The Jonsson School

Solving the Reversible Computing Puzzle

Professor Develops Innovative Skyrmion Logic

For decades, physicists and engineers have tried to create a fully reversible computing system based on conservative logic that computes without dissipating energy through the creation and destruction of information. Dr. Joseph Friedman, assistant professor of electrical and computer engineering at the Erik Jonsson School of Engineering and Computer Science at The University of Texas at Dallas has proposed a method to use skyrmions, nanoscale whirls that occur in magnetic materials, to construct a reversible computing system. Friedman’s research was recently named an Editor’s Suggestion in Physical Review Applied.

Dr. Joseph Friedman

“We’re exploiting the rich physics of magnetic skyrmions to elegantly solve problems,” Friedman said. “Skyrmions don’t want to go straight. Let them do what they’re good at. Let’s not try to force a square peg into a round hole.”

The new logic family is significant, particularly because of the low energy required to run the new reversible computing system. Reversible computing results in no increase in physical entropy or loss of energy, and conservative logic retains all information-carrying signals. Both have been unworkable concepts, until now.

Skyrmions Explained

A skyrmion is, essentially, a stable region of a material that is magnetized in the opposite direction of the remainder of the magnet. These magnetic skyrmions can be pushed with an electrical current, but they end up traveling at an angle instead of going straight.

“It’s similar to how when you kick a soccer ball with spin, the ball curves,” Friedman explained.

Micromagnetic simulations show how the skyrmions execute a reversible function within a gate.

Currently, physicists attempt to minimize the effect in an attempt to force the skyrmions to go straight. Instead of eliminating the Magnus force, named for the German physicist who studied it, Friedman uses this odd behavior to his advantage.

“We send the skyrmions along tracks that force them to travel in a straight line, and then periodically allow them to use this Magnus force to switch tracks,” Friedman said. “But skyrmions repel each other, so they can only switch when there is no skyrmion in the other track. This is the essence of the logic operations.”

Strange Collisions in a Conservative System

While skyrmions themselves are a complex physical phenomenon, they may be understood in a simplified manner and manipulated for Boolean logic. Because skyrmion motion involves the movement of magnetization rather than the movement of actual particles, they can be transported with minimal energy dissipation.

“There’s an important idea in computing systems, that there’s always information and energy lost during computation, as explained via Claude Shannon’s information theory,” Friedman said. “The minimum energy required for computation is called the Landauer limit, developed more than 50 years ago by Rolf Landauer. Ten years later, Edward Fredkin and Tommaso Toffoli proposed reversible computing and conservative logic as approaches to overcome this limit and illustrated their model with elastic collisions of billiard balls. There’s always the same number of billiard balls at the beginning and at the end, and therefore no energy lost. Remarkably, by using the skyrmions, we’re proposing the first nanoscale implementation of reversible computing.”

This simulation illustrates how skyrmions are used in a full adder electronic circuit.

Skyrmions behave in four important ways. First, they tend to stay stable once they are generated. They can be pushed in a particular direction with an electrical current, but curve like a spinning soccer ball due to the Magnus force. They also do not like to be near other skyrmions. Once the skyrmions are generated, Friedman has successfully manipulated them to execute computations.

“The skyrmions never get destroyed, but flow and collide like the billiard balls in Fredkin and Toffoli’s conservative logic,” Friedman explains. “It’s also related to information conservation, or energy conservation.”

Taking Skyrmions to Scale

Friedman is currently working with partners to determine next steps to apply the logic concept to large systems

“We’re looking at low energy, high speed computing,” Friedman said. “For now, we’re looking specifically at Boolean logic. But we’ve also developed a skyrmion Fredkin gate, which forces us to think about extensions to quantum computing.”

Importantly, Friedman’s article also includes a solution to one potential pitfall of skyrmion interactions, namely that timing affects the skyrmions’ behavior.

“The timing or clocking is especially important for a large system,” Friedman said. “Since they repel one another, a skyrmion might go up or down in the gate depending upon whether another skyrmion is present. We developed a clocking mechanism to ensure the skyrmions interact at the right moment.”

Friedman has taken his concept to collaborators to determine next steps so the nanoscale technology can be applied to large scale computer systems.

“We’re working with collaborators to build prototypes, to scale them up. Essentially, we need to resolve issues with robustness, tolerances, and other details.”

While there may be some changes to how the skyrmions are used in a large-scale system, Friedman’s creation of a reversible, conservative computing system at the nanoscale is a major breakthrough.