  We study a single-mode cavity weakly coupled to a voltage-biased quantum
point contact. In a perturbative analysis, the lowest order predicts a thermal
state for the cavity photons, driven by the emission noise of the conductor.
The cavity is thus emptied as all transmission probabilities of the quantum
point contact approach one or zero. Two-photon processes are identified at
higher coupling, and pair absorption dominates over pair emission for all bias
voltages. As a result, the number of cavity photons, the cavity damping rate
and the second order coherence $g^{(2)}$ are all reduced and exhibit less
bunching than the thermal state. These results are obtained with a Keldysh path
integral formulation and reproduced with rate equations. They can be seen as a
backaction of the cavity measuring the electronic noise. Extending the standard
$P(E)$ theory to a steady-state situation, we compute the modified noise
properties of the conductor and find quantitative agreement with the
perturbative calculation.
