Kai Sun of the University of Michigan is a humble physics professor with ambitious goals.
"I'm mainly a paper-and-pencil type of theorist, doing analytical calculations mostly," Sun said. "My interests are pretty broad, but basically searching for new fundamental principles and new phenomena, especially new phenomena and new physics previously believed to be impossible."
While his newest study doesn't quite hit that impossible threshold, it does still update our conception of physical possibilities. A quantum behavior that was thought to be possible only sometimes can actually be readily realized, according to new work from Sun and his colleagues published in the journal Physical Review X.
Taking advantage of this behavior could help manipulate light and other quantum particles in new ways, which could find applications in emerging fields like quantum computing.
The study was funded, in part, by the Office of Naval Research. U-M research fellow Kai Zhang and graduate student Chang Shu also contributed to the work.
Stay weird, quantum mechanics
While classical physics—the set of natural laws governing the majority of what we see and feel in our everyday lives—tends to be black and white, quantum mechanics is famous for being mushier.
For example, in classical physics, waves and particles are different things. But in the ultratiny quantum realm, things like light and electrons act as both waves and particles. In conventional computers, a bit has a value of either zero or one. In quantum computers, quantum bits act as combinations of ones and zeros.
Sun and his colleagues' new study keeps with quantum's penchant for finding a blurrier middle ground between either-or binaries.
Previously, scientists had thought there were two typical ways an energetic wave or particle—remember, they aren't exactly distinct in quantum mechanics—exist inside materials. To envision these states or modes, think about holding a long rubber band that's been snipped, so it's a straight line instead of a loop.
If you pinched the band at two points near the center and pulled it taut, then had someone pluck it like a guitar string, that gives you one of the states. The energy is contained in the string moving up and down between your fingers, forming a standing wave that doesn't travel along the string.
That's compared with a traveling wave, which would be more like flicking the band like a whip to send a ripple traveling along the length of the band.
"If we use quantum terminology, one is confined or localized. The other is a propagating wave," Sun said.
Researchers had known there was a third, in-between state that's partially, but not completely localized. The problem, or so they thought, was that these states were very persnickety.
I have the power (law)
In the rubber band example of a localized wave, your pinching fingers act as barriers preventing energy from traveling. In real materials, such barriers can be presented by edges or irregularities in their microscopic structures. Confined states or modes can wiggle within those boundaries, but their energy vanishes very quickly outside. To be mathematically precise, that fast decay is exponential.
For propagating waves, there are no such obstacles and no fast decays. But in the in-between, partially confined state, there is a decay that isn't as severe as exponential. That decay is described mathematically by what's called a power law.
"A power law is a much slower decay than exponential, but it's much faster than no decay," Sun said.
Researchers have observed situations with power law decays in real world experiments before, but these situations were thought to be tricky to establish and maintain.
"It was possible, but it needed some kind of fine-tuning," Sun said. "In this work, the fun part is we find a family of systems where all the modes are power law and they're extremely robust. They don't need any fine-tuning."
The paper points to new design considerations that could make accessing these states easier and more reliable moving forward. One of the keys to making this discovery is that, previously, most researchers had focused on one dimensional problems, like the rubber band.
Sun and colleagues considered what happens in two or more dimensions and found cases where power-law decays are the norm near the boundaries or the "skin" of materials. They also found that the behavior of these skin modes was very sensitive to a material's shape, specifically its aspect ratio, which had not been shown before.
Sun said it's exciting both to uncover new physics and to begin imagining applications in areas such as quantum computing. For example, bits might host confined modes for calculations while still allowing power-law modes to transmit information between them.
"This work reveals novel concepts on the fundamental side, while also opening new opportunities for future applications," Sun said.
