  Lasers have revolutionized optical science and technology, but their
microwave counterpart, maser, has not realized its great potential due to its
demanding work conditions (high-vacuum for gas maser and liquid-helium
temperature for solid-state maser). Room-temperature solid-state maser is
highly desirable, but under such conditions the lifetimes of emitters (usually
electron spins) are usually too short (~ns) for population inversion. The only
room-temperature solid-state maser is based on a pentacene-doped p-terphenyl
crystal, which has long spin lifetime (~0.1 ms). This maser, however, operates
only in the pulse mode and the material is unstable. Here we propose
room-temperature maser based on nitrogen-vacancy (NV) centres in diamond, which
feature long spin lifetimes at room temperature (~10 ms), high optical pump
efficiency, and material stability. We demonstrate that under readily
accessible conditions, room-temperature diamond maser is feasible.
Room-temperature diamond maser may facilitate a broad range of microwave
technologies.
