A SQUID (for superconducting quantum interference device) is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions.
SQUIDs are sensitive enough to measure fields as low as 5 aT (5×10−18 T) within a few days of averaged measurements. Their noise levels are as low as 3 fT·Hz-½. For comparison, a typical refrigerator magnet produces 0.01 tesla (10−2 T), and some processes in animals produce very small magnetic fields between 10−9 T and 10−6 T. Recently invented SERF atomic magnetometers are potentially more sensitive and do not require cryogenic refrigeration but are orders of magnitude larger in size (~1 cm3) and must be operated in a near-zero magnetic field.
History and design
There are two main types of SQUID: direct current (DC) and radio frequency (RF). RF SQUIDs can work with only one Josephson junction, which might make them cheaper to produce, but are less sensitive.
The DC SQUID was invented in 1964 by Robert Jaklevic, John J. Lambe, James Mercereau, and Arnold Silver of Ford Research Labs after Brian David Josephson postulated the Josephson effect in 1962 and the first Josephson Junction was made by John Rowell and Philip Anderson at Bell Labs in 1963. It has two Josephson junctions in parallel in a superconducting loop. It is based on the DC Josephson effect. In the absence of any external magnetic field, the input current I splits into the two branches equally. Now, consider if a small amount of external flux is applied to the superconducting loop. This results in the screening currents that generate the magnetic field to cancel this applied external flux. The current in one of the branches of the superconducting loop is in the direction of I, and is equal to I /2+ Is/2 and in the second branch is in the opposite direction of I and is equal to I /2− Is/2. As soon as the current in any one of the branches exceeds the critical current for the...