https://doi.org/10.1140/epja/s10050-026-01843-2
Invited Viewpoint and Perspective
Few is different: deciphering many-body dynamics in mesoscopic quantum gases
1
Institut für Theoretische Physik, Heidelberg University, Philosophenweg 16, 69120, Heidelberg, Germany
2
Physikalisches Institut der Universität Heidelberg, Im Neuenheimer Feld 226, 69120, Heidelberg, Germany
3
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, OX1 3PU, Oxford, UK
4
Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000, Aarhus C, Denmark
5
Theoretisch Physikalisches Institut, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743, Jena, Germany
6
Department of Physics, Institute of Science Tokyo, Ookayama, Meguro, 152-8551, Tokyo, Japan
7
Department of Physics, The University of Tokyo, Hongo, Bunkyo, 113-0033, Tokyo, Japan
8
Theoretical Physics Department, CERN, 1211, Geneva 23, Switzerland
9
Department of Physics and Astronomy, Washington State University, 99164, Pullman, WA, USA
10
Scientific Computing Center, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany
11
GSI Helmholtzzentrum für Schwerionenforschung, Planckstraße 1, 64291, Darmstadt, Germany
12
School of Physics and Astronomy, Monash University, 3800, Victoria, Australia
13
Department of Physics, Yale University, 06520, New Haven, CT, USA
14
Yale Quantum Institute, Yale University, 06520, New Haven, CT, USA
15
Department of Physics, Lund University, Box 118, 221 00, Lund, Sweden
16
Mathematical Physics and NanoLund, Lund University, LTH, Box 118, 22100, Lund, Sweden
17
Department of Physics, University of Trieste, 34127, Trieste, Italy
18
CNR-INO – Istituto Nazionale di Ottica, Consiglio Nazionale delle Ricerche, 34149, Trieste, Italy
19
Department of Physics, North Carolina State University, 27695, Raleigh, USA
20
Center for Nuclear Theory, Department of Physics and Astronomy, Stony Brook University, 11794-3800, New York, NY, USA
21
Department of Physics, University of Toronto, M5S 1A7, Ontario, Canada
22
Physics Department, Brookhaven National Laboratory, 11973, Upton, USA
23
Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
24
Istituto Nazionale di Ottica del Consiglio Nazionale delle Ricerche (CNR-INO), 50019, Sesto Fiorentino, Italy
25
European Laboratory for Non-Linear Spectroscopy (LENS), Università di Firenze, 50019, Sesto Fiorentino, Italy
26
Institute for Theoretical Physics, University of Innsbruck, 6020, Innsbruck, Austria
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Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, 6020, Innsbruck, Austria
28
Department of Physics, Harvard University, Cambridge, 02138, Massachusetts, USA
a
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Received:
6
November
2025
Accepted:
19
March
2026
Published online:
12
June
2026
Abstract
Emergent macroscopic descriptions of matter, such as hydrodynamics, are central to our description of complex physical systems across a wide spectrum of energy scales. The conventional understanding of these many-body phenomena has recently been shaken by a number of experimental findings. Collective behavior of matter has been observed in mesoscopic systems, such as high-energy hadron–hadron collisions, or ultracold gases with only a few strongly interacting fermions. In such systems, the separation of scales between macroscopic and microscopic dynamics (at the heart of any effective theory) is inapplicable. To address the conceptual challenges that arise from these observations and explore the universality of emergent descriptions of matter, the EMMI Rapid Reaction Task Force was assembled. This document summarizes the RRTF discussions on recent theoretical and experimental advances in this rapidly developing field. Leveraging technological breakthroughs in the control of quantum systems, we can now quantitatively explore what it means for a system to exhibit behavior beyond the sum of its individual parts. In particular, the report highlights how the (in)applicability of hydrodynamics and other effective theories can be probed across three principal frontiers: the size frontier, the equilibrium frontier, and the interaction frontier.
Communicated by Ulf Meissner.
© The Author(s) 2026
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