Dispersive Microwave Readout of NanoSQUIDs

Superconducting quantum interference devices (SQUIDs) based on Josephson tunnel junctions have long been used as sensitive magnetic flux detectors. NanoSQUIDs, which use submicron weak link junctions for enhanced flux coupling,~are~attractive candidates for magnetic measurements of molecules.~ We present a novel~method~for nanoSQUID readout~which involves embedding the SQUID in a superconducting transmission line cavity operating at microwave frequency. The magnetic flux dependence of the total SQUID inductance modulates the cavity resonant frequency; these frequency changes are determined using microwave reflectometry. This dispersive microwave measurement allows detection of changes in magnetic flux at submicrosecond timescales without creating dissipation in the vicinity of the molecule. Moreover, we can exploit the Josephson nonlinearity of the nanoSQUID for bifurcation amplification to~enhance sensitivity. Optimization of the nanoSQUID design and cavity parameters for maximizing detector sensitivity and bandwidth is discussed. We also discuss the various sources of noise in this measurement scheme and how to minimize their impact. This work is supported by AFOSR and USDOE.