Microwave rf SQUID integrated into a planar YBa2Cu3O7 resonator

Abstract
We fabricated and characterized microwave rf SQUIDs integrated into a planar, S‐shaped λ/2 microstrip resonator. This 3 GHz resonator was fabricated from a pulsed‐laser‐deposited YBa2Cu3O7 epitaxial film. The SQUID structures incorporated double step‐edge junctions and had a loop inductance of 120 pH. Such unoptimized SQUIDs operated between 4.2 and 85 K with dV/dΦ=18–20 μV/Φ0 at 77 K. At that temperature, the energy resolution of (8±2)×10−29 J/Hz above 0.1 Hz (in the best samples) was limited by the white noise, SΦ1/2=(7±1)×10−5 Φ0/Hz1/2. Optimization may increase dV/dΦ and improve the energy resolution by up to an order of magnitude.