A synthetic single crystal diamond material comprising: a first region comprising electron donor defects; a second region comprising quantum spin defects; and a third region between the first and second regions. The second and third regions have a lower concentration of electron donor defects than the first region. The first and second regions are sufficiently close to allow electrons to be donated from the first region to the second region, thus forming negatively charged quantum spin defects in the second and positively charged defects in the first region, and sufficiently far apart to reduce other coupling interactions between the first and second regions which would otherwise unduly reduce the decoherence time of the plurality of quantum spin defects and/or produce strain broaden of a spectral line width of the plurality of quantum spin defects in the second region.