Universal Physics Found in the Dynamics of Quantum Systems

abstract dynamic quantum system concept

Physicists at Penn State have discovered a universal response in quantum systems when they are perturbed by an influx of large amounts of energy. Using ultracold one-dimensional gases, they were able to closely observe this reaction and subsequent steps known as “fluid dynamics,” providing a model for understanding similar quantum systems. The research findings were published in journals. nature.

New experiments on ultracold atomic gases reveal how all interacting quantum systems evolve after a sudden influx of energy.

A new experiment using a one-dimensional gas of ultracold atoms demonstrates the generality of how quantum systems composed of many particles change over time in response to large inputs of energy that throw the system out of equilibrium. A team of Penn State physicists has shown that these gases react instantly and “evolve” into features common to all “many-body” quantum systems that are thrown out of equilibrium in this way. A paper describing the experiment was published in the journal on May 17, 2023. nature.

“Many major advances in physics over the past century have concerned the behavior of quantum systems with many particles,” said David Weiss, a professor of physics at Penn State and one of the team’s leaders. “Despite the remarkable array of various ‘many-body’ phenomena, such as superconductivity, superfluidity and magnetism, the behavior of those close to equilibrium often turns out to be sufficiently similar that they can be classified into small universal classes. In contrast, the behavior of systems far from equilibrium has few such unifying explanations.”

Yuan Le

New experiments with cryogenic atomic gases reveal universal physics in the dynamics of quantum systems. First author of the paper describing the experiment, Penn State graduate student Yuan Le stands near the apparatus she used to create and study one-dimensional gases near absolute zero. Credit: David Weiss, Penn State

These quantum many-body systems are ensembles of atoms-like particles that can move freely with respect to each other, Weiss explained. Quantum mechanics (the fundamental theory that describes the properties of nature at the atomic or sub-atomic scale) is needed to explain the dynamics when it comes to the context-dependent combination of density and coolness.

Dramatically out of balance systems are routinely created in particle accelerators when heavy ion pairs collide at speeds close to the speed of light. crash is[{” attribute=””>plasma—composed of the 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 airflow or other moving fluids—well before the plasma reaches local thermal equilibrium. But what happens in the astonishingly short time before hydrodynamic theory can be used?

“The physical process that occurs before hydrodynamics can be used has been called ‘hydrodynamization,” said Marcos Rigol, professor of physics at Penn State and another leader of the research team. “Many theories have been developed to try to understand hydrodynamization in these collisions, but the situation is quite complicated and it is not possible to actually observe it as it happens in the particle accelerator experiments. Using cold atoms, we can observe what is happening during hydrodynamization.”

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 is thrown of out of equilibrium, but before hydrodynamics can be applied. The first feature is experimental. Interactions in the experiment can be suddenly turned off at any point following the influx of energy, so the evolution of the system can be directly observed and measured. Specifically, they observed the time-evolution of one-dimensional momentum distributions after the sudden quench in energy.

“Ultra-cold atoms in traps made from lasers allow for such exquisite control and measurement that they can really shed light on many-body physics,” said Weiss. “It is amazing that the same basic physics that characterize relativistic heavy ion collisions, some of the most energetic collisions ever made in a lab, also show up in the much less energetic collisions we make in our lab.”

The second feature is theoretical. A collection of particles that interact with each other in a complicated way can be described as a collection of “quasiparticles” whose mutual interactions are much simpler. Unlike in most systems, the quasiparticle description of one-dimensional gases is mathematically exact. It allows for a very clear description of why energy is rapidly redistributed across the system after it is thrown out of equilibrium.

“Known laws of physics, including conservation laws, in these one-dimensional gases imply that a hydrodynamic description will be accurate once this initial evolution plays out,” said Rigol. “The experiment shows that this occurs before local equilibrium is reached. The experiment and theory together, therefore, provide a model example of hydrodynamization. Since hydrodynamization happens so fast, the underlying understanding in terms of quasi-particles can be applied to any many-body quantum system to which a very large amount of energy is added.”

Reference: “Observation of hydrodynamization and local prethermalization in 1D Bose gases” by Yuan Le, Yicheng Zhang, Sarang Gopalakrishnan, Marcos Rigol and David S. Weiss, 17 May 2023, Nature.
DOI: 10.1038/s41586-023-05979-9

In addition to Weiss and Rigol, the research team at Penn State includes Yuan Le, Yicheng Zhang, and Sarang Gopalakrishnan. The research was funded by the U.S. National Science Foundation. Computations were carried out at the Penn State Institute for Computational and Data Sciences.

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