  We have directly measured the quantum noise of a superconducting
single-electron transistor (S-SET) embedded in a microwave resonator consisting
of a superconducting LC tank circuit. Using an effective bath description, we
find that the S-SET provides damping of the resonator modes proportional to its
differential conductance and has an effective temperature that depends strongly
on the S-SET bias conditions. In the vicinity of a double Cooper pair
resonance, when both resonances are red detuned the S-SET effective temperature
can be well below both the ambient temperature and the energy scale of the bias
voltage. When blue detuned, the S-SET shows negative differential conductivity,
