Two-Tone Response in Superconducting Quantum Interference Filters

We successfully exploit the parabolic shape of the dc voltage output dip around B = 0 of a superconducting quantum interference filter (SQIF) to mix weak external r f signals. The two-tone response of weak time harmonic electromagnetic fields has been detected on the spectral voltage output of the SQIF at frequency f
₀
= f
₁
- f
₂
, for various frequencies f
₁
and f
₂
ranging from few MHz up to 20 GHz. The two-tone response is a characteristic function of static magnetic field B and of bias current I
_b
, related to the second derivative of the dc voltage output.