  Nitrogen vacancy (NV) centers in diamond have distinct promise as solid-state
qubits. This is because of their large dipole moment, convenient level
structure and very long room-temperature coherence times. In general, a
combination of ion irradiation and subsequent annealing is used to create the
centers, however for the rigorous demands of quantum computing all processes
need to be optimized, and decoherence due to the residual damage caused by the
implantation process itself must be mitigated. To that end we have studied
photoluminescence (PL) from NV$^-$, NV$^0$ and GR1 centers formed by ion
implantation of 2MeV He ions over a wide range of fluences. The sample was
annealed at $600^{\circ}$C to minimize residual vacancy diffusion, allowing for
the concurrent analysis of PL from NV centers and irradiation induced vacancies
(GR1). We find non-monotic PL intensities with increasing ion fluence,
monotonic increasing PL in NV$^0$/NV$^-$ and GR1/(NV$^0$ + NV$^1$) ratios, and
increasing inhomogeneous broadening of the zero-phonon lines with increasing
ion fluence. All these results shed important light on the optimal formation
conditions for NV qubits. We apply our findings to an off-resonant photonic
quantum memory scheme using vibronic sidebands.
