Feb 12 2021
Superconductors are materials that conduct electricity without any resistance. They have excellent potential and offer a macroscopic glimpse into quantum phenomena, which can generally be observed only at the atomic level.
MIT researchers are developing a superconducting nanowire, which could enable more efficient superconducting electronics. Image Credit: Christine Daniloff, Massachusetts Institute of Technology.
Superconductors not only exhibit physical uniqueness but are also beneficial. They are used in cameras in telescopes, quantum computers and medical imaging.
However, superconducting devices can be demanding. They are usually costly to manufacture and prone to be affected by noise in the surroundings. The results of a study by Karl Berggren’s team in the Department of Electrical Engineering and Computer Science could now change this.
The scientists are designing a superconducting nanowire, which could allow more effective superconducting electronics. The possible advantages of nanowires are derived from their simplicity, stated Berggren. “At the end of the day, it’s just a wire.”
Berggren will describe the study at the IEEE Solid-state Circuits Conference this February.
Resistance is Futile
A majority of the metals tend to lose resistance and turn superconducting at very low temperatures, often just a few degrees above absolute zero. They are utilized for sensing magnetic fields, particularly in highly sensitive conditions such as tracking brain activity. Moreover, they find applications in both classical and quantum computing.
The basis for several of these superconductors is the Josephson junction—a device invented in the 1960s—which typically includes two superconductors isolated by a thin insulator.
That’s what led to conventional superconducting electronics, and then ultimately to the superconducting quantum computer.
Karl Berggren, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology
Berggren added that the Josephson junction “is fundamentally quite a delicate object.” That directly influences the expenses and complications of manufacturing, particularly for the thin insulating layer. Josephson junction-based superconductors might not work well with others.
“If you try to interface it with conventional electronics, like the kinds in our phones or computers, the noise from those just swamps the Josephson junction. So, this lack of ability to control larger-scale objects is a real disadvantage when you're trying to interact with the outside world,” Berggren noted.
To solve these issues, Berggren has been creating a new technology—the superconducting nanowire—with roots that are older compared to the Josephson junction itself.
Cryotron Reboot
In 1956, Massachusetts Institute of Technology (MIT) electrical engineer Dudley Buck described a superconducting computer switch known as the cryotron. The device was a bit more compared to the two superconducting wires: One was straight, and the other one was coiled around it. The cryotron serves as a switch.
This is because when current flows via the coiled wire, its magnetic field decreases the current flowing via the straight wire.
During that time, the cryotron was much smaller compared to other types of computing switches, such as transistors or vacuum tubes, and Buck believed that the cryotron could turn out to be the building block of computers.
However, in 1959, Buck suddenly died at the age of 32, thereby curbing further development of the cryotron. (From that time, transistors have been developed to microscopic sizes and currently constitute the core logic components of computers.)
At present, Berggren is reviving Buck’s concepts regarding superconducting computer switches.
The devices we’re making are very much like cryotrons in that they don’t require Josephson junctions.
Karl Berggren, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology
Berggren named his superconducting nanowire device the nano-cryotron as a tribute to Buck—although it functions somewhat differently compared to the original cryotron.
The nano-cryotron makes use of heat to activate a switch, instead of a magnetic field. In Berggren’s device, current runs via a supercooled, superconducting wire known as the “channel.” That channel is sectioned by an even smaller wire known as a “choke”—similar to how a side road intersects a multilane highway.
Upon transmitting current via the choke, its superconductivity fails and it heats up. As soon as that heat spreads from the choke to the main channel, it makes the main channel lose its superconducting state as well.
Already, Berggren’s team has illustrated proof-of-principle for the nano-cryotron’s use as an electronic component. Adam McCaughan, a former student of Berggren, has designed a device that makes use of nano-cryotrons to add binary digits.
Berggren has been successful in using nano-cryotrons as an interface between superconducting devices and conventional, transistor-based electronics.
According to Berggren, the superconducting nanowire developed by his group could someday complement—or possibly compete with—Josephson junction-based superconducting devices.
“Wires are relatively easy to make, so it may have some advantages in terms of manufacturability,” stated Berggren.
Berggren believes that the nano-cryotron could someday find applications in supercooled electronics for telescopes and superconducting quantum computers. Wires possess low power dissipation, and thus, they might be convenient for energy-intensive applications as well.
“It’s probably not going to replace the transistors in your phone, but if it could replace the transistor in a server farm or data center? That would be a huge impact,” added Berggren.
Apart from particular applications, Berggren takes a wide view of his work on superconducting nanowires.
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.
Karl Berggren, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology
The National Science Foundation offered the initial financial support for nano-cryotron research in the Berggren laboratory.