New experiments using one-dimensional gases of very cold atoms reveal a universality in how quantum systems composed of many particles change over time after a large influx of energy throws the system out of equilibrium. A team of physicists at Penn State has shown that these gases respond instantly, “evolving” with features common to all “multi-body” quantum systems discarded in this way. A paper describing the experiments appears May 17, 2023 in the journal nature.
“Many of the major developments in physics over the past century have concerned the behavior of quantum systems with many particles,” said David Weiss, Distinguished Professor of Physics at Penn State and one of the leaders of the research team. “Despite the dizzying array of diverse ‘many-body’ phenomena such as superconductivity, superfluid, and magnetism, it has been found that their behavior near equilibrium is often similar enough that they can be grouped into a small group of universal categories. In contrast However, the behavior of systems far from equilibrium has led to a few such unified descriptions.”
Weiss explained that these multi-body quantum systems are collections of particles, such as atoms, that are free to move relative to each other. When they are a mixture of dense and cool enough, which can vary depending on context, quantum mechanics—the basic theory that describes properties of nature on the atomic or subatomic scale—is required to describe their dynamics.
Massively out-of-equilibrium systems are routinely created in particle accelerators when pairs of heavy ions collide at speeds approaching the speed of light. The collisions produce a plasma—consisting of subatomic particles quarks and gluons—that emerges very early in the collision and can be described by a hydrodynamic theory—similar to the classical theory used to describe the flow of air or other moving fluids—before the plasma reaches local thermal equilibrium. But what happens in the astoundingly short time before hydrodynamic theory was used?
“The physical process that occurs before hydrodynamics is called ‘hydrodynamics,’” said Marcus Regol, a professor of physics at Penn State and another leader of the research team. “Many theories have been developed to try to understand the hydrodynamics in these collisions, but the situation is too complex to monitor. Exactly as it happens in particle accelerator experiments. Using cold atoms, we can observe what happens during hydrodynamics.”
The Penn State researchers took advantage of two special features of one-dimensional gases, which are trapped and cooled to near absolute zero by lasers, in order to understand the evolution of the system after it has been thrown out of equilibrium, but before hydrodynamics can be applied. The first feature is experimental. The reactions in the experiment can be abruptly stopped at any time after the energy flow, so the evolution of the system can be directly observed and measured. Specifically, they observed the temporal evolution of the one-dimensional momentum distributions after abrupt energy quenching.
“The very cold atoms in the laser traps allow for such wonderful control and scaling that they can really shed light on multibody physics,” said Weiss. “It’s amazing that the same underlying physics that characterizes collisions of relativistic heavy ions, some of the most energetic collisions ever seen in the lab, also emerges in the lower-energy collisions we do in our lab.”
The second advantage is theory. A group of particles that interact with each other in a complex way can be described as a group of “quasiparticles” whose mutual interactions are much simpler. Unlike most systems, the quasi-particle description of one-dimensional gases is mathematically accurate. It allows a very clear description of why energy rapidly redistributes through a system after it has been thrown out of equilibrium.
“The well-known physical laws, including conservation laws, in these one-dimensional gases indicate that the hydrodynamic description will be accurate once this initial evolution has taken place,” said Regolle. “Experiment shows that this occurs before local equilibrium is reached. Thus the experiment and theory together provide a typical example of hydrodynamics. Because hydrodynamics occurs very quickly, the basic understanding in terms of quasiparticles can be applied to any multibody quantum system to which a very large quantity is added of energy.”
In addition to Weiss and Rigol, the Penn State research team includes Yuan Le, Yicheng Zhang, and Sarang Gopalakrishnan. The research was funded by the US National Science Foundation. Calculations were performed at the Penn State Institute for Computing and Data Sciences.