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. ¹⁴N , ¹⁵N , ¹³C) serve as inherently robust qubits, their readout is facilitated via the electron spin in a QND measurement and they exhibit T₁ lifetimes of several minutes. However, for strongly coupled nuclear spins, the coherence time is limited by the electron spin's T₁ 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 NV^- over NV⁰ to 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 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.