In everyday physics, transport describes how things move from one place to another. Electric charge flows through wires, heat spreads through metal, and water travels through pipes. In each case, scientists can measure how easily charge, energy, or mass moves through a material. Under normal conditions, that movement is slowed by friction and collisions, creating resistance that weakens or eventually stops the flow.

Researchers at TU Wien have now demonstrated a rare exception. In a carefully designed experiment, they observed a physical system in which transport does not degrade at all.

An Ultracold Gas With Perfect Flow

The team confined thousands of rubidium atoms so they could only move along a single straight line, using a combination of magnetic and optical fields. This setup produced an ultracold quantum gas in which both energy and mass move with complete efficiency. According to results published in the journal Science, the flow remains steady and unchanged even after countless atomic collisions. The finding reveals a form of transport that behaves very differently from what is seen in ordinary matter.

Two Fundamental Types of Transport

"In principle, there are two very different types of transport phenomena," says Frederik Møller from the Atominstitut at TU Wien. "We speak of ballistic transport when particles move freely and cover twice the distance in twice the time -- like a bullet traveling in a straight line."

The second type is known as diffusive transport, which occurs when motion is dominated by random collisions. Heat conduction is a classic example. When warmer particles interact with cooler ones, energy and momentum are gradually shared until temperatures even out across the system.

"This kind of transport is not linear," says Møller. "To cover twice the distance, you typically need four times as long."

Why Diffusion Breaks Down in This Experiment

The behavior seen in the TU Wien experiment did not follow either familiar pattern. Instead of spreading out through diffusion, the atomic flow stayed sharply defined. "By studying the atomic current, we could see that diffusion is practically completely suppressed," says Møller. "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."

A Quantum Version of Newton's Cradle

The researchers explain this effect using an analogy to a Newton's cradle, the desktop device with a row of suspended metal balls. When one ball is released, its momentum passes straight through the row and sends the ball at the far end swinging outward, while the others barely move.

"The atoms in our system can only collide along a single direction," explains Møller. "Their momenta are not scattered but simply exchanged between collision partners. Each atom's momentum remains conserved -- it can only be passed on, never lost."