Discovery of Universal Physics in the Dynamics of Quantum Systems

A breakthrough has been achieved by physicists at Penn State in unraveling a universal response within quantum systems when they experience a substantial surge of energy. Through meticulous observations of ultra-cold, one-dimensional atomic gases, the scientists closely examined this reaction and the subsequent phase known as “hydrodynamization,” which provides a model for comprehending similar quantum systems. The remarkable findings were recently published in the renowned journal Nature.

By conducting new experiments utilizing ultra-cold atomic gases, researchers have shed light on the evolution of all interacting quantum systems following a sudden influx of energy. The experiments focused on one-dimensional gases composed of ultra-cold atoms and revealed a striking universality in the way these quantum systems, comprising numerous particles, undergo changes over time after being thrown out of equilibrium due to a significant energy disturbance. The team of physicists at Penn State demonstrated that these gases instantaneously respond and “evolve” with characteristics common to all “many-body” quantum systems that experience such out-of-equilibrium scenarios. The details of these experiments were published in Nature on May 17, 2023.

“Throughout the last century, numerous major advancements in physics have revolved around the behavior of quantum systems with a large number of particles,” explained David Weiss, Distinguished Professor of Physics at Penn State and one of the leaders of the research team. “Despite the diverse array of ‘many-body’ phenomena observed, such as superconductivity, superfluidity, and magnetism, it has been discovered that their behavior near equilibrium often exhibits sufficient similarities to be classified into a small set of universal categories. However, when it comes to systems far from equilibrium, there have been few unifying descriptions.”

According to Weiss, these quantum many-body systems consist of ensembles of particles, such as atoms, that are free to move relative to each other. Describing their dynamics often requires the application of quantum mechanics, the fundamental theory that elucidates the properties of nature at the atomic or subatomic scale. The conditions necessary for these systems to manifest include a certain level of density and extreme coldness, which can vary depending on the specific context.

Out-of-equilibrium systems with dramatic deviations from stability are frequently generated in particle accelerators when high-speed collisions occur between heavy ions. These collisions generate a plasma composed of subatomic particles known as “quarks” and “gluons,” which emerges shortly after the collision and can be described by a hydrodynamic theory similar to the classical theory employed to explain fluid flow or other moving fluids. However, what transpires during the incredibly short period before the application of hydrodynamic theory has remained a puzzle.

“The stage prior to the implementation of hydrodynamics has been termed ‘hydrodynamization’,” stated Marcos Rigol, professor of physics at Penn State and another leader of the research team. “Numerous theories have been proposed to comprehend hydrodynamization in these collisions, but the situation is highly complex, and it is impractical to directly observe these processes in particle accelerator experiments. By utilizing cold atoms, we can actually observe what transpires during hydrodynamization.”

The researchers at Penn State took advantage of two unique characteristics of one-dimensional gases, which were trapped and cooled close to absolute zero using lasers, to gain insights into the system’s evolution following an out-of-equilibrium event before hydrodynamics come into play. The first characteristic pertains to the experimental aspect. The researchers had the ability to instantaneously disable interactions at any given point after the energy influx, allowing for direct observation and measurement of the system’s evolution. Specifically, they focused on monitoring the time-dependent changes in one-dimensional momentum distributions subsequent to the sudden surge in energy.

“Ultra-cold atoms trapped using laser techniques enable precise control and measurement, thereby shedding light on the physics of many-body systems,” Weiss remarked. “It is truly remarkable that the same fundamental principles that characterize relativistic heavy ion collisions, some of the most high-energy collisions ever created in a laboratory, are also manifest in the much less energetic collisions we generate in our lab.”

The second characteristic is related to the theoretical aspect. A complex collection of interacting particles can be described as “quasiparticles” whose mutual interactions are significantly simpler. In the case of one-dimensional gases, the quasiparticle description is mathematically exact, setting it apart from most other systems. This description provides a clear understanding of why energy redistributes rapidly across the system after being thrown out of equilibrium.

“The known laws of physics, including conservation laws, in these one-dimensional gases indicate that a hydrodynamic description becomes accurate once the initial evolution takes place,” Rigol explained. “The experiment demonstrates that this occurs before local equilibrium is achieved. Consequently, the experiment and theory together serve as a model example of hydrodynamization. Given the rapid nature of hydrodynamization, the underlying understanding in terms of quasiparticles can be applied to any many-body quantum system that experiences a significant energy injection.”

The research team at Penn State, comprising David Weiss, Marcos Rigol, Yuan Le, Yicheng Zhang, and Sarang Gopalakrishnan, conducted this study. The research was supported by the U.S. National Science Foundation, and the computational aspects were carried out at the Penn State Institute for Computational and Data Sciences.

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• Penn State News: Universal Physics Uncovered in the Dynamics of a Quantum System
• Nature Journal: Observation of hydrodynamization and local prethermalization in 1D Bose gases