rfSQUIDs

rfSQUIDs, or radio-frequency superconducting quantum interference devices, are superconducting magnetometers based on a single Josephson junction embedded in a superconducting loop. The loop inductance and the junction’s critical current define a flux-dependent impedance that is read out at radio frequencies by coupling the loop to a resonant tank circuit. A static magnetic flux biases the loop, while a small RF drive near the tank resonance probes changes in the effective inductance produced by the flux threading the loop. The resulting changes in the amplitude and phase of the reflected RF signal are interpreted as changes in magnetic flux.

In operation, the rf-SQUID is typically biased and driven at cryogenic temperatures to minimize thermal noise. The resonance frequency of the tank circuit, and thus the readout sensitivity, depends on the flux through the loop due to the flux-dependent Josephson inductance. Typical designs achieve wide bandwidth and high sensitivity, with the RF readout enabling fast measurements of magnetic flux. The device can be biased to operate in a non-hysteretic regime, providing a more linear flux response over a useful range.

Applications of rfSQUIDs include ultra-sensitive magnetic field detection, scanning SQUID microscopy, and readout of superconducting qubits and other microwave devices. Compared with dc-SQUIDs, rf-SQUIDs use a single Josephson junction, which can simplify fabrication and enable efficient RF amplification and high-speed readout, albeit with design trade-offs in linearity and dynamic range that depend on the chosen circuit parameters. They are predominantly implemented with superconducting materials such as niobium or aluminum and require cryogenic cooling.