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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.[1] Their noise levels are as low as 3 fHz.[2] 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 Josephson junction, the superconducting ring becomes resistive and a voltage appears across the junction. Now consider if the external flux is further increased and it now exceeds Φ0/2. Since the flux enclosed by the superconducting loop must be an integral number of the flux quanta, in this case the SQUID instead of screening the flux, energetically prefers to increase it to Φ0. The screening current now flows in the opposite direction. Thus the screening current changes direction every time the flux increases by half integer multiples of Φ0. Thus the critical current oscillates as a function of the applied flux. If the input current is more than Ic, then the SQUID always operates in the resistive mode. The voltage in this case is thus a function of the applied magnetic field and the period equal to Φ0. Since the current-voltage characteristics of the DC SQUID is hysteritic, a shunt resistance, R is connected across the junction to eliminate the hysteresis (in the case of copper oxide based high temperature superconductors the junction's own intrinsic resistance is usually sufficient). The screening current is the applied flux divided by the self inductance of the ring. Thus ∆Φ can be estimated as the function of ∆V (flux to voltage converter).[3][4][5].

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