A superconducting quantum interference device (SQUID) is a highly sensitive magnetometer that utilizes the principles of superconductivity and quantum interference to measure extremely weak magnetic fields. It consists of a superconducting loop interrupted by one or two Josephson junctions. The Josephson junction is a weak link between two superconductors, allowing Cooper pairs of electrons to tunnel through it.
Behavior of a SQUID:
Flux quantization: In the absence of an external magnetic field, the magnetic flux through the SQUID loop is quantized due to the quantum mechanical behavior of superconducting electrons. This quantization is responsible for the unique behavior of SQUIDs.
Josephson effect: When a small magnetic field is applied to the SQUID loop, it causes a change in the magnetic flux threading the loop. This change induces a voltage across the Josephson junctions, resulting in an alternating current (AC) in the loop. The voltage oscillates at a frequency directly proportional to the strength of the applied magnetic field.
Flux-to-voltage transfer function: The SQUID's sensitivity and linearity are described by its flux-to-voltage transfer function, which relates the input magnetic flux to the output voltage. This function can be tailored for different applications.
Applications in magnetometry:
Biomagnetism: SQUIDs are widely used to measure the extremely weak magnetic fields generated by biological processes, such as the magnetic fields produced by the human brain and heart. Magnetoencephalography (MEG) and magnetocardiography (MCG) are examples of techniques that utilize SQUIDs for non-invasive measurements of brain and heart activity.
Geophysics: SQUIDs are employed in geophysical surveys to measure variations in the Earth's magnetic field, helping in mineral exploration, studying tectonic processes, and detecting buried structures.
Materials characterization: SQUIDs are used in material science to study the magnetic properties of various materials, such as superconductors and magnetic nanoparticles.
Non-destructive testing: SQUIDs can be utilized to detect flaws or defects in materials without damaging them, making them valuable tools for quality control and inspections in various industries.
Fundamental research: SQUIDs play a crucial role in fundamental research, particularly in condensed matter physics, where they help study exotic states of matter, quantum phase transitions, and superconductivity phenomena.
Due to their high sensitivity, SQUIDs are among the most precise magnetic field detectors available, making them indispensable tools in a wide range of scientific and industrial applications.