Glossary · Fundamentals
SQUIDs measure magnetic flux through superconducting interference
SQUID stands for superconducting quantum interference device. Its superconducting loop and Josephson weak links convert changes in magnetic flux into an electrical signal. NIST describes applications in metrology, biomagnetism, and geophysics. [1][2]
- Published
- Reading time
- 2 min
- Cited
- 2 sources
- DefinitionSQUID
- A superconducting quantum interference device: a magnetic-flux sensor that combines a superconducting loop with Josephson junctions. Magnetic flux changes its electrical response, allowing sensitive magnetometry with suitable readout electronics.
A loop makes magnetic flux measurable
Magnetic flux describes the magnetic field passing through an area. In a SQUID, flux threading the superconducting loop changes the phase relationships across its weak links. The resulting interference changes the current or voltage response. The device therefore measures flux directly; its conversion to magnetic field depends on the pickup geometry. [2]
Sensors describes a direct-current SQUID with two Josephson junctions in a superconducting loop [2]. Radio-frequency SQUIDs use a different arrangement and readout. NIST’s account of Jim Zimmerman traces the development of the radio-frequency device and subsequent magnetometers. The acronym includes these different implementations of superconducting quantum interference. [1]
Feedback turns a periodic response into a useful signal
The response repeats as applied flux changes by a flux quantum. A raw voltage reading therefore does not uniquely identify an arbitrary flux over a wide range. A flux-locked loop applies compensating flux through a feedback coil and holds the sensor near a selected operating point. The feedback signal tracks the input. [2]
Pickup coils or flux transformers can couple a larger sensing area to the small SQUID loop. Their dimensions, inductance, and coupling affect field sensitivity. Readout noise, bandwidth, and unwanted coupling between channels also matter. A quoted flux-noise value must be interpreted alongside this complete measurement arrangement rather than treated as a universal field resolution. [2]
Biomagnetic sensing detects currents without electrical contact
Electrical activity in the brain and heart produces magnetic fields outside the body. SQUID magnetometers can detect these weak signals. Magnetoencephalography measures fields associated with neuronal currents, using sensor arrays to sample their spatial pattern. Interpreting those measurements requires models of the sources and their positions, in addition to sensitive hardware. [1][2]
The primary Sensors study describes an integrated magnetoencephalography system operating in a magnetically shielded clinical environment. Its sensor array, cryogenic enclosure, feedback electronics, and shielding are parts of the reported instrument. That example demonstrates an application of SQUID technology without making every SQUID configuration equivalent to a clinical imaging system. [2]
Sensitivity makes environmental control essential
A sensor sensitive to biological fields also detects unwanted fields from buildings, equipment, and electrical infrastructure. Magnetic shielding, reference channels, and gradiometer arrangements help distinguish the target signal from background changes. Mechanical motion and the distance between the pickup and source can also affect the measured signal. [1][2]
The superconducting sensor needs suitable cooling, even though the measured object can remain outside the cold enclosure. A practical comparison should identify operating temperature, pickup area, noise spectrum, bandwidth, and shielding conditions. These details explain what a magnetometer can resolve in a particular setting and how it connects to the biomagnetic-sensing application. [2]
SuperMatics: AI built for superconductors.