Cooling a microwave mode below the thermal noise floor with a diamond NV ensemble

An electromagnetic mode in the microwave band has a significant thermal photon occupation at room temperature. This thermal noise floor poses a limit for sensing and coherent manipulation in fundamental research, as well as applications ranging from wireless communications to position, navigation, and timing. We overcome this barrier in continuously cooling a 2.9 GHz cavity mode below the thermal noise limit by coupling it to an ensemble of optically spin-polarized nitrogen-vacancy (NV) centers in a room-temperature diamond. The NV spins are pumped into a low entropy state via a green laser and act as a heat sink to the microwave mode through their collective interaction with microwave photons. Using a simple detection circuit we report a peak noise reduction of -2.1 ± 0.1 dB and minimum cavity mode temperature of 178.3 ± 5.4 K. We present also a linearized model to identify the important features of the cooling, and demonstrate its validity through magnetically tuned, spectrally resolved measurements.