EducationSuperconducting quantum interference device SQUID - Construction, Operation and Applications

Superconducting quantum interference device SQUID – Construction, Operation and Applications

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SQUID Superconducting quantum interference device is a very sensitive, weak magnetic field meter. The device is built using superconducting loops with Josephson junctions. Josephson junctions Josephson junction (Josephson junction) was designed upon the theory of B.D. Josephson (b. 1940) was the author of an article published in 1962. Josephson discussed how current can be flowed between two superconductors that are separated by a thin layer of insulator. The effect is now referred to by the Josephson effect. The SQUID is made up of an ultraconducting loop as well as the two Josephson junctions. The currents flowing through these connectors increase and cause interference. When the loop is within even a small magnetic field outcome of the interference may be different because of the shift in phase between the currents induced due to the magnetic field.

The term SQUID refers to an abbreviation of its full title: Superconducting Quantum Interference Device. Superconducting signifies this SQUID is made from superconducting materials, that is, material with zero electrical resistance and is able to conduct current with no loss. What is known as Quantum Interference is much more obscure and is connected to the physical foundation for the SQUID. It is essentially a device or instrument. In this category is an electronic device that can measure the field of magnetic energy with precision which is unattainable with other sensors.

Construction of SQUID

It is made from superconducting wire that is connected to form a ring. In two locations, the circuit is cut and divided by an insulator layer insulation. This creates two superconductor-insulator-superconductor junctions, called Josephson junctions. The entire system is completed by two leads, which link both the SQUID to the electronic circuit.

Construction of SQUID
Figure 1: Superconductor Ring that has two isolating connectors: DC SQUID

But, creating a high quality circuit isn’t that easy. The typical loop of an SQUID is less than 0.1 millimeters in surface area, and is composed from pure Niobium. The junctions are constructed of niobium oxide. They are around 1 nanometer in thickness and comprise only the atoms of a few layers. This means that the whole process is typically made by the process of lithography just like semiconductor processors are manufactured. A completed SQUID is available for purchase as any other electronic component, though the costs are rather high. For Oxford Instruments, for example the chip made using superconductors at low temperatures costs around $3000; to buy one that is made of high-temperature superconductors, you will pay around $7000. The prices don’t include the electronics required that cost about a tenth of a second more.

SQUID Operating principle

The mechanism of SQUID is built on the concept of interference, similar to the interfering of light in the famous experiment of Young’s two slits. In SQUID however the interference does not occur between two light beams however, but rather between the two wave acts in the two superconducting portions of the rings. In both instances, the effect is the same and the degree to which the interference is destructive or constructive varies on the mutual frequency of the two waves. In SQUID it is the frequency of the waves that are correlated to magnetic fields flowing across the loop.

Dependence of the current I on the magnetic field passing through the SQUID
Figure 2: Dependence of the current I on the magnetic field passing through the SQUID

The total flow of current I through the SQUID with a constant voltage, varies according to the field of magnetic. In the event that the magnetic Ph that flows across the loop (that is the product of the field induction and the size that the loop) is an integer multiple of the quantum flux

then the current is the maximum. If

Then, minima are seen. This causes an oscillation in the current in relation to of the field external to it. The curvature’s envelope is a result of diffracted light that is related to the dimensions of Josephson junctions, which is similar to the way light scatters across the slice.

Magnetic field sensors

Based on the result that was previously described magneto-field sensors are now being designed with sensitivities as large as a few Femtotesla (10-15). In real-world applications the magnetic field measured is not directly transmitted via the loop of SQUID instead, a superconductor circuit is utilized. It is constructed to determine the magnetic field or it could be wound in order to serve as a gradiometer. That is, to determine the change in the spatial field. The magnetic field changes and creates a constant current within the superconducting circuit which then creates an electric field within the coil. It is the field that is detected through the SQUID. The SQUID’s features allow for measurements of weak, changing signals in a large but steady field.

Difficulties in SQUID

The primary issue with utilizing SQUIDs is that the main issue with using SQUID can be found in the fact that it has to be fabricated out of superconductor. Sadly, despite decades of study, it has not been able to develop an element that can conduct at temperatures of room temperature. The most effective SQUIDs are composed from materials that conduct only at temperatures lower than 10 Kelvin (-263oC). This means cooling the system using liquid helium, which is a significant expense. SQUIDs created several years ago were made are made from ultra-high temperature superconductors (HTSCs) are also cooling-related due to the fact that in contrast to what they are called they are superconducting at temperatures under the temperature of -150oC. It is enough to utilize liquid nitrogen, which is less expensive than liquid helium for cooling in this instance. Liquid nitrogen-cooled equipment can be mobile and are crucial for certain applications for geophysical measurement, for instance. However, SQUIDs that have HTSC have a lot more sensitive than those that are cooled using Helium. Another issue in measuring extremely fragile fields can be interference. The most common source of interference is the proximity of electrical wires, motors transformers, and electromagnets. Additionally the earth’s magnetic field isn’t constant and can cause a lot of disturbance. The typical fields are derived are derived from a variety of sources. However, the interference is much higher than signals from the most intriguing research objects. To prevent interference, shielding constructed of specific materials is employed. To stop the slow-variable fields of magnetic field, shields made from materials that have a very high magnetic susceptibility at the beginning such as mu-metal, which is the preferred choice is used. To block out fields that are fast-variable shields constructed of strong conductors, where the generated eddy currents don’t let the magnetic field alternating traverse, are adequate. In the most sensitive of measurements the sensor, along with the patient (patient) is placed within a space made of a thick shield walls, which protect any magnetic signals coming from outside.

