A research team in Vienna has shown that a gas of rubidium atoms, cooled to billionths of a degree and confined to a thin one-dimensional line, can move mass and energy with almost no resistance. They measured the Drude weight, a number that indicates how much of a system’s transport behaves like a perfect conductor, and found that it remains strong even when the atoms collide and the temperature is above absolute zero.
The experiment relies on a one-dimensional Bose gas held in a narrow trap just above absolute zero. Under these conditions, the atoms move in highly controlled ways, allowing researchers to test theories of quantum transport. The team focused on a property called integrability, which appears when a system has many conserved quantities.
In such systems, collisions do not cause particles to lose track of their motion. While most materials quickly lose this order, the Vienna setup holds it long enough for direct observation. The atoms form a quasi-condensate at these temperatures. It resembles a Bose-Einstein condensate but includes some thermal motion, and even with strong interactions the gas continues to flow with almost no slowing.
To test this, the researchers used two methods. In the first, they applied a steady force by tilting the trap. Instead of the current rising and then leveling off, atoms kept shifting to one side at an accelerating rate. This shows that the current increased at a constant rate, which signals a finite Drude weight. The second method prepared the gas in two sections with slightly different densities.
When the barrier was removed, atoms streamed from the higher-density region to the lower-density one, forming two clear waves moving in opposite directions. A machine-learning model that followed conservation laws helped reconstruct the flow and extract the Drude weight again. Both approaches closely matched predictions from generalized hydrodynamics, which treats the gas as a set of fast-moving quasiparticles.
The results show that particle transport scales with density divided by mass, as expected for free particles, even though the atoms interact strongly. For energy transport, the Drude weight grows slowly at low densities and almost linearly at higher densities. Slightly lower values compared with theory come from small distortions in the trap that act like defects, but the overall agreement remains close.
The behavior seen here extends beyond ultracold atoms. Similar physics appears in quantum magnets, spin chains, and certain strongly correlated materials. A system like this gives researchers a clear way to test their models and to study how perfect transport breaks down when small disturbances are added.
As experimental tools improve, scientists can now watch quantum transport unfold in real time, something that was not possible two decades ago. Future work may increase interactions, add spin, or push the system far from equilibrium to see how it responds.
Source: Characterising transport in a quantum gas by measuring Drude weights