Research news

The emergence of simplicity

Photo: personal archive

Publish Date: 12.02.2021

Category: Researchers in focus , ERC & MSCA

An experiment sheds light on how quantum matter evolves from complexity to simplicity. By cooling atoms to temperature very close to absolute zero where quantum effects are important and using magnetic traps produced by a special electronic chip and lasers to control them, the researchers have been able to accurately monitor the atoms in time. The results have uncovered the precise physical mechanism that leads from a complex initial state to a much simpler final state, providing hints on how nature tends to equilibrium, a long-standing open question of theoretical physics.

The results were developed within the Marie Sklodowska Currie Seal of Excellence project of dr. Spyros Sotiriadis with a title Quenches, Transport and Entanglement in one-dimensional quantum systems out-of-equilibrium performed at Faculty of Mathematics and Physics, University of Ljubljana, under the supervision of prof. dr. Tomaž Prosen and funded by the Slovenian Research Agency. The work, based on a collaboration between the Technical University of Vienna (TUW), the Freie Universität Berlin (FUB) and the University of Ljubljana (UL), has been recently published in the journal Nature Physics [1].

Nature as we encounter it undeniably features a rich phenomenology. It is the primary task of physics to describe this phenomenology. It provides models for it and captures the physical world in terms of basic laws. It aims at understanding how constituents interact and what emergent properties these interactions give rise to. Quantum physics is the best physical theory we have available today to describe nature on a fundamental level. So, in one way or the other, these interacting systems will ultimately follow dynamical laws within quantum theory. Given a physical model, quantum physics predicts how the system under consideration will evolve in time. This information is expressed as the probability of finding a specific value when measuring a physical quantity at a given time. It is encoded in a mathematical object called the “wave function”, which provides a complete description of a physical system.

Strikingly, very simple models happen to describe a wealth of physical situations very well. These are so-called Gaussian states and models. While this may sound abstract, it may be sufficient to say that Gaussian states describe a physical situation in terms of simple Gaussian probability distributions. Such distributions, named after the mathematician Carl Friedrich Gauss, are ubiquitous in statistics and in nature. Physical systems that interact very little can be described by such Gaussian quantum states to a very good approximation.

This is all fine, but these insights seem to miss an explanation how quantum systems that have interacted in the past ultimately end up in such Gaussian states. Where does the simplicity come from? Theoretical work has long predicted notions of “Gaussification”, that is, the dynamical emergence of Gaussian states in physical systems. Prof. Jens Eisert (FUB) has suggested similar phenomena theoretically as early as in 2008 [2,5], and Dr. Spyros Sotiriadis (UL) has investigated the physical conditions and the underlying mechanism that explains these phenomena [3,4]. But experimental evidence has been missing. Now a team of experimentalists led by Prof. Jörg Schmiedmayer (TUW) and theoretically supported by Thomas Schweigler (TUW), Marek Gluza (FUB), Spyros Sotiriadis and Jens Eisert, has set out to experimentally probe the question how quantum systems ultimately approach Gaussian quantum states. This question is rooted in and related to the question how thermal equilibrium would ultimately emerge. Placing atoms cooled to extremely low temperatures on top of a precisely designed and controlled chip, the team has been able to approach this long-standing question that has already puzzled the forefathers of quantum mechanics under extremely accurate experimental conditions.

Indeed, in this experiment, one sees equilibrium properties as described by Gaussian states to emerge dynamically, accurately monitored in time. After some while, in other words, one encounters how nature tends to equilibrium, a state that is captured by simple and economic physical descriptions: Simplicity emerges dynamically.

Vakuumska komora

Image: Vacuum chamber with atomic chip, an electronic device that produces a magnetic trap that can be used to limit, control and manipulate ultra-cold atomic gas (Photo: Thomas Schweigler, TUW) 



1.  Decay and recurrence of non-Gaussian correlations in a quantum many-body system, Thomas Schweigler, Marek Gluza, Mohammadamin Tajik, Spyros Sotiriadis, Federica Cataldini, Si-Cong Ji, Frederik S. Møller, João Sabino, Bernhard Rauer, Jens Eisert, Jörg Schmiedmayer, Nature Physics 17 (2021).

2.    Exact relaxation in a class of nonequilibrium quantum lattice systems, M. Cramer, C. M. Dawson, J. Eisert, T. J. Osborne, Phys. Rev. Lett. 100, 030602 (2008).

3.     Validity of the GGE for quantum quenches from interacting to noninteracting models, S. Sotiriadis, P. Calabrese, J. Stat. Mech.: Theory Exp. 2014, P07024 (2014).

4.     Equilibration in one-dimensional quantum hydrodynamic systems, S. Sotiriadis, J. Phys. A: Math. Theor. 50 424004 (2017).

5.    Equilibration towards generalized Gibbs ensembles in non-interacting theories, Marek Gluza, Jens Eisert, and Terry Farrelly, SciPost Phys. 7, 38 (2019).

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