From an Unearthed Time Capsule, MIT Discovers Replacement for Josephson Junctions

February 15, 2021 by Jake Hertz

Termed the "tiny switch that almost made it big," MIT's long-forgotten cryotron has inspired a possible replacement to Josephson junctions for quantum computing.

In 1956, MIT researcher Dudley Buck published a paper on a superconducting switch called the cryotron, which was meant to compete with contemporary vacuum tube switches. When Buck passed in 1959, the technology more or less died with him and, as we all know, semiconductors became the replacement for vacuum tubes. 


Dudley Buck, inventor of the cryotron

Dudley Buck, inventor of the cryotron. Image used courtesy of MIT Technology Review

The cryotron remained forgotten for almost 60 years until in 2015 while digging the foundation of MIT's nanotechnology center, workers unearthed a time capsule. Amongst its contents was information about the cryotron, bringing the technology back to life in modern times

Since this miraculous event, the cryotron has become a muse for quantum computing research. Today, MIT researchers have taken inspiration from the cryotron for a new alternative to Josephson Junctions in superconducting electronics. 


A Quick Background on Superconductivity 

Superconducting technology is based on the fact that when materials reach a certain “critical temperature” the metal transitions from a normal state to a superconducting state where the material has no resistance. This happens because, in the superconducting state, electrons experience a slight attractive force, causing them to pair up, which brings them to a lower energy state.

As a result of being in this lower energy state, electrons are not scattered by the material’s ionic lattice, which is the fundamental mechanism that describes electrical resistance. 


Superconductors have zero resistance below a critical temperature point

Superconductors have zero resistance below a critical temperature point. Image used courtesy of CERN

When the amount of current through a superconductor is beneath some “critical current” level, the voltage on the superconductor is zero. However, once this critical current is exceeded, the voltage is no longer zero, but instead oscillates with time. 


What Is a Josephson Junction? 

In the world of superconducting electronics, the Josephson Junction (JJ) has become a hot research topic because it makes a good candidate for qubits in quantum computing. The technology detects the change in superconducting states based on the critical current and the state of the superconductor’s voltage as bits. 


Josephson Junction working principle

Josephson Junction working principle. Image used courtesy of HyperPhysics

In a simplified explanation, a JJ is used to describe a device that is made of a non-superconducting material placed in between two superconducting materials. The technology is named after Brian Josephson, the founder of the Josephson Effect which describes how, in a JJ, superconducting electron pairs can tunnel through the insulating material from one superconductor to another. 

One limiting disadvantage of a JJ is that it is extremely sensitive to outside noise, making the technology very difficult to interface with contemporary electronics. Along with this, they’re very difficult and expensive to manufacture. 


Nano-Cryotron Research 

Now, inspired by the cryotron, MIT researchers are developing a new device to rival the JJ. 

The new device has been dubbed the nano-cryotron, consisting of a superconducting wire called the “channel,” which is intersected by a smaller wire called the “choke.” When the choke conducts a certain current (presumably above the critical current), it loses its superconductivity and begins to heat up. This heat spreads to the channel, causing it to lose its superconductive state as well. 


Nano-cryotrons are partly inspired by Buck's original cryotron from the 50s

Nano-cryotrons are partly inspired by Buck's original cryotron from the 50s. Image used courtesy of Wikimedia Commons

Since the new device undergoes heat-based state changes, it is far less susceptible to outside noise compared to the JJ, making it more easily interfaced with classical electronics. Further, since the device consists of nanowires instead of an elaborate junction, the device is easier to manufacture. 

According to MIT, the researchers have already a shown proof-of-concept for the device using the nano-cryotron to add binary digits. They also claim to have successfully interfaced the technology with classical electronics. 


A Replacement for the JJ? 

While the technology is still in its infancy, MIT researchers are hopeful that the nano-cryotron can push quantum computing forward, offering an inexpensive and less sensitive alternative to the JJ. 

Beyond this, the researchers plan to explore new methods of computing. Karl Berggen, head researcher on this project, says, “We’re doing fundamental research, here. While we’re interested in applications, we’re just also interested in: What are some different kinds of ways to do computing? As a society, we’ve really focused on semiconductors and transistors. But we want to know what else might be out there.”