Broad-Band Coupling With Improved Efficiency Between the Nano-SQUID and Coplanar Waveguide

Abstract

Besides its well-known magnetic sensitivity, the nano superconducting quantum interference device (SQUID) has an almost instantaneous response time. However, transferring an ultrafast magnetic signal to a nano-SQUID is a technical challenge. We describe a very efficient direct-coupling structure between the SQUID and the coplanar waveguide (CPW). The center conductor of the CPW and its grounding planes are connected by a short line. We fabricated devices having direct and indirect coupling between the short line and nano-SQUIDs. The coupling efficiencies of both schemes are compared quantitatively. By injecting microwaves of frequencies 0.1-18 GHz through the CPW, we found that microwave power of 100 nW and 158 μW is required to suppress the critical current of nano-SQUIDs for the direct and indirect-coupling devices, respectively. By applying a dc current to the short line, we were able to measure the flux-modulation curves of SQUIDs. To achieve a flux quanta in SQUIDs requires currents of 0.46 and 11 mA for the direct- and indirect-coupling devices, respectively. Therefore, the direct-coupling scheme improves the efficiency by a factor of 39 from 0.1 to 18 GHz and 24 at dc, respectively. Furthermore, while sending a pulse-modulated signal of 1 μW power, a SQUID incorporating the direct-coupling scheme produced a working flux modulation.