Columbia Physicists Make Ripples With Their Theory Explaining How Quantum Particles Get Stuck
Waves rippling through a body of water will go with the flow. Quantum particles, which behave like waves, do the same—usually. But as Columbia theoretical physicists Boris Altshuler and Igor Aleiner, along with Princeton’s David Huse, discovered, sometimes systems of quantum particles can get profoundly stuck in a process called many-body localization, which could have important applications for protecting the fragile state of qubits.
For their foundational work, the trio received one of the American Physical Society’s (APS) highest honors, the Lars Onsager Prize.
The work is profound and transformative, said Leo Radzihovsky, a physicist at the University of Colorado-Boulder and a member of the prize selection committee. “Boris and Igor launched a whole new field of quantum nonequilibrium dynamics, which David Huse has taken in a new range of directions,” he said. “Their many-body localization discovery has stimulated extensive theoretical and experimental research that is only growing.”
The story began at Bell Labs in the 1950s with Philip Anderson, who would go on to win the Nobel Prize in Physics. Inspired by experiments suggesting that quantum particles could be constrained in a material, particularly if random defects were strewn about, Anderson published his theory of localization in 1958. He described two distinct states: a localized state, in which a single quantum particle can become trapped in one place, and a delocalized state in which the wave unfurls as expected. He theorized that increasing disorder would increase the fraction of localized states, but that delocalized states would still occur in the system.
Decades later, Altshuler began thinking about multiple particles—also known as many bodies—in a system. In particular, he was interested in how negatively charged electrons repel each other and influence a system. In 2006, after Altshuler moved to Columbia, he, Aleiner, and Columbia postdoc, Denis Basko, showed that interacting electrons could produce complete localization. "That statement—that every single state could be localized—was a new and profound insight that required a major rearrangement of our mental furniture,” said Andrew Millis, a theoretical physicist at Columbia.
That statement—that every single state could be localized—was a new and profound insight that required a major rearrangement of our mental furniture.
Many-body localization represents an entirely new phase of quantum matter with unique properties. Huse showed that like glass, a material that should flow like a liquid but is instead “frozen” in place as a solid, a many-body localized system does not reach a state of equilibrium (think of a hot cup of coffee cooling to room temperature). In a delocalized state, the particles in a system eventually disperse into every configuration possible. Eventually, the system “forgets” what it once was. Materials in a many-body localized phase, however, can “remember.”
This memory has potential quantum information applications. For example, the quantum bits that makeup quantum computers need to be isolated lest they lose the information encoded in them. Localization might help protect the fragile quantum states of qubits. As a unique form of non-equilibrium quantum matter, many-body localization has also inspired recent work into time crystals, an uncannily stable material that can oscillate without using energy.
As the details and implications of many-body localization continue to be explored, a new community of scientists inspired by Altshuler, Aleiner, and Huse has begun to apply their ideas to investigate non-equilibrium quantum states and their potential for innovative quantum technologies. “The work has opened up entirely new ways of thinking about quantum computers and quantum devices,” said Millis. “As with most of the revolutionary ideas in science, the field has grown far beyond its initial context. We’re all looking to what comes up next.”