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A superconducting quantum interference device (SQUID) is a very sensitive device for measuring weak magnetic fields. It is capable of measuring magnetic fields as low as 5 aT (5×10-18 T). Due to their sensitivity, SQUIDs are widely used in research, biological studies and other ultrasensitive electronic and magnetic measurements where faint signals cannot be sensed using conventional measurement instruments.
A SQUID is constructed of a superconducting loop containing one or more Josephson junctions.
There are two types of SQUID, the radio frequency (RF) SQUID consisting of only one Josephson junction, and the direct current (DC) SQUID with two or more junctions. The RF is cheaper in construction but less sensitive compared to the DC.
A typical DC SQUID has two parallel junctions inserted in a superconducting loop. Without a magnetic field, the input current splits equally between the branches. This maintains an externally connected tank circuit at resonance. Any external magnetic field causes a change in the resonant frequency in the tank circuit, and a current imbalance that leads to a voltage across the Josephson junction. The voltage is a function of the magnetic flux and can therefore be measured and used to calculate the magnetic flux.
The superconducting materials used for low-temperature SQUIDs are pure niobium or lead alloys. The device is cooled with liquid helium to maintain superconductivity. High-temperature SQUIDs are made from high-temperature superconductors such as yttrium barium copper oxide (YBCO) and cooled with the cheaper and readily available liquid nitrogen. However, they are not as sensitive as the low-temperature models, but are good enough for certain applications.
A SQUID is very sensitive in detection of magnetic energy fields, as low as 100 billion times smaller in magnitude than the energy that moves a compass needle. This extreme sensitivity makes them ideal for highly sensitive applications in research, biological studies and medical tests where the magnetic fields present cannot be measured using conventional instruments.
For example, SQUIDS are used in measuring faint signals in the human brain or heart by sensing the magnetic fields created by the neurological currents. Other applications include the construction of highly sensitive gradiometers, magnetometers and voltmeters.