Extended Data Figure 1: Diamond sample geometry and orientation.

a, The diamond is a rectangular cuboid with dimensions 2.1 mm × 2.1 mm × 2.6 mm. The main faces (pink) are {100}, with the {111} faces (to which the NV〈111〉 directions are normal) depicted in blue. b, Side view of the diamond oriented within the maser cavity, with static magnetic field B applied across the NV〈111〉 direction. The microwave magnetic field is perpendicular to the N–V defect axis and static magnetic field direction. c, Top view of the diamond when it is oriented inside the cavity. d, Top view of the diamond placed within the single-crystal sapphire ring within the maser cavity. The depiction of the structure of diamond within the crystals (carbon atoms, bonds and tetrahedral) are for illustrative purposes only.