The ultimate sensitivity of a high-temperature microwave SQUID magnetometer operating in the hysteresis regime

A method for the determination of the noise spectral density in a high-temperature microwave SQUID operating in the hysteresis regime is developed. Under these conditions, the reflection coefficient serves as an output signal. It is shown that if a directional coupler used for extracting the reflected wave is placed as close to the SQUID loop as possible, the magnetometer can be designed as a microwave integrated circuit with a noise flux spectral density S_F ^1/2 < 10 ^{- 5} F₀ /textHz^text0text.5 ,textwhere F_text0 S_Phi ^{1/2} < 10^{ - 5} Phi _0 /{text{Hz}}^{{text{0}}{text{.5}}} ,{text{where }}Phi _{text{0}} , is the magnetic flux quantum.