# What Are Superconducting Quantum Interference Devices or SQUIDs?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

There is no denying the complexity of quantum mechanics, but that can become even more complicated when we bring electronics into the mix. This does give us interesting situations that have such implications we give them their own field of study. Such is the case with Superconducting Quantum Interference Devices, or SQUIDs.

The first SQUID was constructed in 1964 after work for their existence was published in 1962 by Josephson. This revelation was called a Josephson junction, a critical component to our SQUIDs. He was able to demonstrate that given two superconductors separated via an insulating material would allow for a current to be exchanged. This is very weird because by nature an insulator should prevent this from happening. And it does…directly, that is. As it turns out, quantum mechanics predicts that given a sufficiently small insulator, a quantum tunneling effect occurs that sends my current to the other side without actually traveling through the insulator. This is the wacky world of quantum mechanics in full force. Those probabilities of unlikely things do happen sometimes, in unexpected ways (Kraft, Aviv).

## SQUIDs

When we start combining Josephson Junctions in parallel, we develop a direct current SQUID. In this set-up, our current faces two of our Junctions in parallel, so the current splits down each path to preserve our voltage. This current would be correlated to the “phase difference between the two superconductors” with respect to their quantum wave functions, which has a relation to magnetic flux. Therefore, if I can find my current I could essentially figure out the flux. This is why they make great magnetometers, figuring out magnetic fields over a given area based on this tunneled current. By placing the SQUID in a known magnetic field, I can determine the magnetic flux going through the circuit via that current, as before. Hence the name of SQUIDs, for they are made of Superconductors with a split current caused by QUantum effects which results in an Interference of the phase changes in our Device (Kraft, Nave, Aviv).

Is it possible to develop a SQUID with just a single Josephson junction? For sure, and we call it a radio frequency SQUID. In this, we have our Junction in a circuit. By placing another circuit near this we can gain an inductance which will fluctuate our resonant frequency for this new circuit. By measuring these frequency changes I can then back track and find the magnetic flux of my SQUID (Aviv).

## Applications and the Future

SQUIDs have many uses in the real world. For one, magnetic systems often have underlying patterns to their structure so SQUIDs can be used to find phase transitions as our material changes. SQUIDs are also useful in measuring the critical temperature at which any superconductor at that or below such temperature will prevent other magnetic forces from impacting by countering with an opposite force courtesy of the current rotating through it, as determined by the Meissner effect (Kraft).

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SQUIDs can even be useful in quantum computing, specifically in generating qubits. The temperatures needed for SQUIDs to operate are low since we need the superconductor properties, and if we get low enough then quantum mechanical properties become greatly magnified. By alternating the direction of the current through the SQUID I can change the direction of my flux, but at those supercool temperatures the current has probabilities of flowing in either direction, creating a superposition of states and therefore a means of generating qubits (Hutter).

But we have hinted at a problem with SQUIDs, and it is that temperature. Cold conditions are hard to produce, much less make available at a reasonable operating system. If we could find high-temperature SQUIDs then their availability and use would grow. A group of researchers from the Oxide Nano Electronics Laboratory at the University of California in San Diego set out to try and develop a Josephson junction in a known (but difficult) high temperature superconductor, yttrium barium copper oxide. Using a helium beam, researchers were able to fine-tune the nanoscale insulator needed as the beam acted like our insulator (Bardi).

Are these objects complicated? Like many topics in physics, yes they are. But it reinforces the depth of the field, the opportunities for growth, for learning new things otherwise unknown. SQUIDs are but one example of the joys of science. Seriously.

## Works Cited

Aviv, Gal. “Superconducting Quantum Interference Devices (SQUIDs).” Physics.bgu.ac.il. Ben-Gurion University of the Negev, 2008. Web. 04 Apr. 2019.

Bardi, Jason Socrates. “Fabricating inexpensive, high-temp SQUIDs for future electronic devices.” Innovatons-report.com. innovations report, 23 Jun. 2015. Web. 04 Apr. 2019.

Hutter, Eleanor. “Not Magic…Quantum.” 1663. Los Alamos National Laboratory, 21 Jul. 2016. Web. 04 Apr. 2019.

Kraft, Aaron, and Christoph Rupprecht, Yau-Chuen Yam. “Superconducting Quantum Interference Device (SQUID).” UBC Physics 502 Project (Fall 2017).

Nave, Carl. “SQUID Magnetometer.” http://hyperphysics.phy-astr.gsu.edu. Georgia State University, 2019. Web. 04 Apr. 2019.

© 2020 Leonard Kelley