What is a SQUID?

A Superconducting Quantum Interference Device consists of two Josephson tunnel junctions in parallel, connected by a superconducting ring. If the SQUID is driven with a constant electric current, the voltage across the SQUID is a sensitive measure of magnetic flux threading the ring. Since magnetic fields can be created with electric currents, the SQUID is also sensitive to electric currents.

Cooper pairs form when an electron traveling through a lattice structure polarizes the lattice and sets up a phonon wave. Another electron will experience this disturbance and react to reduce the potential energy, thus causing effective attraction, which overcomes the Coulomb repulsion. L. Cooper described this process in 1956 in an attempt to explain the little understood quantum phenomena associated with superconducting states. He formulated a theory by which two electrons could couple to form an effective new particle, even under a very weak attractive force. It was then shown that the most energetically favorable situation for this to occur was when the two electrons had a total spin of zero together with equal and opposite wave functions. A superconductor consists of electrons occupying one-electron states and what is known as the condensate. The condensate is composed of Cooper pairs, each having the same energy, momentum and invariant phase relationship. This condensate behaves as a separate entity and is very coherent state, that is, all Cooper pairs in the condensate can be described with a single wave function and consequently, in a sense, they act as on particle. The state can be defined with an amplitude and phase .

In 1962, while conducting research for his Ph.D., B.D. Josephson noticed two effects. First, a dc electric current can flow across a junction with no resulting DC voltage up to some limit called the critical current. Second, an ac current through the junction results from application of a dc voltage across the junction. DC SQUIDs use two Josephson junctions in parallel and rely on the interference of the currents from each junction for their ability to produce a voltage across the SQUID. The interference is a result of the modulation of the supercurrent by an applied magnetic fields passing through the superconducting ring. This field is coupled to the ring by means of an AC waveform through an inductor that is fabricated on the SQUID chip. This interference occurs because the magnetic field changes the phases of the wave functions across the junctions, and hence the currents through them. The wave function's amplitude squared corresponds to the probability that an electron pair will cross the junction; therefore the resistance of the SQUID to the passage of a supercurrent depends on quantum interference. This resistance causes a voltage to develop across the SQUID, which can be measured. The quantum interference pattern is periodic and therefore the voltage developed across the SQUID repeats regularly with magnetic flux. The period of the variation is the flux quantum (approximately weber). SQUIDs measure in units of this natural fundamental constant and their sensitivity can be very stable.

SQUIDs operate as sensitive flux-to-voltage or current-to-voltage transducers. They are parametric amplifiers, and internal oscillations occur at frequencies proportional to the DC voltage according to the AC Josephson relation. These frequencies are typically above 1 GHz, and can be as high as 100 GHz or more.

The output impedance of a DC SQUID is typically on the order of an Ohm and is poorly matched to the input impedance of room temperature amplifiers, which generally have much higher impedance. In the past, restrictions imposed by the impedance matching network have limited the attainable bandwidth, ever though the intrinsic bandwidth of a SQUID is very high. Recently, uniform arrays of SQUIDs have allowed us to attain much higher bandwidth and investigation into SQUID operational amplifiers has shown that linear, predictable analog circuit response can be attained without using a room temperature amplifier to provide feedback, thus increasing bandwidth and decreasing power requirements (which simplifies cost and design).