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Recent research from University of Michigan theorists illustrates a physical phenomenon that could pave the way for innovations in areas like quantum computing

Kai Sun from the University of Michigan is an unassuming physics professor with lofty aspirations.

“I primarily rely on paper and pencil as a theorist, focusing mostly on analytical calculations,” Sun remarked. “While my interests are quite diverse, they essentially revolve around discovering new fundamental principles and phenomena, particularly those that were previously thought to be unattainable.”

While his latest research doesn’t exactly reach that unattainable threshold, it does enhance our understanding of physical possibilities. A quantum behavior once believed possible only under specific conditions can, in fact, be easily realized, according to new findings from Sun and his team published in the journal Physical Review X.

Leveraging this behavior could enable the manipulation of light and other quantum particles in innovative ways, which could have implications for burgeoning fields like quantum computing.

The study received partial funding from the Office of Naval Research. U-M research fellow Kai Zhang and graduate student Chang Shu also played significant roles in this research.

Embrace the unusual, quantum mechanics

While classical physics—the framework of laws governing most of our observable reality—tends to be straightforward, quantum mechanics is renowned for its complexity.

For instance, in classical physics, waves and particles are distinct entities. However, in the minuscule quantum domain, elements such as light and electrons behave both as waves and particles. In traditional computers, a bit can have a value of either zero or one. In quantum systems, quantum bits can represent combinations of ones and zeros.

Sun and his colleagues’ recent research continues with quantum’s tendency to occupy an ambiguous intermediary space between binary options.

Previously, scientists believed there were typically two fundamental modes through which an energetic wave or particle—remember, they are not strictly separate in quantum mechanics—exists within materials. To visualize these states or modes, picture holding a long rubber band that has been cut, forming a straight line rather than a loop.

If you pinched the band at two points near the center and pulled it taut, having someone pluck it like a guitar string would demonstrate one of the states. The energy remains contained in the string’s up-and-down motion between your fingers, forming a standing wave that does not travel along the band.

This contrasts with a moving wave, which would be akin to snapping the band like a whip to create a ripple that travels along its length.

“In quantum vernacular, one is confined or localized, while the other is a propagating wave,” Sun explained.

Researchers were aware of a third, intermediate state that is partially, but not entirely localized. The challenge, or so they believed, was that these states tended to be quite fragile.

I possess the power (law)

In the rubber band analogy for a localized wave, your pinching fingers serve as barriers inhibiting energy from spreading. In physical materials, such barriers can manifest as edges or irregularities in their microscopic configurations. Confined states or modes can vibrate within those boundaries, but their energy dissipates rapidly outside. To be precise, that swift decay is exponential.

For propagating waves, there are no such constraints and no rapid decays. However, in the in-between, partially confined state, there is a decay that is less severe than exponential. Mathematically, that decay is characterized by what’s termed a power law.

“A power law represents a significantly slower decay than exponential, yet is still faster than no decay at all,” Sun stated.

Researchers have previously witnessed situations with power law decays in real-world experiments, but these scenarios were regarded as complex to establish and maintain.

“It was feasible, but required a certain degree of fine-tuning,” Sun said. “In this study, the intriguing part is that we discovered a range of systems where all the modes exhibit power-law behavior and are exceptionally resilient. They don’t require any fine-tuning.”

The paper suggests new design considerations that may facilitate easier and more dependable access to these states in the future. A crucial aspect of this discovery is that, up till now, most researchers had focused predominantly on one-dimensional issues, like the rubber band.

Sun and his team explored the implications in two or more dimensions and uncovered instances where power-law decays are prevalent near the edges or the “skin” of materials. They also identified that the behavior of these skin modes is highly sensitive to a material’s shape, particularly its aspect ratio—something not reported previously.

Sun expressed enthusiasm about both uncovering new physics and envisioning potential applications in areas such as quantum computing. For instance, bits might contain confined modes for computations while concurrently allowing power-law modes to relay information between them.

“This research unveils groundbreaking concepts on a fundamental level while also unlocking new possibilities for future applications,” Sun concluded.

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