Bench-Top Cooling of a Microwave Mode Using an Optically Pumped Spin Refrigerator

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 narrowband cryogenic load, removing photons from the otherwise room-temperature mode via stimulated absorption. The noise temperature of the microwave mode dropped to 50_{-32}^{+18}  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 the possibility of the mode's temperature sinking to ∼10  K (corresponding to ∼140 microwave photons). These observations, when combined with engineering improvements to deepen the cooling, identify the system as a narrowband 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.