  We experimentally demonstrate the temporary removal of thermal photons from a
microwave mode at 1.45 GHz through its interaction with the spin-polarized
triplet states of photo-excited pentacene molecules doped within a p-terphenyl
crystal at room temperature. The crystal functions electromagnetically as a
narrow-band cryogenic load, removing photons from the otherwise
room-temperature mode via stimulated absorption. The noise temperature of the
microwave mode dropped to 35 K (as directly inferred by noise-power
measurements) while the metal walls of the cavity enclosing the mode remained
at room temperature. Simulations based on the same system's behavior as a maser
(which could be characterized more accurately) indicate that the mode's true
temperature sank to ~10 K (corresponding to ~140 microwave photons), below the
noise floor of our room-temperature instrumentation. These observations, when
combined with engineering improvements to deepen the cooling, identify the
system as a narrow-band yet extremely convenient platform -- free of
cryogenics, vacuum chambers and strong magnets -- for realizing low-noise
detectors, quantum memory and quantum-enhanced machines (such as heat engines)
based on strong spin-photon coupling and entanglement at microwave frequencies.
