Phases on the Move: A Quantum Game of Catch

The world is out-of-equilibrium, said JILA Fellow Ana Maria Rey. All around us systems are constantly in flux, from our living bodies to the weather and the stock market. These systems aren't settled in a final state.

Equilibrium systems are stationary; they don't change in time, said JILA Fellow James Thompson. And after a hundred years of research, we have developed tools to understand what happens in a system at equilibrium, Rey added. But when it comes to understanding an out-of-equilibrium system, many questions remain.

"We are out of theory," Rey said. "One of the challenges that we have at the moment is trying to understand out-of-equilibrium matter and how it tends to organize."

Rey and Thompson teamed up to create a controllable, non-equilibrium macroscopic system in the lab, so they can study how it behaves when you tune individual parameters. What they found could pave the way for a new foundation in our basic understanding of physics.

Atoms playing catch

Studying the organization principles of out-of-equilibrium systems requires a highly tunable system, Rey said, so Thompson put a million atoms in a one-dimensional lattice inside an optical cavity. In this setup, the atoms have the opportunity for all kinds of interactions.

By shining a laser onto the cavity, they are able to inject photons that act like baseballs, and the atoms start playing a quantum game of catch. If they catch the photon-baseballs, they start spinning. If these atoms couldn't interact with each other, they would spin freely, flipping over, Rey explained. But each atom is interacting with all others in the array-and if their partners aren't catching the photons and flipping over, then they won't either.

In this case, atomic interactions occur by exchanging virtual cavity photons, which act slightly different than the injected photon-baseballs.

"Essentially an atom throws a photon into the cavity mode. It bounces back and forth and any atom is able to reach up and grab it," Thompson said, and throw it back.

Interactions between atoms want to keep them pointing all at the same direction. "If he's down, I want to be down. So, in principle, the atoms want to be aligned," Rey explained.

In a quiet cavity the atoms don't want to play catch. But when the intensity of the injected photons (i.e. the number of photon-baseballs per second) increase, the atoms' behavior changes.

"If you just throw enough photons, eventually atoms start catching these photons, ignoring others. Catching the baseball is equivalent to start spinning up and down," Thompson explained. "If they don't have a ball in their hand, they prefer to remain aligned with their peers and point down."

Changing phases

When the atoms decide to spin, they do so simultaneously and instantaneously. That abrupt change in behavior is interesting, Rey said. It is a dynamical phase transition. In many respects, it resembles the transitions we are used to seeing in our daily life but in a dynamical system.

"Think of it like trying to understand what you need to change to turn water into ice or into a gas," Rey elaborated. You fiddle with the temperature or the pressure until you reach a critical point where the water freezes, boils, or melts.

But in this non-equilibrium system, the starting phase really affects what the atoms would do next. Watching and understanding how these transitions happen helps shed light on organizing principles of systems in motion.

As the team turned the dials, they could identify the complete phase diagram-all of the different phases this system could display-all of which Rey's group had predicted. To confirm the dynamical nature of the phases, Thompson and Rey showed a surprising symmetry; changing the sign of the interactions-plus to minus or vice versa-does not affect the phase diagram. That doesn't happen in equilibrium systems, Rey explained. This demonstrated that "the organization principles of out of equilibrium matter can be very different to what we're used to," Rey summarized. "There are different rules that govern out-of-equilibrium systems and how they organize."

Learning new rules

These results give scientists new clues to the rules that govern an out-of-equilibrium system. And coming up with rules to explain these complex systems is exactly the point of basic research like this, Thompson added.

"When thermodynamics was being developed, they didn't know PV=NkT [the ideal gas law]. They had to figure that out," Thompson said. "What are the rules of the game for this kind of regime that we're operating in? What are the emergent properties that describe these systems?"

Rey hopes that this type of experiment can help them understand more complex systems featuring chaos and information scrambling behavior also exhibited by black holes but at extreme scales.

There are a lot of opportunities to build on this experiment, Rey and Thompson pointed out, but this is a crucial starting point.

"This is just a stepping stone," Rey concluded. "We're paving the ground for something really cool to happen."

The study was published in Nature on April 30, 2020, and was supported by the Physics Frontier Center Grant from the National Science Foundation.

Written by Rebecca Jacobson