  Using a quantum detector, a superconductor-insulator-superconductor junction,
we probe separately the emission and absorption noise in the quantum regime of
a superconducting resonant circuit at equilibrium. At low temperature the
resonant circuit exhibits only absorption noise related to zero point
fluctuations whereas at higher temperature emission noise is also present. By
coupling a Josephson junction, biased above the superconducting gap, to the
same resonant circuit, we directly measure the noise power of quasiparticles
tunneling through the junction at two resonance frequencies. It exhibits a
strong frequency dependence, consistent with theoretical predictions.
