  Possible light bosonic dark matter interactions with the Standard Model
photon have been searched by microwave resonant cavities. In this paper, we
demonstrate the cryogenic readout system calibration of a 7.138 GHz copper
cavity with a loaded quality factor $Q_l=10^4$, operated at 22 mK temperature
based on a dilution refrigerator. Our readout system consists of High Electron
Mobility Transistors as cryogenic amplifiers at 4 K, plus room-temperature
amplifiers and a spectrum analyzer for signal power detection. We test the
system with a superconducting two-level system as a single-photon source in the
microwave frequency regime and report an overall 95.6 dB system gain and -71.4
dB attenuation in the cavity's input channel. The effective noise temperature
of the measurement system is 7.5 K.
