SQUID

A SQUID (for superconducting quantum interference device) is a very sensitive magnetometer used to measure extremely subtle magnetic fields, based on superconducting loops containing Josephson junctions. SQUIDs are sensitive enough to measure fields as low as 5 a T (5×10−18 T) with 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. There are two main types of SQUID: direct current (DC) and radio frequency (RF). RF SQUIDs can work with only one Josephson junction (superconducting tunnel 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 {displaystyle I} splits into the two branches equally. If a small external magnetic field is applied to the superconducting loop, a screening current, I s {displaystyle I_{s}} , begins circulating in the loop that generates a magnetic field canceling the applied external flux. The induced current is in the same direction as I {displaystyle I} in one of the branches of the superconducting loop, and is opposite to I {displaystyle I} in the other branch; the total current becomes I / 2 + I s {displaystyle I/2+I_{s}} in one branch and I / 2 − I s {displaystyle I/2-I_{s}} in the other. As soon as the current in either branch exceeds the critical current, I c {displaystyle I_{c}} , of the Josephson junction, a voltage appears across the junction. Now suppose the external flux is further increased until it exceeds Φ 0 / 2 {displaystyle Phi _{0}/2} , half the magnetic flux quantum. Since the flux enclosed by the superconducting loop must be an integer number of flux quanta, instead of screening the flux the SQUID now energetically prefers to increase it to Φ 0 {displaystyle Phi _{0}} . The current now flows in the opposite direction, opposing the difference between the admitted flux Φ 0 {displaystyle Phi _{0}} and the external field of just over Φ 0 / 2 {displaystyle Phi _{0}/2} . The current decreases as the external field is increased, is zero when the flux is exactly Φ 0 {displaystyle Phi _{0}} , and again reverses direction as the external field is further increased. Thus, the current changes direction periodically, every time the flux increases by additional half-integer multiple of Φ 0 {displaystyle Phi _{0}} , with a change at maximum amperage every half-plus-integer multiple of Φ 0 {displaystyle Phi _{0}} and at zero amps every integer multiple. If the input current is more than I c {displaystyle I_{c}} , 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 Φ 0 {displaystyle Phi _{0}} . Since the current-voltage characteristic of the DC SQUID is hysteretic, a shunt resistance, R {displaystyle 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 Δ Φ {displaystyle Delta Phi } can be estimated as the function of Δ V {displaystyle Delta V} (flux to voltage converter) as follows: The discussion in this Section assumed perfect flux quantization in the loop. However, this is only true for big loops with a large self-inductance. According to the relations, given above, this implies also small current and voltage variations. In practice the self-inductance L {displaystyle L} of the loop is not so large. The general case can be evaluated by introducing a parameter with i c {displaystyle i_{c}} the critical current of the SQUID. Usually λ {displaystyle lambda } is of order one.