The study led by researchers at his former institution, the University of Maryland, uncovers the first evidence of many-body localization of particles without disorder.
Pagano explained the discovery has relevance for quantum computing and, in general, materials that don’t follow the rules of classical mechanics.
“Many-body localization is a complicated phenomenon that takes place in systems of particles that have both disorder, like many real-life materials, and also interact with each other,” Pagano said. “Localization means the system will not thermalize (that is, relax into thermal equilibrium). In this case, even if the system starts in an initial highly excited state, it preserves memory of that state for a very long time without relaxing to an equilibrium state, as predicted by thermodynamics.
“There have been many studies about the interplay between disorder and interaction in quantum systems,” he said. “In this paper, we remove one ingredient: Disorder. The main novelty is that we impose a new type of localization -- Stark localization -- in which we still have interactions among the particles, but the disorder is replaced by a large gradient across the system.
The researchers found the effect using a chain of trapped ytterbium ions as a quantum simulator of a spin system and used lasers to control their long-range spin-spin coupling while local magnetic fields controlled the spin of individual ions, creating the gradient. This combination showed that the ions could preserve memory of the initial state, a signature of many-body localization, but not relax into a thermal equilibrium state.
“The gradient means a particle would have to pay a huge energetic cost to hop from one state to another,” he said. “Our big question was, does the system thermalize? And it seems that, for large enough gradients, it does not, at least for the system sizes and the timescales we were able to study. This is a new phenomenon and it is generating a lot of interest, since systems that can avoid thermalization are not so common.”
Pagano said that the system could thermalize over longer time scales than his team’s experiments could perform. “There are practical limitations to observing thermalization over infinite time,” he said. “We can probe the quantum evolution up to a certain time, and we pushed this one as far as we could, but not to the time that theorists want to dissipate any doubt on the question of long-time thermalization.”
He noted that joining Rice in 2020 could have knocked him off of the project, but because the pandemic put every team meeting on Zoom, he was able to keep pace with the collaboration.
Pagano continues to assemble his Brockman Hall lab for further study of quantum phenomena, including work that will contribute to the goals of the Rice Quantum Initiative. “A big goal at Rice is to build a system that is as clean and controllable as possible so that we can probe and physically simulate these quantum spin systems for longer to be more up to the expectation of the theorists,” he said.