  Microwave cavities have been widely used to investigate the behavior of
closed few-level systems. Here, we show that they also represent a powerful
probe for the dynamics of charge transfer between a discrete electronic level
and fermionic continua. We have combined experiment and theory for a carbon
nanotube quantum dot coupled to normal metal and superconducting contacts. In
equilibrium conditions, where our device behaves as an effective quantum
dot-normal metal junction, we approach a universal photon dissipation regime
governed by a quantum charge relaxation effect. We observe how photon
dissipation is modified when the dot admittance turns from capacitive to
inductive. When the fermionic reservoirs are voltage biased, the dot can even
cause photon emission due to inelastic tunneling to/from a
Bardeen-Cooper-Schrieffer peak in the density of states of the superconducting
contact. We can model these numerous effects quantitatively in terms of the
charge susceptibility of the quantum dot circuit. This validates an approach
that could be used to study a wide class of mesoscopic QED devices.
