RFSQUID

RFSQUID, short for Radio-Frequency Superconducting Quantum Interference Device, is a superconducting magnetometer that uses a single Josephson junction in a superconducting loop to detect extremely small magnetic flux changes. It is a variant of the SQUID family designed for radio-frequency readout, enabling rapid signal extraction and broad bandwidth.

The device operates by exploiting flux quantization in a superconducting loop. A magnetic flux threading the loop modulates the current through the Josephson junction, producing a flux-dependent inductance. The loop is part of a resonant tank circuit driven at RF; changes in flux shift the resonance, which is detected as a change in the reflected or transmitted RF signal. A feedback scheme such as a flux-locked loop linearizes the response and enhances dynamic range.

Performance figures vary with design, but rf-SQUIDs can achieve flux sensitivities on the order of a few micro-Φ0 per root Hz, where Φ0 is the magnetic flux quantum, and bandwidths up to several tens to hundreds of megahertz in well-designed systems. They typically require cryogenic cooling to superconducting temperatures, and magnetic shielding to minimize environmental noise.

Applications include high-sensitivity magnetometry for biomagnetism, geophysics, scanning SQUID microscopy, nondestructive evaluation, and detection of small signals in NMR/MRI contexts. Compared with dc-SQUIDs, rf-SQUIDs offer simpler RF readout and potential for higher bandwidth, but can be more susceptible to flux noise in some configurations.

RFSQUIDs were developed in the late 20th century as part of the broader SQUID technology program and remain a topic of research for ultra-sensitive magnetic sensing and cryogenic microwave electronics.