A SQUID (Superconducting Quantum Interference Device) is a highly sensitive magnetometer used to detect minute magnetic fields—often as small as a few femtotesla (10⁻¹⁵ T). It operates on the principles of quantum interference and superconductivity, relying on the behavior of Josephson junctions.
How SQUIDs Work:
A basic SQUID consists of a superconducting loop interrupted by one or two Josephson junctions. Because superconductors can carry current without resistance and exhibit quantum coherence, the current through a SQUID loop is sensitive to the magnetic flux passing through it.
Due to the quantization of magnetic flux, only discrete amounts of magnetic flux (multiples of the magnetic flux quantum, ϕ₀ ≈ 2.07 × 10⁻¹⁵ Wb) are allowed in the loop. Any change in magnetic field alters the phase difference across the junctions, which in turn changes the current or voltage in a predictable way.
Two Types of SQUIDs:
- DC SQUID: Uses two Josephson junctions and is extremely sensitive. It detects changes in magnetic flux by monitoring voltage variations as a function of magnetic field.
- RF SQUID: Uses a single Josephson junction and operates with a radio-frequency signal. It’s simpler in design but generally less sensitive than the DC type.
Applications:
- Medical imaging: Used in MEG (magnetoencephalography) to measure magnetic activity in the brain.
- Geophysics: To study magnetic properties of rocks and minerals.
- Quantum computing: As readout devices for superconducting qubits.
- Fundamental physics: For detecting small magnetic signals in experiments requiring extreme precision.
Because of their incredible sensitivity and speed, SQUIDs are considered one of the most powerful tools for detecting weak magnetic signals—capable of observing even the magnetic field produced by the firing of a single neuron.