Superconducting quantum interference device instruments and applications


The capability to measure extremely low magnetic field strength has yielded applications beyond the field of physics, but also geophysics and medicine. Of greatest interest is magnetoencephalography (MEG), which is the monitoring of human brain function by measuring the magnetic field produced by nerve impulses. The strength of the field generated by a neuron group can be as high as 50 to 500 femtotesla without the skull. A collection of up to hundreds of sensors are situated at liquid helium temperatures inside a heat shield. The device, along with the patient is encased within a space with several tons of magnetic shields. All signals from sensors are sent to the outside to be amplified, recorded and analysed using specialized software. With the help of a magnetic map that is measured on the skull’s surface you can trace the pattern of the electrical currents that generate these magnetic fields.

The information gathered from MEG is different from information obtained through magneto-resonance imaging or tomography. These two methods provide details about anatomy, whereas MEG offers a view of the activity of organs. The temporal resolution of magnetoencephalography is on the order of 1 millisecond, which makes it possible to accurately read the nervous system’s responses to stimuli in real time. The spatial resolution is quite good, making it possible to pinpoint which brain region is displaying activity. This allows us to determine which brain part reacts in response to stimuli from the visual and or auditory stimuli, and which is where memory is and where the capability to speak is situated.

Geophysical research

In certain rocks, information is recorded on the direction of Earth’s magnetic field was at the time the rock was solidified. The reason for it is the following. Iron at temperatures higher than 770oC is paramagnetic, and therefore cannot create a magnetic field on its own. If the iron-rich rock is cooled below 770oC it becomes ferromagnetic. Its spontaneous magnetic field follows its direction from the field externally. The whole rock is an inert magnet. Today, geologists can obtain rock samples making sure to record their location relative to regions of the globe and in a laboratory, they can determine by using a magnetometer which direction is the magnetization for the sample. It is apparent that earth’s magnetic field hasn’t always been the way it is now. Over the course of 70 million years the magnetic poles changed about 170 times. Today, it remains unclear what leads to such a switch.

New materials, magnetometers

Scientists are finding, or making, a variety of exciting and new materials. These include newly synthesized chemical compounds as well as manufactured miniature structures. When conducting research on these types of structures magnetometers are frequently required. It is important to let the significance of magnetic properties be proven by the reality how CD-ROMs operate as well as headphones for walkman are built on the most recent materials and the same magnetic properties. Magnetometers are instruments used to determine the properties of magnetic materials.

SQUID on spy duty

Finding enemies (or allies) submarines is a problem for intelligence services for many years. One method for detecting vessels is to measure the strength of the magnetic field on Earth by a satellite with low flight. Submarines, just like all ferrous objects, alter the magnetic field that is in the vicinity. The alteration of the earth’s magnetic field is significant enough that it is observable at a great height with an SQUID-based magnetometer. In the ocean ships are the only big iron structures. So, if a perturbation of earth’s fields is occurring while there is nothing visible at the top of it, this means that something is underground. The research on these techniques is classified, and it is difficult to determine if it is true or not.

Importance of superconducting quantum interference devices

Superconducting Quantum Interference Devices (SQUIDs) play an enormously crucial role due to their unique properties and capabilities. Here are a few reasons why SQUIDs should be given greater consideration:

1. High Sensitivity: SQUIDs are highly sensitive magnetometers capable of measuring even very weak magnetic fields – down to 1015 Tesla! They make SQUIDs indispensable tools in many scientific and technological applications.

2. Non-destructive Measurement: SQUIDs provide non-destructive magnetic field measurements without direct contact to objects being measured, making them suitable for applications such as biomagnetism where measuring brain activity or detecting small magnetic signals from within humans is a necessity.

3. Broad Frequency Range: SQUIDs can operate across an extensive frequency spectrum, enabling them to measure both static and dynamic magnetic fields. DC SQUIDs excel at low frequency applications like geophysics and material characterization while rf SQUIDs offer precise magnetic field measurement in rapidly shifting fields such as nuclear magnetic resonance (NMR) imaging or magnetic resonance imaging (MRI).

4. Quantum Sensing: SQUIDs are devices based on superconductivity and quantum interference principles, making them an excellent example of quantum sensing devices. Utilizing the quantum properties of superconducting materials and Josephson junctions as conduits into quantum space, SQUIDs offer unique windows into this realm that make them valuable tools for researching fundamental physics phenomena as well as quantum computing research.

5. Diverse Applications: SQUIDs have many different uses across many disciplines. Physics researchers utilize SQUIDs for researching superconductivity, magnetic properties of materials and quantum phenomena; SQUIDs can be utilized in magnetoencephalography (MEG), which maps brain activity to diagnose neurological disorders; they’re even employed geophysics research for mapping underground mineral deposits and studying Earth’s magnetic field variations as well as nondestructive testing, quality control and industrial applications.

6. Scientific Discoveries and Technological Advancements: SQUIDs have played a significant role in many scientific breakthroughs and technological developments. Their contributions include breakthroughs in understanding superconductivity, magnetic phenomena and quantum effects as well as aiding medical diagnostic techniques and magnetic field sensing imaging technologies.

SQUIDs have immense importance because of their extraordinary sensitivity, quantum nature and wide applicability; making them invaluable tools across scientific, medical and industrial domains.


Electronics and Telecommunications engineer with Electro-energetics Master degree graduation. Lightning designer experienced engineer. Currently working in IT industry.