Quadratic Mixing of Radio Frequency Signals using Superconducting   Quantum Interference Filters
  The authors demonstrate quadratic mixing of weak time harmonic electromagnetic fields applied to Superconducting Quantum Interference Filters, manufactured from high-$T_{\mathrm{c}}$ 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 \emph{quadratically} two external rf-signals, at frequencies $f_{\mathrm{1}}$ and $f_{\mathrm{2}}$ well below the Josephson frequency $f_{\mathrm{J}}$, and detect the corresponding mixing signal at $| {f_{1}-f_{2}}| $. 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. 
P. Caputo, J. Tomes, J. Oppenlaender, Ch. Haeussler, A. Friesch, T.   Traeuble, N. Schopohl