  We present general guidelines for finding solid-state systems that could
serve as coherent electron spin-photon interfaces even at relatively high
temperatures, where phonons are abundant but cooling is easier, and show that
transition metal ions in various crystals could comply with these guidelines.
As an illustrative example, we focus on divalent nickel ions in magnesium
oxide. We perform electron spin resonance spectroscopy and
polarization-sensitive magneto-optical fluorescence spectroscopy of a dense
ensemble of these ions and find that (i) the ground-state electron spin stays
coherent at liquid-helium temperatures for several microseconds, and (ii) there
exists energetically well-isolated excited states which can couple to two
ground state spin sub-levels via optical transitions of orthogonal
polarizations. The latter implies that fast, coherent optical control over the
electron spin is possible. We then propose schemes for optical initialization
and control of the ground-state electron spin using polarized optical pulses,
as well as two schemes for implementing a noise-free, broadband quantum-optical
memory at near-telecom wavelengths in this material system.
