How do quantum systems evolve during energy surges and system imbalances Nature Express
New experiments using a one-dimensional gas of ultracold atoms reveal a generality: how quantum systems of many particles change over time when a large influx of energy throws the system out of equilibrium.
A team of physicists at Pennsylvania State University has shown that these gases immediately respond by "evolving" common features of related "many-body" quantum systems. A paper describing these experiments was published May 17, 2023, in the journal Nature.
The results of the study, titled "Observation of hydrodynamization and local prethermalization in 1D Bose gases," were published May 17 in the journal Nature.
Penn State graduate student Yuan Le, the first author of the paper describing these experiments, stands near the apparatus she used to create and study one-dimensional gases near absolute zero.
Many of the major advances in physics over the last century have involved the behavior of quantum systems with many particles," said David Weiss, Penn State Distinguished Professor of Physics and one of the leaders of the research team. Despite the astounding variety of 'many-body' phenomena: superconductivity, superfluidity and magnetism, it has been found that their behavior as they approach equilibrium is often similar enough that they can be classified into a small set of general categories. In contrast, the behavior of systems far from equilibrium is rarely described in such a uniform way."
Weiss explained that these quantum many-body systems are collections of particles that are free to move around each other. When they are some combination of density and coldness (which varies by environment), quantum mechanics begins to describe their dynamics.
In a particle gas pedal, when a pair of heavy ions collide at speeds approaching the speed of light, a severely out-of-equilibrium system is usually created. The plasma produced by the collision - consisting of subatomic particles "quarks" and "gluons" - appears early in the collision and can be described in terms of hydrodynamic theory (similar to that used to describe air flow or gluons). (similar to the classical theory used to describe air flow or other moving fluids) well before the plasma reaches local thermal equilibrium.
But what happened in the short period before hydrodynamic theory became available?
The physical process that occurs before hydrodynamics becomes available is called 'hydrodynamization,'" says Marcos Rigol, a professor of physics at Penn State and another leader of the research group. Many theories have been developed to try to understand the hydrodynamics in these collisions, but the actual situation is quite complex and it is impossible to actually observe it as it happens in particle gas pedal experiments."
"Using cold atoms, we will be able to observe what happens during hydrodynamics."
This time, the Penn State researchers took advantage of two special features of one-dimensional gases that are trapped and cooled to near absolute zero by lasers to understand the evolution of the system after it is thrown out of equilibrium, but before hydrodynamics can be applied. The interactions in the experiment can be abruptly turned off at any time after the energy inflow, so the evolution of the system can be directly observed and measured. Specifically, they observe the time evolution of the one-dimensional momentum distribution after a sudden quenching of energy.
Evolution of a homogeneous TG gas after Bragg quenching.
Ultracooled atoms in traps made by lasers allow such exquisite control and measurements that they can really shed light on many-body physics," Weiss said. Surprisingly, the fundamental physics that characterizes relativistic heavy-ion collisions (i.e., some of the most energetic collisions performed in the laboratory) also shows up in the much less energetic collisions we perform in the laboratory."
The second feature is theoretical. A collection of particles interacting in a complex way can be described as a collection of "particle-like" particles whose interactions are much simpler. Unlike most systems, the quasiparticle description of a one-dimensional gas is mathematically precise. It allows a very clear description of why the energy is rapidly redistributed after the system is thrown out of equilibrium.
"The known laws of physics in these one-dimensional gases, including conservation laws, mean that the hydrodynamic description will be accurate once this initial evolution plays out." Rigol said, "The experiment shows that this occurs before local equilibrium is reached. Thus, the experiment and theory together provide a model example of hydrodynamicization. Because hydrodynamicization occurs so quickly, the fundamental understanding in terms of quasiparticles can be applied to any many-body quantum system that incorporates very large amounts of energy."
Reference links:
[1] https://phys.org/news/2023-05-uncovering-universal-physics-dynamics-quantum.html
[2]https://www.nature.com/articles/s41586-023-05979-9
[3] https://samacharcentral.com/uncovering-universal-physics-in-the-dynamics-of-a-quantum-system/
