The Physics of Superconducting Quantum Interference Devices (SQUIDs)

Introduction

Superconducting Quantum Interference Devices, or SQUIDs, are highly sensitive magnetometers used to measure extremely subtle magnetic fields. They are based on superconducting loops containing Josephson junctions. SQUIDs are among the most sensitive magnetic field detectors available, with noise levels as low as 3 fT Hz−½.

Principle of Operation

The basic principle of operation of a SQUID is the flux quantization in superconducting loops, and the Josephson effect. When a superconducting loop is exposed to a magnetic field, the change in the field induces a current in the loop, according to Faraday's law of electromagnetic induction. In a SQUID, the current thus induced produces a magnetic field that opposes the change in the applied field, thus keeping the total magnetic flux through the loop constant.

Types of SQUIDs

There are two main types of SQUIDs: DC SQUIDs and RF SQUIDs.

DC SQUIDs

DC SQUIDs have two Josephson junctions in parallel in the superconducting loop. The current-voltage characteristic of a DC SQUID is periodic in the applied magnetic flux, with a period equal to the flux quantum.

RF SQUIDs

RF SQUIDs, on the other hand, have only one Josephson junction in the loop. They operate on the principle of the resonance of the LC circuit formed by the loop and a capacitor.

Applications

SQUIDs have a wide range of applications, from fundamental physics experiments to industrial and medical applications. They are used in Magnetic Resonance Imaging (MRI) for their ability to detect extremely small changes in magnetic fields, in geophysics for detecting variations in the Earth's magnetic field, and in astrophysics for measuring the properties of superconductors.