Optical manipulation of single spins in diamond

Nitrogen-vacancy centers in diamond typically have spin-conserving optical transitions, a feature which allows for optical detection of the long-lived electronic spin states through fluorescence detection. However, by applying stress to a sample it is possible to obtain spin-nonconserving transitions in which a single excited state couples to multiple ground states. Here we describe two-frequency optical spectroscopy on single nitrogen-vacancy centers in a high-purity diamond sample at low temperature. When stress is applied to the sample it is possible to observe coherent population trapping with a single center. By adjusting the stress it is possible to obtain a situation in which all of the transitions from the three ground sublevels to a common excited state are strongly allowed. These results show that all-optical spin manipulation is possible for this system, and we propose that that by coupling single centers to optical microcavities, a scalable quantum network could be realized for photonic quantum information processing.