Verifying the radiative cooling of a superconducting resonator with a qubit spectrum analyzer*

Cavity electro-optomechanical systems are among the leading candidates for transducing quantum signals between microwave and optical frequencies. In such a photon converter, high-fidelity quantum state transfer is possible only if the electrical resonator is close to its quantum ground state in thermal equilibrium. However, the temperature dependence of the mechanical quality factor often may require the operation of the hybrid device at temperatures with non-negligible thermal photon populations at microwave frequencies. To resolve this conflict, the electrical resonator at a higher temperature can predominantly be coupled to, and thus radiatively cooled by the black-body radiation at a much lower temperature. In this talk, we will introduce an experiment in which a 10 GHz superconducting resonator anchored to the 1 K stage of a dilution refrigerator is overcoupled to a 20 mK environment anchored to the mixing chamber stage of the refrigerator. To verify this cooling mechanism, we use a transmon qubit as a quantum spectrum analyzer and measure the thermal noise coming from the resonator at 1 K through the photon-induced qubit dephasing. Preliminary results will be presented.