• SQUID is an acronym of superconducting Quantum Interference Device .
• A SQUID is a superconducting device that can measure an extremely small magnetic field, voltage or current. It is a very sensitive magnetometer in which a superconducting loop is used with one or more Josephson junctions.
• A schematic diagram of SQUID is shown, the SQUID is kept in a magnetic field. A DC super current I enters the device through port C, gets divided into two ports I_1 and I_2. The current I_1 and I_2 undergoes a phase shift while crossing the Josephson junctions P and Q and become I_1^' and I_2' respectively. The super currents I_1^' and I_2' interface at port D.
• In superconductors the current is caused by the Cooper pairs. So the interfering waves are the de Broglie waves of the Cooper pairs. The phase shifts of the waves occur due to the applied magnetic field. In the absence of the magnetic field the phase shift and the phase difference are zero.

• The resultant current at port D oscillates between maxima and minima. The maxima occurs when the magnetic flux increases by one quantum given by $φ_o= h/2e=2.06 ×10^{(-15)} wb$
• In practice instead of the current the voltage V across the SQUID is measured. The voltage also oscillates with the changing magnetic field.
• Thus the SQUID is a flux-to-voltage transducer which converts a small change in magnetic flux into voltage.
• Because of their extreme sensitivity to magnetic field SQUIDS have application in many field like geology, medicine, engineering, etc.

Some of them are listed below:-

1. In measuring rock magnetism and continental drift SQUIDs are used.
2. The human brain generates magnetic fields of about 〖10〗^(-14)wb/m^2 . to detect them SQUID are used in brain imaging.
3. The SQUIDs are used in non destructive testing like testing of the aluminium sheets to be used for aircrafts.