  Nitrogen-vacancy centers possessing nuclear spins are promising candidates
for a novel nuclear spin gyroscope. Preparation of a nuclear spin state is a
crucial step to implement a sensor that utilizes a nuclear spin. In a low
magnetic field, such a preparation utilizes population transfer, from polarized
electronic spin to nuclear spin, using microwave pulses. The use of the
narrowband microwave pulse proposed earlier is inefficient when magnetic
transitions are not well resolved, particularly when applied to diamond with a
natural abundance of carbon atoms or dense ensembles of nitrogen-vacancy
centers. In this study, the authors performed optimization of the pulse shape
for 3 relatively easily accessible pulse shapes. The optimization was done for
a range of magnetic transition linewidths, corresponding to the practically
important range of nitrogen concentrations (5-50 ppm). It was found that, while
at low nitrogen concentrations, optimized pulse added very little to simple
square shape pulse, and in the case of dense nitrogen-vacancy ensembles, with a
rather wide magnetic transition width of 1.5 MHz optimal pulses, a factor of
15% improvement in the population of the target state was observed.
