Nanowire could provide a stable, easy-to-make superconducting transistor

-By Nikita Vijay Biliye

Firstly, let us briefly discuss what superconductors are. Superconductors are substances or materials that conduct electricity with almost zero resistance. Superconductors conduct electricity when they become colder than a critical temperature. At this temperature, the electrons move freely through the materials without resistance. Hence there is no release of heat, noise, or any other form of energy.

Superconductors are useful in applications that require a strong magnetic field. Example: Magnetic Resonance Imaging (MRI), mineral separation in industries, quantum computers, cameras in the telescope, etc.

In the present time, superconductors do not show conductivity unless they are cooled, below room temperature. Therefore, scientists are using quantum physics to search for a superconductor that will work at room temperature and is also easy to use.

But the devices showing superconductivity can be a great fuss as they are expensive to manufacture and energy-intensive to maintain. To overcome these drawbacks, Karl Berggren’s group in the Department of Electrical Engineering and Computer Science are researching in the field. The researchers are working on creating a superconducting nanowire that could help more efficient superconducting electronics.

Karl Berggren says that the possible benefits of the nanowire will originate from its simplicity.

Berggren will present a description of this research at the IEEE Solid-state Circuits Conference.


Resistance is futile

As we know, every standard cable or electronic device has some amount of electric resistance. However, some superconducting metals have the potential to conduct electricity with zero resistance at extremely low temperatures or even at absolute zero temperatures.

These superconducting metals are used to sense magnetic fields in highly- sensitive situations like monitoring brain activity. Superconducting metals have applications in quantum as well as classical computing.

It can be a challenging task to find a material acting as a superconductor at room temperature. This is because of the unclear understanding of the quantum effects related to superconductivity. Finding a material that will work as a superconductor could result in a wide range of applications in technology and medicine-related fields. An ultimate cause of these superconductors is a device named Josephson junction

Josephson Junction:

The device Josephson Junction invented in the 1960s consists of a thin layer of the insulator. The insulator is sandwiched between two superconductors. This device led to the conventional superconducting electronics giving rise to superconducting quantum computers, says Berggren. This device is a strong applicant for the construction of quantum bits for quantum computers. 

Adding to it, Berggren said that- it was not the best superconducting device as it was fundamentally quite a delicate object. These devices have high costs of production and are difficult to manufacture. 

The Josephson junction-based devices do not play well when associated with conventional electronics like the one in our phones and computers.

Therefore, the major disadvantage of the Josephson junction-based superconductor is-its lack of ability to control large-scale objects. 

Hence, to overcome these limitations, Karl Berggren is developing a superconducting nanowire with its roots older than the Josephson junctions itself.

  • Cryotron reboot

In 1965, Dudley Buck, an electrical engineer at the Massachusetts Institute of Technology (MIT), published a description of the superconducting computer switch. This superconducting computer switch was called the Cryotron. 

Cryotron consisted of two superconducting wires with different critical temperatures. As described in his book, one wire is wrapped around another straight wire in a single-layer coil. Here both the wires are electrically isolated from each other. Cryotron acts as a switch that operates on superconductivity. The working principle of cryotron is that the magnetic field destroys superconductivity. When current flows through the coiled wire, it leads to the magnetic field reducing the current flowing through the straight wire.

The cryotron was much smaller compared to other switches like vacuum tubes or transistors. Because of its smaller size, Dudley Buck believed that cryotron could turn out to be a building block for computers. 

In 1959, Buck died at the age of 32. Because of his death, there was a pause in the further development of the cryotron. After which, transistors were developed in microscopic sizes, which is now a core logic component of computers.


Presently, Berggren is regenerating Buck’s ideas about the superconducting computer switches. 

Karl Berggren has named his superconducting wire ‘Nano-cryotron’ as a tribute to Dudley Buck. But, the operating functions of the Nano-cryotron are different from that of a cryotron.

Working of Nano-cryotron:

Instead of a magnetic field, the Nano-cryotron uses heat to activate the switch. This device consists of a superconducting, supercooled wire called a “channel” through which the current flows. This channel is again sectioned by an even smaller wire called a “choke.”

This intersection is similar to the sectioning of a multilane highway by a side road. Once the currents flow through the choke, its superconductivity fails, and the choke heats up. The heat then spreads from the choke to the main channel resulting in the main channel losing its superconducting state too.

The proof of this concept is already been demonstrated by Berggren’s group. Also, Berggren’s former student developed a device that makes use of Nano-cryotrons to add binary digits.

The National Science Foundation helped by offering the initial financial support for the research of nano-cryotron in the Berggren laboratory.

Benefits of Nano-cryotron:

  • Nano-cryotron can be used as an interface between superconducting devices and classical, transistor-based electronics.
  • The nanowires are easy to develop and hence have a manufacturability advantage.
  • Nano-cryotron has a scope to find applications in supercooled electronics for telescopes and superconducting quantum computers.
  • Since wires have low power dissipation, they might be used for energy-intensive applications as well.
  • The transistors used in a server farm or data center could be replaced by Nano-cryotron. 




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