Highly coherent nitrogen-vacancy centres in diamond via rational high-pressure and high-temperature synthesis and treatment

Nitrogen-vacancy (NV^-) centres in diamond offer an excellent platform for device design and development in the emerging fields of quantum sensing and quantum network, but fabrication of quantum-grade diamonds with highly coherent NV- centres needed for advanced applications presents a pressing challenge. Here we report a major advance in generating NV^- centres in diamonds via tailored high-pressure and high-temperature (HPHT) synthesis and post-grown annealing treatment. The resulting well dispersed single NV^- centres in type-IIa diamonds exhibit long spin coherence times reaching the highest levels achieved in diamonds prepared by more elaborate and restrictive chemical vapor deposition techniques. Moreover, we produce high-density NV^- ensembles in <100>-grown type-Ib diamonds with superb zero-phonon lines considerably sharper than those of native NV^- centres in as-grown diamonds. These findings demonstrate a superior synthesis and optimization protocol for creating top-quality NV^- centres to meet the most stringent requirements for a wide range of emerging quantum technologies.