optical properties of these
compounds, paving the way for designer quantum systems synthesized from the
bottom-up.
