An atomic system acts like a quantum Newton’s cradle
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Atoms in a one-dimensional quantum gas behave like a Newton’s cradle toy, transferring energy from atom to atom without dissipation. Developed by researchers at the TU Wien, Austria, this quantum fluid of ultracold, confined rubidium atoms can be used to simulate more complex solid-state systems. By measuring transport quantities in this “perfect” atomic system, the team aims to gain a deeper understanding of how transport phenomena and thermodynamics behave at the quantum level.
Physical systems transport energy, charge, and mass in various ways. Electrical currents streaming along a wire, heat flowing through a solid, and light travelling down an optical fibre are just three examples. How readily these quantities move within a material depends on the resistance they encounter; collisions and friction slow them and cause them to decay. This level of resistance largely determines whether the material is classed as an insulator, a conductor, or a superconductor.
The mechanisms behind such transport fall into two main categories. The first is ballistic transport, which involves linear motion without loss, as a bullet travels in a straight line. The second is diffusive transport, in which the quantity is subject to numerous random collisions. A good example is heat conduction, in which heat moves through a material gradually, travelling in many directions simultaneously.
Breaking the rules
Most systems are strongly affected by diffusion, which makes it surprising that the TU Wien researchers could build an atomic system where mass and energy flowed freely without it. According to study leader Frederik Møller, the key to this unusual behaviour is the magnetic and optical fields that keep the rubidium atoms confined to one dimension, “freezing out” interactions in the atoms’ two transverse directions.
Because the atoms can only move along a single direction, Møller explains, they transfer momentum perfectly, without scattering their energy as would be the case in normal matter. Consequently, the 1D atomic system does not thermalize despite undergoing thousands of collisions.
To quantify the transport of mass (charge) and energy within this system, the researchers measured Drude weights, fundamental parameters that characterize ballistic, dissipationless transport in solid-state systems. According to these measurements, one-dimensional interacting bosonic atoms indeed exhibit perfect dissipationless transport. The results also agree with the generalized hydrodynamics (GHD) framework, which describes the large-scale, inhomogeneous dynamics of one-dimensional integrable quantum many-body systems, such as ultracold atomic gases and certain spin chains.
A Newton’s cradle for atoms
According to team leader Jörg Schmiedmayer, the experiment is analogous to a Newton’s cradle toy, which consists of a row of metal balls suspended on wires (see below). When the ball on one end of the row is made to collide with the one next to it, its momentum transfers straight through the other balls to the ball on the opposite end, which swings out. Schmiedmayer adds that the system enables transport to be studied under perfectly controlled conditions and could open new avenues for understanding how resistance emerges or disappears at the quantum level. “Our next steps are applying the method to strongly correlated transport and to transport in a topological fluid,” he tells Physics World.
Karèn Kheruntsyan, a theoretical physicist at the University of Queensland, Australia, who was not involved in this research, calls it “a significant step for studying quantum transport”. He states that the team’s work clearly demonstrates ballistic (dissipationless) transport at finite temperature, providing an experimental benchmark for theories of integrability and disorder. The work also validates the thermodynamic meaning of Drude weights, while confirming that linear-response theory and GHD accurately describe transport in quantum systems.Read more
In Kheruntsyan’s view, though, the team’s most significant achievement is the quantitative extraction of Drude weights that characterize atomic and energy currents, with “excellent agreement” between experiment and theory. This, he says, shows truly ballistic transport in an interacting many-body system. One caveat, however, is that the system’s limited spatial resolution and near-ideal integrability preclude its use to explore diffusive regimes or more substantial interaction effects, leaving unresolved microscopic dynamics such as dispersive shock waves.
The study is published in Science.
from physicsworld.com 20/12/2025
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