A fundamental result of quantum mechanics is that the fluctuations of a bosonic field are given by its temperature
T. An electromagnetic mode with frequency
ω in the microwave band has a significant thermal photon occupation at room temperature according to the Bose-Einstein distribution
nˉ=kBT/ℏω. The room temperature thermal state of a 3 GHz mode, for example, is characterized by a mean photon number
nˉ∼2000 and variance
Δn2≈nˉ2. This thermal variance sets the measurement noise floor in applications ranging from wireless communications to positioning, navigation, and timing to magnetic resonance imaging. We overcome this barrier in continuously cooling a
∼3 GHz cavity mode 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
−2.3±0.1dB and minimum cavity mode temperature of
150±5K. We present also a linearized model to identify the important features of the cooling, and demonstrate its validity through magnetically tuned, spectrally resolved measurements. The realization of efficient mode cooling at ambient temperature opens the door to applications in precision measurement and communication, with the potential to scale towards fundamental quantum limits.