Quadratic Mixing of Radio Frequency Signals using Superconducting Quantum Interference Filters

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The authors demonstrate quadratic mixing of weak time harmonic electromagnetic fields applied to Superconducting Quantum Interference Filters, manufactured from high-Tc grain boundary Josephson junctions and operated in active microcooler. The authors use the parabolic shape of the dip in the dc-voltage output around B=0 to mix quadratically two external rf-signals, at frequencies f₁ and f₂ well below the Josephson frequency fⱼ, and detect the corresponding mixing signal at | f₁-f₂|. Quadratic mixing takes also place when the SQIF is operated without magnetic shield. The experimental results are well described by a simple analytical model based on the adiabatic approximation.