Double transformer coupling to a very low noise SQUID

We demonstrate a new way to couple efficiently to a low-inductance, low-noise SQUID. We have built and tested a planar dc SQUID with an integral matching transformer. The measured coupling agrees with our calculations. We demonstrate that this configuration can efficiently couple a 1 μH signal source to a 16 pH SQUID loop. We have also built an uncoupled SQUID of this design that has an energy sensitivity, referred to the SQUID inductor, of 1.3 × 10^-32J/Hz = 20 h over a flux range of about 0.15 φ0.