Investigating the positively charged nitrogen-vacancy center in diamond as a long lived quantum memory

The nitrogen-vacancy (NV) defect in diamond is one of the major candidates for a solid-state quantum processor. Its electron spin is readout and initialized optically. Proximal nuclear spins (e.g. $^{14}\text{N}$, $^{15}\text{N}$, $^{13}\text{C}$) serve as inherently robust qubits, their readout is facilitated via the electron spin in a QND measurement and they exhibit $\text{T}_1$ lifetimes of several minutes. However, for strongly coupled nuclear spins, the coherence time is limited by the electron spin's $\text{T}_1$ lifetime (˜ 5ms @ roomtemperature). In Si:P, this obstacle is overcome by ionizing the P donor into a spinless charge-state. In this work, we employ in-plane gate structures on the diamond surface for deterministic charge state switching of near-surface NVs from $\text{NV}^-$ over $\text{NV}^0$ to $\text{NV}^+$, while investigating the electron spin properties using the nitrogen nuclear spin as a probe. The positive charge state happens to have no unpaired electrons, therefore the nuclear spin coherence time is prolonged beyond the 5ms-limit imposed by the $\text{NV}^-$ electron spin. Proper charge state control removes an important roadblock for achieving minute-long coherence times at room-temperature and deterministic quantum system initialization.