  A solid-state system combining a stable spin degree of freedom with an
efficient optical interface is highly desirable as an element for integrated
quantum optical and quantum information systems. We demonstrate a bright color
center in diamond with excellent optical properties and controllable electronic
spin states. Specifically, we carry out detailed optical spectroscopy of a
Germanium Vacancy (GeV) color center demonstrating optical spectral stability.
Using an external magnetic field to lift the electronic spin degeneracy, we
explore the spin degree of freedom as a controllable qubit. Spin polarization
is achieved using optical pumping, and a spin relaxation time in excess of 20
$\mu$s is demonstrated. Optically detected magnetic resonance (ODMR) is
observed in the presence of a resonant microwave field. ODMR is used as a probe
to measure the Autler-Townes effect in a microwave-optical double resonance
experiment. Superposition spin states were prepared using coherent population
trapping, and a pure dephasing time of about 19 ns was observed. Prospects for
realizing coherent quantum registers based on optically controlled GeV centers
are discussed.
