A One-Dimensional Quantum Gas Challenges Classical Transport Rules

Frederik Møller, a physicist at the Atominstitut of the Vienna University of Technology, is part of a research team that has demonstrated an unusual form of transport in an ultracold quantum gas. In their recent experiment, atoms confined to a single spatial dimension were shown to carry both mass and energy without measurable resistance, even while undergoing frequent collisions. The result offers a clear example of a quantum regime where familiar assumptions from classical physics no longer hold.

Schüttelkopf, P., Tajik, M., Bazhan, N., Cataldini, F., Ji, S.-C., Schmiedmayer, J., & Møller, F. (2025). Characterizing transport in a quantum gas by measuring Drude weights. Science. https://doi.org/10.1126/science.ads8327

Transport processes underpin many areas of science and engineering. Electric current in a wire, heat flow through a solid, and fluid motion in a pipe all depend on how particles move and interact. Under ordinary conditions, collisions between particles create resistance, causing energy to dissipate and currents to weaken over time. Even materials considered highly conductive are ultimately limited by scattering events.

Frederik Møller, a physicist at the Atominstitut of the Vienna University of Technology stated,

“By studying the atomic current, we could see that diffusion is practically completely suppressed. The gas behaves like a perfect conductor; even though countless collisions occur between the atoms, quantities like mass and energy flow freely, without dissipating into the system.”

The system studied at TU Wien behaves differently. By forcing atoms to move only along a single line, the researchers created conditions in which collisions no longer lead to dissipation. Instead, mass and energy continue to flow steadily, suggesting a near ideal conductor at the quantum scale.

The experiment relied on a gas of rubidium atoms cooled to temperatures close to absolute zero. Magnetic and optical fields were used to restrict the atoms’ motion so tightly that movement was effectively limited to one dimension. In this geometry, atoms cannot pass around one another and can only interact through direct collisions along the line of motion.

Measurements showed that atomic currents remained constant over time, even after the atoms experienced a very large number of collisions. This behavior runs counter to expectations from classical transport theory, where repeated collisions typically result in diffusion and energy loss. In the one dimensional quantum gas, the usual link between collisions and resistance breaks down.

In classical physics, transport is often described as either ballistic or diffusive. Ballistic transport occurs when particles move freely and maintain their velocity. Diffusive transport arises when random collisions scatter particles, causing motion to spread out and slow. The atomic gas created at TU Wien does not fit neatly into either category. Although collisions are frequent, the overall transport resembles ballistic motion.

This behavior can be traced to conservation laws that dominate in one dimensional quantum systems. When atoms collide in this constrained geometry, they exchange momentum rather than dissipating it. Momentum remains within the system and is passed from one atom to another without being randomized. As a result, the collective flow of mass and energy is preserved.

The researchers liken this process to a Newton’s cradle, where momentum transfers through a line of spheres with little apparent loss. In the atomic system, momentum similarly propagates along the chain of atoms. Because there is no transverse direction for momentum to scatter into, it remains confined, allowing motion to persist indefinitely.

This explains why the gas does not thermalize in the usual way. Instead of distributing energy evenly through random motion, the system maintains structured currents over long times. The findings provide direct experimental evidence of dissipation free transport in a many body quantum system.

From an engineering and applied physics perspective, the work offers a valuable platform for studying how resistance emerges from microscopic interactions. By examining a system where resistance is effectively absent, researchers can better isolate the mechanisms that normally lead to energy loss in real materials.

The results also have implications for future quantum technologies. One dimensional systems are central to proposed quantum wires, atomic circuits, and low dimensional materials. Understanding how transport behaves in these regimes may inform strategies for controlling heat and particle flow at very small scales.

The research team plans to explore how slight changes in confinement or interaction strength affect transport behavior. Introducing controlled perturbations could reveal how and when dissipation begins to appear. Although the experimental system is highly idealized, it provides a clean and controlled environment for testing fundamental ideas that are difficult to isolate in conventional materials.

By demonstrating that resistance is not an inevitable consequence of collisions, the study highlights how dimensionality and quantum effects can reshape the basic rules governing motion. Under carefully controlled conditions, quantum systems can behave in ways that challenge long standing expectations rooted in classical physics.