Date Time Current trend reversed The demonstration that a tiny cloud of atoms can be turned from a heat engine into a cooler by cranking up the interactions between the particles provides both deep fundamental insight and a possible template for more efficient thermoelectric devices. Artistic impression of the experiment in which Häusler and colleagues first heat one of two quantum-gas clouds and then connect them with a two-dimensional channel, such that they can equilibrate. (Graphics: D. Husmann & S. Häusler, ETH Zurich) When a piece of conducting material is heated up at one of its ends, a voltage difference can build up across the sample, which in turn can be converted into a current. This is the so-called Seebeck effect, the cornerstone of thermoelectric effects. In particular, the effect provides a route to creating work out of a temperature difference. Such thermoelectric engines do not have any movable part and are therefore convenient power sources in various applications, including propelling NASA’s Mars rover Perseverance. The Seebeck effect is interesting for fundamental physics, too, as the magnitude and sign of the induced thermoelectric current is characteristic of the material and indicates how entropy and charge currents are coupled. Writing in Physical Review X, the group of Prof. Tilman Esslinger at the Institute for Quantum Electronics now reports on the controlled reversal of such a current by changing the interaction strength among the constituents of a quantum simulator made of extremely cold atoms trapped in shaped laser fields. The capability to induce such a reversal means that the system can be turned from a thermoelectric engine into a cooler.