LC-Resonant voltage response of superconducting quantum interference filters

The voltage vs. magnetic field relation V(B) of superconducting interferome ters consisting of two or more Josephson junctions is the basic characteris tic of their operation as highly sensitive magnetometers and amplifiers. Th e conversion efficiency partial derivativeV/partial derivativeB of interfer ometers containing two Josephson junctions, i.e. de SQUIDs, decreases signi ficantly with increasing loop inductance L. Therefore, the range of SQUID l oop sizes for which the SQUID operates properly is very restricted. However , for a sufficient coupling between the SQUID and the signal coil a large v alue of L is desirable. In this work theoretical and experimental results o n the voltage response function of quantum interference filters (SQIFs) are presented. A SQIF consists of N Josephson junctions connected in parallel to form a 1D array. The array loop sizes are chosen such that the voltage r esponse becomes a unique function around B = 0. It is shown that for McCumb er parameters beta (C) > 0.5 and sufficiently large array inductances the V (B) relation of SQIFs shows LC-resonances that lead to two pronounced minim a in the response function. In the LC-resonant operation mode the conversio n efficiency of SQIFs is not degraded even for large array inductances. By this, a very strong coupling to signal coils can be achieved. The experimen tal results agree very well with the theoretical predictions on which basis the SQIF has been fabricated.