What is a SQUID

Superconducting QUantum Interference Device (SQUIDs) is one of the most sensitive detectors of magnetic flux and field known, with an equivalent energy sensitivity that approaches the quantum limit. Due to their unique properties, SQUID devices are widely used in several applications like biomagnetism, magnetic microscopy, non-destructive evaluation, geophysics, quantum information, nanoscience and in recent interesting basic physic experiments like the detection of axion dark-matter, Hawking radiation, dynamical Casimir effect, Majorana fermions, Gravitational wave, Sunyaev-Zeldovich effect. A SQUID sensor is basically a magnetic flux-voltage converter having an extremely low magnetic flux noise. The physical quantities (magnetic field, current, voltage, displacements, etc.) to be detected are converted in a magnetic flux by using suitable flux transformer circuits. The operation principle of a SQUID is based on the Josephson effect and the flux quantization in a superconducting ring. The device consists of a superconducting loop interrupted by two Josephson junctions and is schematically shown in figure according to the resistively shunted junctions model. In order to obtain a non hysteretic current-voltage characteristics, it is needed to place in parallel with each Josephson junction a shunt resistor R. Biasing a SQUID device with a constant current greater than the sum of the Josephson critical currents of the two junctions, the output voltage across the Josephson junctions is a non linear periodic function of the magnetic flux through the superconductive loop having the period equal to a flux quantum [2.07x10-15 Wb].

So, it is a flux-to-voltage converter with a non linear response. If the signals to measure are much smaller than the flux quantum, the SQUID can work in small signal mode provided that it is flux biased in a linear, although small, range of the flux to voltage characteristic having a maximum slope, so that if the amplitude of the input signal fall within that range of linearity, the response of the SQUID is linear with the external magnetic flux.

Readout scheme

In order to linearize the SQUID response a Flux-Locked-Loop (FLL) configuration is often used.In such scheme the output voltage is converted into a current by a resistor and fed back into the SQUID as a flux, via a coil coupled to the sensor, nulling the input magnetic flux. So, the SQUID works as a null detector of magnetic flux. The voltage across the feedback resistor is proportional to the magnetic flux input.

The FLL linearizes the SQUID output increasing the linear dynamic range. Due to the SQUID's very low output voltage noise, direct voltage readout mode generally leads to a reduction of intrinsic SQUID sensitivity. In order to solve this problem, complicated schemes such as ac-flux modulation in combination with an impedance matching were often used. In recent years, a second generation of SQUID sensors, with a large flux-to-voltage transfer factor, has been proposed to allow a direct-coupled readout scheme without flux modulation. In comparison with the standard electronics, the direct-coupled readout schemes are simpler, more compact and less expensive. In particular circuits based on Additional Positive Feedback (APF) are very effective increasing, asymmetrically, the slope of the flux to voltage characteristic so as to make the amplifier noise contribution negligible.

Main Configurations

Since the dc-SQUID is a magnetic flux to voltage converter, its sensitivity is related to the flux capture area. Nevertheless, the sensitivity improvement cannot be obtained by simply increasing the SQUID superconducting loop dimension because this leads an increase of SQUID inductance and subsequent loss of performance.

Usually, an efficient way to increase the sensitivity consists of using a proper detection circuit depending on the application. In the case of the magnetic field measurement, as in the case of biomagnetism, the detection circuit is designed to measure magnetic field (magnetometer) or its gradient (gradiometer). In both case, they consist of a series of pickup coil having a flux capture area much higher than the SQUID one, and an input coil inductively coupled to the SQUID loop.

The SQUID can be used also in a voltmeter configuration in which the voltage to measure is converted in a current by a resistor. Such current flows in an input coil inducing a magnetic flux into the SQUID loop.