Charge state control of nitrogen-vacancy centers in diamond

The nitrogen-vacancy (NV) center in diamond is considered one of the most promis- ing candidates for the realization of a quantum computer. Also, it shows an ex- traordinary potential as a sensor for magnetic or electric fields. In this context, a reliable control of the NV charge state is highly desired. We addressed this issue by first investigating the role of the diamond surface termination, implantation en- ergy and nitrogen impurity concentration on the NV charge state. We could show that the charge state of NV centers in the vicinity of the diamond surface changes from mostly negatively charged to a neutral or even non-fluorescent charge state when the surface is changed from an oxygen termination to a hydrogen termi- nation. Further, we successfully employed the concept of an electrolyte gate to reversibly change the charge state of single NV centers from a non-fluorescent state to the neutral charge state NV0. Full charge state control could only be achieved for ensembles of NV centers, where some centers could be changed from a non-fluorescent state to NV0 and others from NV0 to the negative charge state NV−. Finally, using an in-plane gate structure full charge state control was even achieved for individual centers. With the same technique we could also show how a nanometer-sized in-plane gate structure serves as a large, transparent top gate, that enables the control of the charge state of NV centers far away from the gate structure. All results could be understood in the frame of a charge transition level model that included the energy level at which an NV changes its charge state into the energy bands of the diamond. The charge state of an NV is determined by the relative position of the charge transition level with respect to the Fermi-level, which could be controlled by surface termination, an electrolyte gate or an in- plane gate. All mechanisms were simulated with the Schröedinger-Poisson solver nextnano3, confirming our observations in a semi-quantitative way. In summary, different methods to control the charge states of NV centers in di- amond were developed and demonstrated. They could enable sophisticated ways of quantum information processing with NV centers, which involve a controlled change of the NV charge state. Furthermore, our technology could assist in dis- covering the hitherto unobserved positive charge state NV+.