Progress towards scanning SQUID imaging with dispersive readout

Superconducting QUantum Interference Device (SQUID) microscopy has proven useful for imaging magnetic fields, currents, local susceptibility, and noise, and could be even more useful at microwave frequencies (2-12 GHz) which can offer higher bandwidth and lower noise. In this talk I will detail our latest efforts in imaging with radio frequency (RF) SQUIDs using dispersive readout with circuit QED-based methods. We use capacitively shunted RF SQUIDs with sub-micron spatial resolution, similar to those previously designed and reported in Ref [1] which demonstrated a tunable resonance frequency above 5 GHz and ~200 MHz bandwidth, and flux noise as low as 80 nΦ₀/√Hz at 4 K with potential improvements at mK temperatures via Josephson junction-based parametric amplification. We will describe recent work towards implementing RF-SQUIDs in a dry fridge for magnetic imaging with optimized measurement circuitry. We will further discuss their potential for local noise spectroscopy, considering the Berezinskii-Kosterlitz-Thouless transition as an example.

[1] F. Foroughi, J.-M. Mol, T. Müller, J. R. Kirtley, K. A. Moler, H. Bluhm, Appl. Phys. Lett. 112, 252601 (2018).