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Wikipedia / SQUID

A SQUID (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 magnetic fields as low as 5×10−18 T with a few days of averaged measurements (see orders of magnitude (magnetic field)). Their noise levels are as low as 3 fT·Hz−. For comparison, a typical refrigerator magnet produces 10−2 T, and some processes in animals produce very small magnetic fields between 10−9 T and 10−6 T. SERF atomic magnetometers, invented in the early 2000s are potentially more sensitive and do not require cryogenic refrigeration but are orders of magnitude larger in size (~1cm3) 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 (superconducting tunnel junction), which might make them cheaper to produce, but are less sensitive.

DC SQUID

The DC SQUID was invented in 1964 by Robert Jaklevic, John J. Lambe, James Mercereau, and Arnold Silver of Ford Research Labs after Brian 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., begins to circulate the loop that generates the magnetic field canceling the applied external flux, and creates an additional Josephson phase which is proportional to this external magnetic flux., of the Josephson junction, a voltage appears across the junction.

, half the magnetic flux quantum and at zero amps every integer multiple.

is connected across the junction to eliminate the hysteresis (in the case of copper oxide based high-temperature superconductors (flux to voltage converter) as follows:

is the self inductance of the superconducting ring

of the loop is not so large. The general case can be evaluated by introducing a parameter

is of order one.

RF SQUID

The RF SQUID was invented in 1967 by Robert Jaklevic, John J. Lambe, James E. Mercereau, Arnold Silver, and James Edward Zimmerman at Ford. It is based on the AC Josephson effect and uses only one Josephson junction. It is less sensitive compared to DC SQUID but is cheaper and easier to manufacture in smaller quantities. Most fundamental measurements in biomagnetism, even of extremely small signals, have been made using RF SQUIDS. The RF SQUID is inductively coupled to a resonant tank circuit.. For a precise mathematical description refer to the original paper by Erné et al.

Materials used

The traditional superconducting materials for SQUIDs are pure niobium or a lead alloy with 10% gold or indium, as pure lead is unstable when its temperature is repeatedly changed. To maintain superconductivity, the entire device needs to operate within a few degrees of absolute zero, cooled with liquid helium.

High-temperature SQUID sensors were developed in the late 1980s. They are made of high-temperature superconductors, particularly YBCO, and are cooled by liquid nitrogen which is cheaper and more easily handled than liquid helium. They are less sensitive than conventional low temperature SQUIDs but good enough for many applications.

In 2006, A proof of concept was shown for CNT-SQUID sensors built with an aluminium loop and a single walled carbon nanotube Josephson junction. The sensors are a few 100nm in size and operate at 1K or below. Such sensors allow to count spins.

In 2022 a SQUID was constructed on magic angle twisted bilayer graphene (MATBG)

Uses

SQUID magnetometer

A very common use case of SQUIDs is in magnetometry, where the magnetization of bulk materials is measured as a function of external parameters (such as temperature, magnetic field, pressure etc.). There are several commercial systems available which allow measurements in the temperature range from 300 mK to 400 K and in magnetic fields up to 7 T. In these systems a sample is moved through a setup of pickup-coils, which are coupled to the SQUID sensor. This setup allows the measurement of the magnetic flux Φ inside the pickup coils by fitting of a so called dipole function to the registered signal as a function of sample location relative to the coils. Since Φ is proportional to the magnetic moment of the sample μ, the exact proportionality is determined using a calibration sample with known moment μcalib.

In practice the pickup coil consists of multiple windings in a gradiometer setup (usually of 2nd order), which measures not the moment but its spatial derivative. The reason for this is the fact that environmental noise signals are spatially more homogenous than the sample's signal and can thus be filtered out better.

In healthcare

The extreme sensitivity of SQUIDs makes them ideal for studies in biology. Magnetoencephalography (MEG), for example, uses measurements from an array of SQUIDs to make inferences about neural activity inside brains. Because SQUIDs can operate at acquisition rates much higher than the highest temporal frequency of interest in the signals emitted by the brain (kHz), MEG achieves good temporal resolution. Another area where SQUIDs are used is magnetogastrography, which is concerned with recording the weak magnetic fields of the stomach. A novel application of SQUIDs is the magnetic marker monitoring method, which is used to trace the path of orally applied drugs. In the clinical environment SQUIDs are used in cardiology for magnetic field imaging (MFI), which detects the magnetic field of the heart for diagnosis and risk stratification.

For example, SQUIDs are being used as detectors to perform magnetic resonance imaging (MRI). While high-field MRI uses precession fields of one to several teslas, SQUID-detected MRI uses measurement fields that lie in the microtesla range. In a conventional MRI system, the signal scales as the square of the measurement frequency (and hence precession field): one power of frequency comes from the thermal polarization of the spins at ambient temperature, while the second power of frequency comes from the fact that the induced voltage in the pickup coil is proportional to the frequency of the precessing magnetization. In the case of untuned SQUID detection of prepolarized spins, however, the NMR signal strength is independent of precession field, allowing MRI signal detection in extremely weak fields, on the order of Earth's magnetic field. SQUID-detected MRI has advantages over high-field MRI systems, such as the low cost required to build such a system, and its compactness. The principle has been demonstrated by imaging human extremities, and its future application may include tumor screening.

Transition-edge sensors

One of the largest uses of SQUIDs is to read out superconducting transition-edge sensors. Hundreds of thousands of multiplexed SQUIDs coupled to transition-edge sensors are presently being deployed to study the cosmic microwave background, for X-ray astronomy, to search for dark matter made up of weakly interacting massive particles, and for spectroscopy at synchrotron light sources.

In physics

Another application is the scanning SQUID microscope, which uses a SQUID immersed in liquid helium as the probe. With the decreasing size of SQUID sensors, they can equip the tip of an AFM probe. Such device allows simultaneous measurement of roughness of the surface of a sample and the local magnetic flux.

The use of SQUIDs in oil prospecting, mineral exploration, earthquake prediction and geothermal energy surveying is becoming more widespread as superconductor technology develops; they are also used as precision movement sensors in a variety of scientific applications, such as the detection of gravitational waves. A SQUID is the sensor in each of the four gyroscopes employed on Gravity Probe B in order to test the limits of the theory of general relativity. Advanced SQUIDS called near quantum-limited SQUID amplifiers form the basis of the Axion Dark Matter Experiment (ADMX) at the University of Washington. Axions are a prime candidate for cold dark matter.

An RF-biased DC SQUID was used to observe the dynamical Casimir effect for the first time.

SQUIDs constructed from super-cooled niobium wire loops are used as the basis for D-Wave Systems 2000Q quantum computer.

Proposed uses

A potential military application exists for use in anti-submarine warfare as a magnetic anomaly detector (MAD) fitted to maritime patrol aircraft.

SQUIDs are used in superparamagnetic relaxometry (SPMR), a technology that utilizes the high magnetic field sensitivity of SQUID sensors and the superparamagnetic properties of magnetite nanoparticles. These nanoparticles are paramagnetic; they have no magnetic moment until exposed to an external field where they become ferromagnetic. After removal of the magnetizing field, the nanoparticles decay from a ferromagnetic state to a paramagnetic state, with a time constant that depends upon the particle size and whether they are bound to an external surface. Measurement of the decaying magnetic field by SQUID sensors is used to detect and localize the nanoparticles. Applications for SPMR may include cancer detection.