  We demonstrate the cooling of a microwave mode at 2872 MHz through its
interaction with optically spin-polarized NV$^-$ centers in diamond at zero
applied magnetic field, removing thermal photons from the mode. By
photo-exciting (pumping) a brilliant-cut red diamond jewel with a
continuous-wave 532-nm laser, outputting 2 W, the microwave mode is cooled down
to a noise temperature of 188 K. This noise temperature can be preserved
continuously for as long as the diamond is optically excited and kept cool. The
latter requirement restricted operation out to 10 ms in our preliminary setup.
The mode-cooling performance of NV$^-$ diamond is directly compared against
that of pentacene-doped para-terphenyl, where we find that the former affords
the advantages of cooling immediately upon light excitation without needing to
mase beforehand (or at all) and being able to cool continuously at
substantially lower optical pump power.

Quantum Physics;Applied Physics;Instrumentation and Detectors;Optics