  The ST1 center is a point defect in diamond with bright fluorescence and a
mechanism for optical spin initialization and readout. The center has
impressive potential for applications in diamond quantum computing as a quantum
bus to a register of nuclear spins. This is because it has an exceptionally
high readout contrast and, unlike the well-known nitrogen-vacancy center, it
does not have a ground state electronic spin that decoheres the nuclear spins.
However, its chemical structure is unknown and there are large gaps in our
understanding of its properties. We present the discovery of ST1 centers in
natural diamond. Our experiments identify interesting power dependence of the
center's optical dynamics and reveal new electronic structure. We also present
a theory of its electron-phonon interactions, which we combine with previous
experiments, to shortlist likely candidates for its chemical structure.
