Superconducting RFTES Detector at Milli-Kelvin Temperatures

We describe the first phase of experimental study of the superconducting bridge RFTES detector at temperatures 20-300 mK including the measurement of its thermal conductance, which fits the model of electron gas heating. We discuss the idea of the RFTES scheme, which is based on the probing of microwave loss near superconducting transition of the bridge. The heat applied to the bridge is generated by the probing signal at the frequency of the high-Q resonator. Since the real part is dominating in the nonlinear impedance of the bridge, the applied heat provides merely amplitude modulation of Q suggesting the suppression of phase jitter of the resonator. The bridge was made from a 50-nm-thick hafnium film (T
_C
≈ 380 mK) sized to 2.5 μm × 2.5 μm. The resonator and the rest of the circuit were made from 200-nm-thick film of niobium (T
_C
≈ 9 K) demonstrating the loaded Q-factor up to and above 10 000 at 1.5 GHz. A cryogenic semiconductor amplifier was used in the readout circuit. The thermal conductance was measured using the steady Q regime of the resonator and found to follow T
⁵
down to and below G ≈ 1×10
^-13
W/K. The NEP below 10
^-18
W/√Hz is estimated for the electron temperature of the bridge about 300 mK.