  Identifying and designing physical systems for use as qubits, the basic units
of quantum information, are critical steps in the development of a quantum
computer. Among the possibilities in the solid state, a defect in diamond known
as the nitrogen-vacancy (NV-1) center stands out for its robustness - its
quantum state can be initialized, manipulated, and measured with high fidelity
at room temperature. Here we describe how to systematically identify other deep
center defects with similar quantum-mechanical properties. We present a list of
physical criteria that these centers and their hosts should meet and explain
how these requirements can be used in conjunction with electronic structure
theory to intelligently sort through candidate defect systems. To illustrate
these points in detail, we compare electronic structure calculations of the
NV-1 center in diamond with those of several deep centers in 4H silicon carbide
(SiC). We then discuss the proposed criteria for similar defects in other
tetrahedrally-coordinated semiconductors.
