Figure 1: Scheme of room-temperature diamond maser.

(a) The diamond maser system. A diamond sample is fixed inside a high-quality sapphire microwave dielectric resonator loaded in a coaxial cylindrical cavity. Microwave signal outputs from the loop coupling to the TE_01δ mode magnetic field (dashed red circles). The Halbach magnet array (the outer cylindrical wall) provides a uniform external magnetic field along the NV axis, which is set perpendicular to the cavity axial direction. The NV centres are pumped by a 532-nm light (green arrow). (b) The energy levels of an NV centre spin as functions of a magnetic field B. The zero-field splitting at B=0 is about 2.87 GHz. The magnetic field is set such that the transition frequency ω_S between the states and is resonant with the cavity mode frequency ω_c. (c) The pump scheme. After the optical excitation by a 532-nm light (green arrows), the excited-state ³E can directly return to the ground-state ³A₂ via spin-conserving photon emission at a rate of ∼70 μs⁻¹, but the excited states can also decay to the singlet state ¹A₁ via inter-system crossing at a rate of ∼50 μs⁻¹ and quickly decay to the metastable state ¹E, then relax back to the three different ground states at a rate of ∼1 μs⁻¹ in each pathway.