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 -32 J/Hz = 20 h over a flux range of about 0.15 φ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> .