Two-tone response of radiofrequency signals using the voltage output of a superconducting quantum interference filter

In the presence of weak time-harmonic electromagnetic fields, superconducting quantum interference filters (SQIFs) show the typical behavior of nonlinear mixers. The SQIFs are manufactured from high-T c grain boundary Josephson junctions and operated in active microcooler. The dependence of dc voltage output V dc versus static external magnetic field B is nonperiodic and consists of a well pronounced unique dip at zero field, with marginal side modulations at higher fields. We have successfully exploited the parabolic shape of the voltage dip around B = 0 to mix quadratically two external time-harmonic rf-signals, at frequencies f 1 and f 2 below the Josephson frequency f J, and detect the corresponding mixing signal at |f 1- f 2|. When the mixing takes place on the SQIF current-voltage characteristics, the component at 2f 2-f 1 is present. The experiments suggest potential applications of a SQIF as a nonlinear mixing device, capable to operate at frequencies from dc to few gigahertz with a large dynamic range.