  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 $\omega$ in the microwave band has a significant thermal photon
occupation at room temperature according to the Bose-Einstein distribution
$\bar{n} = k_BT / \hbar\omega$. The room temperature thermal state of a 3 GHz
mode, for example, is characterized by a mean photon number $\bar{n} \sim 2000$
and variance $\Delta n^2 \approx \bar{n}^2$. 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 ${\sim} 3$ 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 $-2.3 \pm 0.1 \, \textrm{dB}$ and minimum
cavity mode temperature of $150 \pm 5 \textrm{K}$. We present also a linearized
model to identify the important features of the cooling, and demonstrate its
validity through magnetically tuned, spectrally resolved measurements. 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.
